feat(pds): Einladungscodes für createAccount

Die Instanz soll öffentlich erreichbar werden. createAccount hatte bis
jetzt keinerlei Schranke: kein Code, kein Rate-Limit, describeServer
meldete invite_code_required hartkodiert false. Jeder hätte beliebig viele
Konten anlegen können, jedes mit eigenem Repo, Blöcken und
Firehose-Events.

Zwei Tabellen: invite_codes trägt Zähler und Sperre, invite_code_uses
protokolliert, welcher DID welchen Code eingelöst hat — ein Code kann
mehrere Konten wert sein, also ist "wer hat ihn benutzt" eine Menge. Die
DID hat bewusst keinen Fremdschlüssel: das Protokoll soll das Konto
überleben.

Der Kern ist das Einlösen ohne Rennen. Geprüft wird nicht vorher, sondern
im Schreiben selbst:

  UPDATE invite_codes SET used_count = used_count + 1
   WHERE code = $1 AND NOT disabled AND used_count < max_uses
  RETURNING used_count, max_uses

Der Verlierer zweier gleichzeitiger Registrierungen blockiert auf der
Zeilensperre, liest danach die committete Zeile neu, wertet die
WHERE-Klausel erneut aus, trifft nichts und bekommt 400. Ein
Lesen-dann-Schreiben hätte beide durchgelassen — ich habe genau das
probeweise eingebaut, woraufhin die Nebenläufigkeitstests umfielen
("a one-use code let 5 concurrent registrations through").

Das Einlösen ist die erste Anweisung in der bestehenden Transaktion von
create_account: die Zeilensperre hält über die Konto-Inserts, und ein
Rollback gibt den Code wieder frei. Wer ein Handle-Rennen verliert,
verliert nicht auch noch seine Einladung.

Alle Fehlerfälle — fehlend, leer, unbekannt, gesperrt, aufgebraucht —
liefern dieselbe Meldung, damit der unauthentifizierte Endpoint kein
Orakel zum Abtasten des Code-Raums wird.

Codes erzeugt das Binary selbst, statt dafür einen Endpoint zu öffnen:
`pds-server invite create --count 5 --uses 1`, dazu list und disable.

PDS_INVITE_REQUIRED steht auf false per Default — sonst brechen die
Integrationstests und jede Dev-Instanz. Der Server warnt beim Start
deutlich, solange es aus ist, und describeServer meldet jetzt den
tatsächlichen Wert.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013HC9HLrUU1LNwkzp8nkDLX

.env.example

@@ -61,6 +61,14 @@ S3_BUCKET_APPVIEW=maarcadetweet-appview
 PLC_DIRECTORY_URL=https://plc.directory
 # PLC_DIRECTORY_URL=http://127.0.0.1:2582
 
+# --- Registrierung ---
+# Verlangt createAccount einen Einladungscode? Default false, damit Dev-
+# Instanzen und die Integrationstests frei Konten anlegen können — eine
+# öffentlich erreichbare PDS gehört auf true gestellt, sonst kann jeder
+# beliebig viele Repos anlegen. Der Server warnt beim Start, solange es
+# aus ist. Codes erzeugen: `pds-server invite create --count 5 --uses 1`
+PDS_INVITE_REQUIRED=false
+
 # --- AppView ingest auth (optional, dev ok if unset) ---
 # Wenn gesetzt, muss die PDS denselben Wert als Header
 # `X-Ingest-Secret` mitschicken; ist er nicht gesetzt, nimmt

crates/at-shared/src/config.rs

@@ -33,6 +33,36 @@ fn default_pds_firehose_enabled() -> bool {
     true
 }
 
+/// Default for `PDS_INVITE_REQUIRED`.
+///
+/// `false` — `com.atproto.server.createAccount` stays open unless the
+/// operator says otherwise. This is the one security switch in this file
+/// that fails *open*, and it does so for a concrete reason: dozens of
+/// integration tests across `pds-server` and `appview` create throwaway
+/// accounts against a locally running PDS, and every dev instance is
+/// bootstrapped the same way. Defaulting to `true` would break all of
+/// them on the next `cargo test`, and the usual reflex to a suite that
+/// suddenly fails is to switch the new thing off — which lands you at
+/// `false` anyway, only with the flag now looking like the thing that
+/// was in the way rather than the thing that protects the server.
+///
+/// The cost of that choice is that an operator who exposes the PDS
+/// publicly without setting the variable gets an open registration
+/// endpoint. That is paid for at startup: `pds-server` logs a loud
+/// warning on every boot where this is `false`, in the same spirit as
+/// `appview`'s `log_startup_posture`. A warning you have to read once
+/// per restart is the trade for a test suite that keeps working.
+///
+/// That warning also covers the other way this fails open:
+/// [`parse_bool_env`] reads anything it doesn't recognise as `false`, so
+/// `PDS_INVITE_REQUIRED=ture` leaves registration open. The operator
+/// who typed it sees the same startup warning as the operator who never
+/// set the variable at all, which is the only signal that distinguishes
+/// "I meant to leave it open" from "I thought I had closed it".
+fn default_pds_invite_required() -> bool {
+    false
+}
+
 #[derive(Debug, Clone, Deserialize)]
 pub struct AppConfig {
     pub pds_host: String,
@@ -99,6 +129,24 @@ pub struct AppConfig {
     /// [`default_pds_firehose_enabled`] for why.
     #[serde(default = "default_pds_firehose_enabled")]
     pub pds_firehose_enabled: bool,
+    /// Whether `com.atproto.server.createAccount` demands a valid invite
+    /// code. Default `false` — see [`default_pds_invite_required`] for
+    /// why this switch, alone among the security switches here, fails
+    /// open.
+    ///
+    /// When `true`, a request without an `invite_code` (or its
+    /// camelCase `inviteCode` spelling), or with one that is unknown,
+    /// disabled or already used up, is rejected with
+    /// `400 InvalidInviteCode`. The value is also what
+    /// `describeServer` reports as `invite_code_required`, so a client
+    /// can find out before it asks the user for a handle.
+    ///
+    /// Codes are minted out of band with `pds-server invite create`;
+    /// there is no HTTP endpoint that creates them, on purpose — an
+    /// open PDS's registration gate should not come with a second
+    /// public surface that hands out keys to it.
+    #[serde(default = "default_pds_invite_required")]
+    pub pds_invite_required: bool,
 }
 
 impl AppConfig {
@@ -147,6 +195,10 @@ impl AppConfig {
                 .ok()
                 .map(|s| parse_bool_env(&s))
                 .unwrap_or_else(default_pds_firehose_enabled),
+            pds_invite_required: std::env::var("PDS_INVITE_REQUIRED")
+                .ok()
+                .map(|s| parse_bool_env(&s))
+                .unwrap_or_else(default_pds_invite_required),
         })
     }
 

crates/pds-server/src/invite.rs

@@ -0,0 +1,684 @@
+//! Invite codes: minting, listing, and the single redeem operation that
+//! `com.atproto.server.createAccount` calls.
+//!
+//! # Why this exists
+//!
+//! `createAccount` had no gate at all. That was fine while the PDS only
+//! answered on `127.0.0.1:2583`; it is not fine on a public name, because
+//! each accepted account allocates a repo head, a server-held key pair, an
+//! MST that grows with every write, and a stream of firehose events every
+//! subscribed AppView is obliged to index. Invite codes are the smallest
+//! gate that turns "anyone with curl" into "anyone the operator handed a
+//! string to".
+//!
+//! The gate is off by default (`PDS_INVITE_REQUIRED`, see
+//! [`at_shared::config`]) so the existing test suites and dev instances
+//! keep creating throwaway accounts; `main` warns loudly on every boot
+//! where it is off.
+//!
+//! # Where codes come from
+//!
+//! Nowhere over HTTP. Minting lives in the `pds-server invite` subcommand
+//! ([`run_cli`]), which the operator runs on the box. Adding a
+//! `createInviteCode` endpoint would mean the thing that guards
+//! registration is itself reachable by whoever can reach registration —
+//! at which point it guards nothing, and the only question left is
+//! whether *its* auth has a hole. A subcommand has no attack surface to
+//! get wrong.
+//!
+//! # The one interesting piece of code in here
+//!
+//! [`redeem`] is a single conditional `UPDATE … RETURNING`, not a
+//! `SELECT`-then-`UPDATE`. See its docs and
+//! `migrations/pds/0004_invite_codes.sql` for why that distinction is the
+//! whole feature.
+
+use rand::rngs::OsRng;
+use rand::RngCore;
+use sqlx::{PgPool, Postgres, Transaction};
+
+/// Alphabet the codes are drawn from: Crockford base32, lowercased.
+///
+/// Exactly 32 symbols, which is the property that matters — it lets each
+/// character consume exactly 5 bits of entropy with no modulo bias, so
+/// every code in the space is equally likely. A 31- or 36-character
+/// "human friendly" alphabet would need rejection sampling to say the
+/// same thing, and the usual `byte % len` shortcut would quietly make
+/// some characters more probable than others.
+///
+/// The excluded letters are Crockford's: `i`, `l`, `o` and `u`. The
+/// first three are the ones people mistype as `1`, `1` and `0` when
+/// copying a code out of a chat message; `u` is dropped so a random draw
+/// cannot spell something the operator has to apologise for.
+const CODE_ALPHABET: &[u8; 32] = b"0123456789abcdefghjkmnpqrstvwxyz";
+
+/// Characters per group, and groups per code. Two groups of five is
+/// 50 bits of entropy — far past anything an online guesser can reach
+/// against a database round-trip per attempt, and short enough to read
+/// aloud.
+const GROUP_LEN: usize = 5;
+const GROUPS: usize = 2;
+
+/// Fixed prefix so a code is recognisable as one when it turns up out of
+/// context (a support ticket, a pasted log line) and so it cannot be
+/// confused with a handle or a DID.
+const CODE_PREFIX: &str = "mt";
+
+/// Why a redemption was refused.
+///
+/// Deliberately coarse. The route maps [`RedeemError::Invalid`] to a
+/// single `400 InvalidInviteCode` with one fixed message, so an
+/// unauthenticated caller cannot use the error text to distinguish
+/// "no such code" from "that code exists but is used up" — which would
+/// turn the endpoint into an oracle for probing the code space.
+#[derive(Debug)]
+pub enum RedeemError {
+    /// Unknown, disabled, or already at its use limit. One variant on
+    /// purpose: see the type docs.
+    Invalid,
+    /// The database itself failed. Distinct from [`RedeemError::Invalid`]
+    /// because this is a `500`, not a `400` — refusing a legitimate code
+    /// because Postgres hiccuped would be a lie to the user.
+    Db(sqlx::Error),
+}
+
+impl std::fmt::Display for RedeemError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            RedeemError::Invalid => write!(f, "invite code is not valid"),
+            RedeemError::Db(e) => write!(f, "invite lookup failed: {e}"),
+        }
+    }
+}
+
+/// Normalise a client-supplied code into the form stored in the table.
+///
+/// Trims surrounding whitespace (people paste codes with a trailing
+/// newline out of a terminal) and lowercases. Every generated code is
+/// already lowercase ASCII, so this is an exact normalisation — which is
+/// what lets [`redeem`] look the code up with a plain `code = $1` and
+/// hit the primary-key index, instead of `lower(code) = $1`, which
+/// would force a sequential scan on the one query that runs per
+/// registration attempt.
+///
+/// Returns `None` for a code that is empty after trimming, so "field
+/// present but blank" and "field absent" reach the route as the same
+/// case.
+pub fn normalize(raw: &str) -> Option<String> {
+    let trimmed = raw.trim();
+    if trimmed.is_empty() {
+        return None;
+    }
+    Some(trimmed.to_ascii_lowercase())
+}
+
+/// Generate one cryptographically random invite code, e.g.
+/// `mt-7k3qw-z9d2m`.
+///
+/// Randomness comes from [`OsRng`] — the same source
+/// `keys::generate_user_keys` and `password::hash_password` already use
+/// in this crate, i.e. the OS CSPRNG, never a seeded or thread-local
+/// generator. A code is a bearer credential for creating an account on
+/// this server; a predictable one is the same bug as a predictable
+/// password-reset token.
+///
+/// Entropy: [`GROUPS`] × [`GROUP_LEN`] characters × 5 bits = 50 bits.
+pub fn generate_code() -> String {
+    let total = GROUPS * GROUP_LEN;
+    let mut bytes = vec![0u8; total];
+    OsRng.fill_bytes(&mut bytes);
+
+    let mut out = String::with_capacity(CODE_PREFIX.len() + total + GROUPS);
+    out.push_str(CODE_PREFIX);
+    for chunk in bytes.chunks(GROUP_LEN) {
+        out.push('-');
+        for b in chunk {
+            // Take the low 5 bits of a uniformly random byte. The
+            // alphabet is exactly 32 symbols, so this is a bijection
+            // from 5 bits onto it — no bias, no rejection loop.
+            out.push(CODE_ALPHABET[(*b & 0b0001_1111) as usize] as char);
+        }
+    }
+    out
+}
+
+/// Consume one use of `code` on behalf of the account `did` / `handle`.
+///
+/// **This must be called with the same transaction that creates the
+/// account.** The whole point is that a code is spent if and only if an
+/// account was actually created: if the caller's transaction rolls back
+/// — handle taken, key generation failed, anything — the counter goes
+/// back with it and the code is still redeemable. `create_account`
+/// therefore begins its transaction, redeems here, inserts `users` and
+/// `repos`, and only then commits.
+///
+/// # The race, and why there isn't one
+///
+/// The tempting implementation is: `SELECT used_count, max_uses …`,
+/// compare in Rust, then `UPDATE`. That is a check-then-act. Two
+/// registrations arriving together on a code with one use left both read
+/// `used_count = 0`, both conclude they may proceed, and both write
+/// `used_count = 1`. Two accounts, one use — and the row afterwards
+/// claims it was redeemed once, so nothing even shows up as wrong.
+///
+/// Instead the check *is* the write:
+///
+/// ```sql
+/// UPDATE invite_codes
+///    SET used_count = used_count + 1
+///  WHERE code = $1 AND NOT disabled AND used_count < max_uses
+/// RETURNING used_count, max_uses
+/// ```
+///
+/// Postgres serialises the two statements on the row lock. The loser
+/// blocks until the winner commits, and then — this is the part that
+/// makes it work — does *not* continue with its old snapshot: it
+/// re-fetches the committed row and re-evaluates the `WHERE` clause
+/// against it (EvalPlanQual). `used_count` is now `1`, the predicate is
+/// false, the row is dropped from the update set, and the statement
+/// affects zero rows. Zero rows is the rejection. This function never
+/// forms an opinion about validity that could be stale by the time it
+/// acts on it, because it never looks before it writes.
+///
+/// The `invite_code_uses` insert that follows is inside the same
+/// transaction and the same row lock, so the counter and the audit rows
+/// cannot drift apart.
+pub async fn redeem(
+    tx: &mut Transaction<'_, Postgres>,
+    code: &str,
+    did: &str,
+    handle: &str,
+) -> Result<(), RedeemError> {
+    let normalized = match normalize(code) {
+        Some(c) => c,
+        None => return Err(RedeemError::Invalid),
+    };
+
+    // One statement, and its row count is the verdict.
+    let claimed: Option<(i32, i32)> = sqlx::query_as(
+        r#"UPDATE invite_codes
+              SET used_count = used_count + 1
+            WHERE code = $1
+              AND NOT disabled
+              AND used_count < max_uses
+        RETURNING used_count, max_uses"#,
+    )
+    .bind(&normalized)
+    .fetch_optional(&mut **tx)
+    .await
+    .map_err(RedeemError::Db)?;
+
+    if claimed.is_none() {
+        return Err(RedeemError::Invalid);
+    }
+
+    // Audit trail: which account this code produced. Same transaction,
+    // so it lands exactly when the counter increment does.
+    //
+    // The `(code, did)` primary key makes a duplicate impossible; a
+    // conflict here would mean the same DID redeemed the same code
+    // twice in one registration, which cannot happen but would corrupt
+    // the counter/uses agreement if it did — so let it be an error
+    // rather than silently ignoring it.
+    sqlx::query(
+        r#"INSERT INTO invite_code_uses (code, did, handle)
+           VALUES ($1, $2, $3)"#,
+    )
+    .bind(&normalized)
+    .bind(did)
+    .bind(handle)
+    .execute(&mut **tx)
+    .await
+    .map_err(RedeemError::Db)?;
+
+    Ok(())
+}
+
+// =====================================================
+// CLI: `pds-server invite …`
+// =====================================================
+
+/// One row of `invite list`, and the shape `create` hands back.
+#[derive(Debug)]
+pub struct InviteRow {
+    pub code: String,
+    pub max_uses: i32,
+    pub used_count: i32,
+    pub disabled: bool,
+    pub note: Option<String>,
+    pub created_at: chrono::DateTime<chrono::Utc>,
+}
+
+/// What a parsed `invite` command line asks for.
+///
+/// Parsed out of `std::env::args` by hand. The workspace has no
+/// argument-parsing dependency and this subcommand is not worth adding
+/// one for: three verbs, three flags, and a hand-rolled parser that is
+/// small enough to unit-test exhaustively (which it is, below) beats a
+/// derive macro plus a new crate in the dependency tree of a server
+/// binary.
+#[derive(Debug, PartialEq)]
+pub enum InviteCommand {
+    /// `invite create [--count N] [--uses N] [--note TEXT]`
+    Create {
+        count: u32,
+        uses: i32,
+        note: Option<String>,
+    },
+    /// `invite list [--all]` — without `--all`, spent and disabled codes
+    /// are hidden, because the question the operator almost always has
+    /// is "what can I still hand out".
+    List { all: bool },
+    /// `invite disable <code>` — stop honouring a code without losing
+    /// the record of which accounts it already created.
+    Disable { code: String },
+}
+
+/// Usage text. Printed for `invite help`, and for anything that fails to
+/// parse.
+pub const INVITE_USAGE: &str = "\
+usage: pds-server invite <command>
+
+  create [--count N] [--uses N] [--note TEXT]
+      Mint N codes (default 1), each good for `--uses` accounts
+      (default 1). Prints one code per line and nothing else, so the
+      output can be piped or pasted directly.
+
+  list [--all]
+      Show codes that can still be redeemed. --all includes spent and
+      disabled ones.
+
+  disable <code>
+      Stop honouring a code. The record of accounts it already created
+      is kept.
+
+The database is the one named by DATABASE_URL_PDS (read from .env like
+the server does). Codes are only meaningful while PDS_INVITE_REQUIRED
+is true.";
+
+/// Parse the arguments after the `invite` verb.
+///
+/// Returns `Err(message)` for anything malformed; the caller prints the
+/// message plus [`INVITE_USAGE`] and exits non-zero. Unknown flags are
+/// an error rather than being ignored — a typo'd `--uses` that silently
+/// became `1` would hand out the wrong codes and the operator would only
+/// find out when the second person to use one got a `400`.
+pub fn parse_invite_args(args: &[String]) -> Result<InviteCommand, String> {
+    let verb = args
+        .first()
+        .map(|s| s.as_str())
+        .ok_or_else(|| "missing invite command".to_string())?;
+    let rest = &args[1..];
+
+    match verb {
+        "create" => {
+            let mut count: u32 = 1;
+            let mut uses: i32 = 1;
+            let mut note: Option<String> = None;
+            let mut i = 0;
+            while i < rest.len() {
+                match rest[i].as_str() {
+                    "--count" => {
+                        let v = rest
+                            .get(i + 1)
+                            .ok_or_else(|| "--count needs a value".to_string())?;
+                        count = v
+                            .parse()
+                            .map_err(|_| format!("--count: not a number: {v}"))?;
+                        if count == 0 {
+                            return Err("--count must be at least 1".to_string());
+                        }
+                        i += 2;
+                    }
+                    "--uses" => {
+                        let v = rest
+                            .get(i + 1)
+                            .ok_or_else(|| "--uses needs a value".to_string())?;
+                        uses = v
+                            .parse()
+                            .map_err(|_| format!("--uses: not a number: {v}"))?;
+                        // Mirrors the table's CHECK (max_uses > 0). Caught
+                        // here so the operator gets a sentence instead of a
+                        // constraint-violation dump.
+                        if uses < 1 {
+                            return Err("--uses must be at least 1".to_string());
+                        }
+                        i += 2;
+                    }
+                    "--note" => {
+                        let v = rest
+                            .get(i + 1)
+                            .ok_or_else(|| "--note needs a value".to_string())?;
+                        note = Some(v.clone());
+                        i += 2;
+                    }
+                    other => return Err(format!("unknown option for `create`: {other}")),
+                }
+            }
+            Ok(InviteCommand::Create { count, uses, note })
+        }
+        "list" => {
+            let mut all = false;
+            for a in rest {
+                match a.as_str() {
+                    "--all" => all = true,
+                    other => return Err(format!("unknown option for `list`: {other}")),
+                }
+            }
+            Ok(InviteCommand::List { all })
+        }
+        "disable" => {
+            let code = rest
+                .first()
+                .ok_or_else(|| "disable needs a code".to_string())?;
+            if rest.len() > 1 {
+                return Err("disable takes exactly one code".to_string());
+            }
+            let code = normalize(code).ok_or_else(|| "disable needs a code".to_string())?;
+            Ok(InviteCommand::Disable { code })
+        }
+        other => Err(format!("unknown invite command: {other}")),
+    }
+}
+
+/// Insert `count` freshly generated codes, each good for `uses`
+/// accounts.
+///
+/// Retries on a primary-key collision. With 50 bits per code a
+/// collision is not something that will happen, but "not something that
+/// will happen" is exactly the class of event that turns into a
+/// confusing `duplicate key` traceback at 2am, and the retry costs three
+/// lines.
+pub async fn create_codes(
+    db: &PgPool,
+    count: u32,
+    uses: i32,
+    note: Option<&str>,
+) -> anyhow::Result<Vec<String>> {
+    let mut out = Vec::with_capacity(count as usize);
+    for _ in 0..count {
+        let mut attempt = 0;
+        loop {
+            let code = generate_code();
+            let inserted = sqlx::query(
+                r#"INSERT INTO invite_codes (code, max_uses, note)
+                   VALUES ($1, $2, $3)
+                   ON CONFLICT (code) DO NOTHING"#,
+            )
+            .bind(&code)
+            .bind(uses)
+            .bind(note)
+            .execute(db)
+            .await?
+            .rows_affected();
+            if inserted == 1 {
+                out.push(code);
+                break;
+            }
+            attempt += 1;
+            if attempt >= 5 {
+                anyhow::bail!("could not find a free invite code after 5 attempts");
+            }
+        }
+    }
+    Ok(out)
+}
+
+/// Read back codes for `invite list`.
+pub async fn list_codes(db: &PgPool, all: bool) -> anyhow::Result<Vec<InviteRow>> {
+    // Two statements rather than one with a `$1`-toggled predicate:
+    // the redeemable filter is exactly the redeem query's `WHERE`
+    // clause, and keeping it spelled the same way makes it obvious that
+    // `list` shows what `redeem` would accept.
+    let sql = if all {
+        r#"SELECT code, max_uses, used_count, disabled, note, created_at
+             FROM invite_codes
+         ORDER BY created_at DESC"#
+    } else {
+        r#"SELECT code, max_uses, used_count, disabled, note, created_at
+             FROM invite_codes
+            WHERE NOT disabled AND used_count < max_uses
+         ORDER BY created_at DESC"#
+    };
+    let rows: Vec<(String, i32, i32, bool, Option<String>, chrono::DateTime<chrono::Utc>)> =
+        sqlx::query_as(sql).fetch_all(db).await?;
+    Ok(rows
+        .into_iter()
+        .map(
+            |(code, max_uses, used_count, disabled, note, created_at)| InviteRow {
+                code,
+                max_uses,
+                used_count,
+                disabled,
+                note,
+                created_at,
+            },
+        )
+        .collect())
+}
+
+/// Flip `disabled` on one code. Returns `false` if there is no such
+/// code, so the CLI can say so instead of reporting a successful no-op.
+pub async fn disable_code(db: &PgPool, code: &str) -> anyhow::Result<bool> {
+    let n = sqlx::query("UPDATE invite_codes SET disabled = TRUE WHERE code = $1")
+        .bind(code)
+        .execute(db)
+        .await?
+        .rows_affected();
+    Ok(n == 1)
+}
+
+/// Run the `invite` subcommand end to end: parse, connect, act, print.
+///
+/// Connects with the same `DATABASE_URL_PDS` and runs the same
+/// migrations as [`crate::main`], so `invite create` works on a fresh
+/// checkout before the server has ever been started — otherwise the
+/// first thing an operator does after deploying would fail with
+/// "relation invite_codes does not exist".
+pub async fn run_cli(args: &[String]) -> anyhow::Result<()> {
+    if matches!(args.first().map(|s| s.as_str()), None | Some("help") | Some("-h") | Some("--help"))
+    {
+        println!("{INVITE_USAGE}");
+        return Ok(());
+    }
+    let cmd = match parse_invite_args(args) {
+        Ok(c) => c,
+        Err(msg) => {
+            eprintln!("pds-server invite: {msg}\n\n{INVITE_USAGE}");
+            std::process::exit(2);
+        }
+    };
+
+    let cfg = at_shared::config::AppConfig::from_env()?;
+    let db = sqlx::postgres::PgPoolOptions::new()
+        .max_connections(2)
+        .acquire_timeout(std::time::Duration::from_secs(10))
+        .connect(&cfg.database_url_pds)
+        .await?;
+    sqlx::migrate!("../../migrations/pds").run(&db).await?;
+
+    match cmd {
+        InviteCommand::Create { count, uses, note } => {
+            let codes = create_codes(&db, count, uses, note.as_deref()).await?;
+            // Bare codes, one per line, nothing else on stdout — the
+            // operator pipes this into a message or a file. Anything
+            // decorative here would have to be stripped by hand.
+            for c in &codes {
+                println!("{c}");
+            }
+            if !cfg.pds_invite_required {
+                // Not an error: minting codes before flipping the switch
+                // is the correct order of operations. But an operator
+                // who thinks they have just closed registration should
+                // find out now.
+                eprintln!(
+                    "note: PDS_INVITE_REQUIRED is not true — createAccount currently \
+                     accepts requests without any code."
+                );
+            }
+        }
+        InviteCommand::List { all } => {
+            let rows = list_codes(&db, all).await?;
+            if rows.is_empty() {
+                eprintln!("no invite codes");
+            }
+            for r in rows {
+                let state = if r.disabled {
+                    "disabled"
+                } else if r.used_count >= r.max_uses {
+                    "spent"
+                } else {
+                    "open"
+                };
+                println!(
+                    "{}  {}/{}  {}  {}  {}",
+                    r.code,
+                    r.used_count,
+                    r.max_uses,
+                    state,
+                    r.created_at.format("%Y-%m-%dT%H:%M:%SZ"),
+                    r.note.as_deref().unwrap_or("")
+                );
+            }
+        }
+        InviteCommand::Disable { code } => {
+            if disable_code(&db, &code).await? {
+                println!("disabled {code}");
+            } else {
+                eprintln!("no such invite code: {code}");
+                std::process::exit(1);
+            }
+        }
+    }
+    Ok(())
+}
+
+// -- tests -------------------------------------------------------------------
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::collections::HashSet;
+
+    #[test]
+    fn generated_codes_use_only_the_safe_alphabet() {
+        let code = generate_code();
+        // `mt-xxxxx-xxxxx`
+        assert!(code.starts_with("mt-"), "code = {code}");
+        let groups: Vec<&str> = code.split('-').collect();
+        assert_eq!(groups.len(), GROUPS + 1, "code = {code}");
+        assert_eq!(groups[0], CODE_PREFIX);
+        for g in &groups[1..] {
+            assert_eq!(g.len(), GROUP_LEN, "group {g} in {code}");
+            for ch in g.chars() {
+                assert!(
+                    CODE_ALPHABET.contains(&(ch as u8)),
+                    "character {ch:?} in {code} is outside the alphabet"
+                );
+            }
+        }
+        // The letters people mistype must never appear.
+        for bad in ['i', 'l', 'o', 'u'] {
+            assert!(!code[3..].contains(bad), "{code} contains {bad}");
+        }
+    }
+
+    #[test]
+    fn generated_codes_do_not_repeat() {
+        // Not a randomness test — a 1000-draw collision would mean the
+        // generator is returning a constant or reusing a seeded RNG,
+        // which is the failure mode that actually happens when someone
+        // swaps `OsRng` for `thread_rng` with a fixed seed in a test
+        // helper.
+        let mut seen = HashSet::new();
+        for _ in 0..1000 {
+            assert!(seen.insert(generate_code()), "duplicate code in 1000 draws");
+        }
+    }
+
+    #[test]
+    fn normalize_trims_and_lowercases() {
+        assert_eq!(normalize("  MT-ABCDE-FGHJK \n").as_deref(), Some("mt-abcde-fghjk"));
+        assert_eq!(normalize("mt-abcde-fghjk").as_deref(), Some("mt-abcde-fghjk"));
+        // Absent and blank must be indistinguishable to the route.
+        assert_eq!(normalize(""), None);
+        assert_eq!(normalize("   "), None);
+        assert_eq!(normalize("\t\n"), None);
+    }
+
+    fn args(v: &[&str]) -> Vec<String> {
+        v.iter().map(|s| s.to_string()).collect()
+    }
+
+    #[test]
+    fn parses_create_with_defaults_and_flags() {
+        assert_eq!(
+            parse_invite_args(&args(&["create"])).unwrap(),
+            InviteCommand::Create {
+                count: 1,
+                uses: 1,
+                note: None
+            }
+        );
+        assert_eq!(
+            parse_invite_args(&args(&["create", "--count", "5", "--uses", "1"])).unwrap(),
+            InviteCommand::Create {
+                count: 5,
+                uses: 1,
+                note: None
+            }
+        );
+        // Order must not matter, and --note takes the next argument
+        // verbatim (spaces included).
+        assert_eq!(
+            parse_invite_args(&args(&["create", "--note", "meetup 2026", "--uses", "3"])).unwrap(),
+            InviteCommand::Create {
+                count: 1,
+                uses: 3,
+                note: Some("meetup 2026".to_string())
+            }
+        );
+    }
+
+    #[test]
+    fn rejects_malformed_create_flags() {
+        // A typo'd flag must not be silently ignored — that would hand
+        // out codes with the default limits.
+        assert!(parse_invite_args(&args(&["create", "--use", "3"])).is_err());
+        assert!(parse_invite_args(&args(&["create", "--count"])).is_err());
+        assert!(parse_invite_args(&args(&["create", "--count", "x"])).is_err());
+        assert!(parse_invite_args(&args(&["create", "--count", "0"])).is_err());
+        // max_uses > 0 is a table constraint; catch it before Postgres does.
+        assert!(parse_invite_args(&args(&["create", "--uses", "0"])).is_err());
+        assert!(parse_invite_args(&args(&["create", "--uses", "-2"])).is_err());
+    }
+
+    #[test]
+    fn parses_list_and_disable() {
+        assert_eq!(
+            parse_invite_args(&args(&["list"])).unwrap(),
+            InviteCommand::List { all: false }
+        );
+        assert_eq!(
+            parse_invite_args(&args(&["list", "--all"])).unwrap(),
+            InviteCommand::List { all: true }
+        );
+        assert!(parse_invite_args(&args(&["list", "--everything"])).is_err());
+        // `disable` normalises the code the same way redeem does, so an
+        // operator pasting a shouted code still disables the right row.
+        assert_eq!(
+            parse_invite_args(&args(&["disable", " MT-ABCDE-FGHJK "])).unwrap(),
+            InviteCommand::Disable {
+                code: "mt-abcde-fghjk".to_string()
+            }
+        );
+        assert!(parse_invite_args(&args(&["disable"])).is_err());
+        assert!(parse_invite_args(&args(&["disable", "a", "b"])).is_err());
+    }
+
+    #[test]
+    fn rejects_unknown_verb_and_empty_args() {
+        assert!(parse_invite_args(&args(&[])).is_err());
+        assert!(parse_invite_args(&args(&["destroy"])).is_err());
+    }
+}

crates/pds-server/src/main.rs

@@ -2,6 +2,7 @@ mod appview_push;
 mod car;
 mod dag_cbor;
 mod firehose;
+mod invite;
 mod jwt_issuer;
 mod keys;
 mod password;
@@ -17,6 +18,16 @@ use serde_json::json;
 use tracing::{info, warn};
 use tracing_subscriber::EnvFilter;
 
+/// Usage line for the binary itself. The subcommands are operator
+/// tooling; the no-argument form is the server, which is what every
+/// deploy script and systemd unit invokes.
+const USAGE: &str = "\
+usage: pds-server [command]
+
+  (no command)   run the PDS server
+  invite …       manage invite codes (see `pds-server invite help`)
+  help           show this message";
+
 #[tokio::main]
 async fn main() -> anyhow::Result<()> {
     // Load `.env` from the working directory (and upwards) if present.
@@ -25,11 +36,57 @@ async fn main() -> anyhow::Result<()> {
     // PDS_HOST`. Real environment variables always win over the file.
     let _ = dotenvy::dotenv();
 
-    tracing_subscriber::fmt()
+    // Argument dispatch, by hand.
-        .with_env_filter(EnvFilter::try_from_default_env().unwrap_or_else(|_| EnvFilter::new("info")))
+    //
-        .init();
+    // The workspace carries no argument-parsing crate and this does not
+    // justify adding one: exactly one subcommand exists, and the
+    // overwhelmingly common invocation is the bare binary. Anything we
+    // do not recognise is an error rather than being ignored — a
+    // mistyped `pds-server invit create` that silently booted a server
+    // would look like it worked and mint no codes.
+    let args: Vec<String> = std::env::args().skip(1).collect();
+    match args.first().map(|s| s.as_str()) {
+        None => {}
+        Some("invite") => {
+            // CLI output is meant to be read and pasted, so keep the
+            // log stream quiet unless the operator asked for it. Without
+            // this, `sqlx::migrate` chatters over the codes.
+            init_tracing("warn");
+            return invite::run_cli(&args[1..]).await;
+        }
+        Some("help") | Some("-h") | Some("--help") => {
+            println!("{USAGE}");
+            return Ok(());
+        }
+        Some(other) => {
+            eprintln!("pds-server: unknown command: {other}\n\n{USAGE}");
+            std::process::exit(2);
+        }
+    }
+
+    init_tracing("info");
 
     let cfg = at_shared::config::AppConfig::from_env()?;
+
+    // Announce the relaxed security posture before we bind a port.
+    //
+    // `PDS_INVITE_REQUIRED` is the only switch in `AppConfig` that
+    // defaults to *open* (so the integration suites and dev instances
+    // can keep creating throwaway accounts), which makes this warning
+    // the only thing standing between "we made the PDS public" and
+    // "anyone on the internet can mint repos on our disk". Mirrors
+    // `appview`'s `auth::log_startup_posture`.
+    if !cfg.pds_invite_required {
+        warn!(
+            "PDS_INVITE_REQUIRED is not true — com.atproto.server.createAccount accepts \
+             ANY caller, and every accepted account allocates a repo, a server-held key \
+             pair and firehose events. Fine on a private/dev instance; on a publicly \
+             reachable PDS set PDS_INVITE_REQUIRED=true and hand out codes with \
+             `pds-server invite create`."
+        );
+    } else {
+        info!("PDS_INVITE_REQUIRED=true — createAccount requires a valid invite code");
+    }
     let db = sqlx::postgres::PgPoolOptions::new()
         .max_connections(32)
         .min_connections(2)
@@ -71,6 +128,21 @@ async fn main() -> anyhow::Result<()> {
     Ok(())
 }
 
+/// Install the tracing subscriber, with `default_filter` as the level
+/// when `RUST_LOG` says nothing.
+///
+/// Factored out because the two entry points want different defaults:
+/// the server wants `info`, the `invite` subcommand wants `warn` so that
+/// migration chatter does not land in the middle of a list of codes the
+/// operator is about to copy.
+fn init_tracing(default_filter: &str) {
+    tracing_subscriber::fmt()
+        .with_env_filter(
+            EnvFilter::try_from_default_env().unwrap_or_else(|_| EnvFilter::new(default_filter)),
+        )
+        .init();
+}
+
 pub fn router(state: AppState) -> Router {
     Router::new()
         .route("/", get(root))
@@ -241,7 +313,12 @@ async fn describe_server(State(state): State<AppState>) -> Json<DescribeServerRe
             .pds_handle_dns_zone
             .trim_start_matches('.')
             .to_string()],
-        invite_code_required: false,
+        // The real switch, not a hardcoded `false`. A client reads this
+        // to decide whether to ask the user for a code *before*
+        // collecting a handle and password — advertising `false` on a
+        // server that then answers `400 InvalidInviteCode` sends the
+        // user back to the start of a form they already filled in.
+        invite_code_required: state.cfg.pds_invite_required,
         links: json!({
             "termsOfService": null,
             "privacyPolicy": null,

crates/pds-server/src/routes/auth.rs

@@ -93,6 +93,55 @@ pub async fn create_account(
 
     let mut tx = state.db.begin().await.map_err(|e| internal(e))?;
 
+    // Invite gate.
+    //
+    // Inside the transaction, and *first* inside it, for two reasons.
+    //
+    // Inside, because the code must be spent if and only if an account
+    // was really created. Redeeming before `begin()` (or in a
+    // transaction of its own) would burn a code every time the INSERT
+    // below hit the `users.handle` unique index — a user who lost a
+    // handle race would also lose their invite, with nothing to show
+    // for it. Everything from here to `tx.commit()` rolls back together.
+    //
+    // First, because `invite::redeem` takes the code row's lock, and
+    // holding it across the account INSERTs is what serialises two
+    // registrations that present the same last remaining use. See
+    // `invite::redeem` for how the conditional UPDATE turns that lock
+    // into a correct decision rather than a stale one.
+    //
+    // Note this runs after the handle/password validation above, so a
+    // malformed request is rejected without touching a code at all.
+    if state.cfg.pds_invite_required {
+        let supplied = req
+            .invite_code
+            .as_deref()
+            .and_then(crate::invite::normalize);
+        match supplied {
+            None => return Err(invalid_invite_code()),
+            Some(code) => {
+                if let Err(e) =
+                    crate::invite::redeem(&mut tx, &code, &did, &req.handle).await
+                {
+                    return match e {
+                        crate::invite::RedeemError::Invalid => {
+                            // Deliberately not logged with the code at
+                            // info level: a public endpoint that echoes
+                            // every guessed code into the log is a way
+                            // to fill the disk from outside.
+                            warn!(
+                                handle = %req.handle,
+                                "createAccount rejected: invite code invalid, disabled or spent"
+                            );
+                            Err(invalid_invite_code())
+                        }
+                        crate::invite::RedeemError::Db(db_err) => Err(internal(db_err)),
+                    };
+                }
+            }
+        }
+    }
+
     sqlx::query(
         r#"INSERT INTO users (did, handle, email, password_hash, signing_key, rotation_key)
            VALUES ($1, $2, $3, $4, $5, $6)"#,
@@ -306,6 +355,28 @@ pub async fn refresh_session(
     }))
 }
 
+/// The one error a failed invite check produces.
+///
+/// Same `(StatusCode, Json<ErrorBody>)` shape every other route in this
+/// module returns, so a client parses it with the code it already has:
+/// `{"error": "InvalidInviteCode", "message": "..."}` under a `400`.
+///
+/// One message for every failure mode — missing, unknown, disabled,
+/// spent — on purpose. A distinct "that code exists but is used up"
+/// would let an unauthenticated caller walk the code space and learn
+/// which strings are real, which is most of the work of stealing one.
+/// The operator can tell the cases apart from `pds-server invite list`;
+/// the internet cannot.
+fn invalid_invite_code() -> (StatusCode, Json<crate::routes::types::ErrorBody>) {
+    (
+        StatusCode::BAD_REQUEST,
+        Json(crate::routes::types::ErrorBody::new(
+            "InvalidInviteCode",
+            Some("a valid invite code is required to create an account on this server".into()),
+        )),
+    )
+}
+
 fn internal(e: impl std::fmt::Display) -> (StatusCode, Json<crate::routes::types::ErrorBody>) {
     (
         StatusCode::INTERNAL_SERVER_ERROR,

crates/pds-server/src/routes/types.rs

@@ -6,6 +6,18 @@ pub struct CreateAccountReq {
     pub email: Option<String>,
     pub password: Option<String>,
     pub did: Option<String>,
+    /// The invite code, when `PDS_INVITE_REQUIRED` is on.
+    ///
+    /// The alias is not cosmetic. This struct — like every other type in
+    /// this module — is snake_case on the wire, which is what our own
+    /// clients send. The AT Protocol lexicon for
+    /// `com.atproto.server.createAccount` spells the field `inviteCode`,
+    /// so every off-the-shelf atproto client sends *that*, and without
+    /// the alias serde would drop it into `None` silently — the account
+    /// would be refused with "an invite code is required" while the user
+    /// is looking at the code they just pasted. Accepting both spellings
+    /// costs one attribute; debugging that report costs an afternoon.
+    #[serde(alias = "inviteCode")]
     pub invite_code: Option<String>,
     pub recovery_key: Option<String>,
 }

crates/pds-server/tests/invite_integration.rs

@@ -0,0 +1,793 @@
+//! Invite-code enforcement on `com.atproto.server.createAccount`.
+//!
+//! # Why this file starts its own PDS
+//!
+//! Every other integration suite in this crate talks to whatever PDS the
+//! developer already has running on `:2583` and skips itself when there
+//! isn't one. That works because those tests only need *a* PDS. These
+//! need a PDS with `PDS_INVITE_REQUIRED=true`, and the ambient one is
+//! (correctly) started with the default `false` — otherwise every other
+//! suite, which creates throwaway accounts with no code, would fail.
+//!
+//! Asking the developer to restart their PDS with a different flag
+//! before this file passes would mean the flag's behaviour is only ever
+//! tested by hand. So each test here spawns its own `pds-server` on a
+//! free port with the flag set the way that test needs it, and kills it
+//! on the way out ([`Pds`]'s `Drop`). `env!("CARGO_BIN_EXE_pds-server")`
+//! is cargo's own path to the binary it just built for this test run, so
+//! the process under test is always the current code.
+//!
+//! The suite still fails open, in the same spirit as its neighbours: if
+//! the child never becomes healthy — no Postgres, no `.env`, no
+//! `DATABASE_URL_PDS` — the tests print why and return green rather than
+//! failing a workstation that simply isn't running the stack.
+
+use serde_json::{json, Value};
+use std::process::{Child, Command};
+use std::time::Duration;
+
+/// A `pds-server` child process bound to its own port, killed when the
+/// test that started it goes out of scope.
+///
+/// The `Drop` impl is the reason this is a struct at all: a test that
+/// panics mid-way must not leave a server holding a port and a pool of
+/// Postgres connections for the rest of the run.
+struct Pds {
+    child: Child,
+    port: u16,
+    http: reqwest::Client,
+}
+
+impl Drop for Pds {
+    fn drop(&mut self) {
+        let _ = self.child.kill();
+        let _ = self.child.wait();
+    }
+}
+
+impl Pds {
+    fn url(&self, path: &str) -> String {
+        format!("http://127.0.0.1:{}{}", self.port, path)
+    }
+
+    async fn create_account(&self, body: Value) -> (u16, Value) {
+        let resp = self
+            .http
+            .post(self.url("/xrpc/com.atproto.server.createAccount"))
+            .json(&body)
+            .send()
+            .await
+            .expect("createAccount request");
+        let status = resp.status().as_u16();
+        let body: Value = resp.json().await.unwrap_or(Value::Null);
+        (status, body)
+    }
+
+    async fn describe(&self) -> Value {
+        self.http
+            .get(self.url("/xrpc/com.atproto.server.describeServer"))
+            .send()
+            .await
+            .expect("describeServer")
+            .json()
+            .await
+            .expect("describeServer json")
+    }
+}
+
+/// Ask the OS for a port nobody is using, then let go of it.
+///
+/// There is a window between the drop and the child's `bind` in which
+/// something else could take the port; on a test machine that window is
+/// theoretical, and the alternative (a fixed port) would make two
+/// concurrently running tests in this file collide *reliably* instead of
+/// never.
+fn free_port() -> Option<u16> {
+    let l = std::net::TcpListener::bind("127.0.0.1:0").ok()?;
+    let p = l.local_addr().ok()?.port();
+    drop(l);
+    Some(p)
+}
+
+/// Start a `pds-server` with `PDS_INVITE_REQUIRED` set to `required`.
+///
+/// Returns `None` when the stack this needs isn't available, which the
+/// callers turn into a skip. The child inherits the ambient environment
+/// (so `DATABASE_URL_PDS` and friends come from `.env` exactly as they
+/// do for the real server — `dotenvy` does not override real variables,
+/// so our overrides below win).
+async fn start_pds(required: bool) -> Option<Pds> {
+    let port = free_port()?;
+    let child = Command::new(env!("CARGO_BIN_EXE_pds-server"))
+        .env("PDS_HOST", "127.0.0.1")
+        .env("PDS_PORT", port.to_string())
+        .env("PDS_PUBLIC_URL", format!("http://127.0.0.1:{port}"))
+        .env("PDS_INVITE_REQUIRED", if required { "true" } else { "false" })
+        // Point the PLC submit at a closed port. `create_account`
+        // tolerates a failed submit by design (the DID is computed
+        // locally), and a connection refused on loopback fails in
+        // microseconds — whereas the real directory would add a network
+        // round-trip to every account this file creates, and might
+        // actually publish throwaway test DIDs.
+        .env("PLC_DIRECTORY_URL", "http://127.0.0.1:1")
+        .env("RUST_LOG", "warn")
+        .stdout(std::process::Stdio::null())
+        .stderr(std::process::Stdio::null())
+        .spawn()
+        .ok()?;
+
+    let http = reqwest::Client::builder()
+        .timeout(Duration::from_secs(30))
+        .build()
+        .ok()?;
+    let mut pds = Pds { child, port, http };
+
+    for _ in 0..80 {
+        // If the child already exited (bad/missing env, no Postgres),
+        // stop waiting — there is nothing to become healthy.
+        if let Ok(Some(_)) = pds.child.try_wait() {
+            return None;
+        }
+        if let Ok(r) = pds.http.get(pds.url("/healthz")).send().await {
+            if r.status().is_success() {
+                return Some(pds);
+            }
+        }
+        tokio::time::sleep(Duration::from_millis(150)).await;
+    }
+    None
+}
+
+async fn db_pool() -> Option<sqlx::PgPool> {
+    let url = std::env::var("DATABASE_URL_PDS")
+        .unwrap_or_else(|_| "postgres://pds:pds@127.0.0.1:5434/pds".to_string());
+    sqlx::postgres::PgPoolOptions::new()
+        .max_connections(4)
+        .acquire_timeout(Duration::from_secs(3))
+        .connect(&url)
+        .await
+        .ok()
+}
+
+/// Put a code straight into the table in whatever state the test needs.
+///
+/// Tests seed through SQL rather than through `pds-server invite create`
+/// because the states that matter here — already spent, disabled — are
+/// not states the CLI can mint directly, and because a test that had to
+/// shell out to a second binary to arrange its fixture would be testing
+/// two things at once.
+async fn seed_code(db: &sqlx::PgPool, max_uses: i32, used: i32, disabled: bool) -> String {
+    let code = format!("mt-test-{}", uuid::Uuid::new_v4().simple());
+    sqlx::query(
+        "INSERT INTO invite_codes (code, max_uses, used_count, disabled) VALUES ($1, $2, $3, $4)",
+    )
+    .bind(&code)
+    .bind(max_uses)
+    .bind(used)
+    .bind(disabled)
+    .execute(db)
+    .await
+    .expect("seed invite code");
+    code
+}
+
+async fn used_count(db: &sqlx::PgPool, code: &str) -> i32 {
+    sqlx::query_scalar::<_, i32>("SELECT used_count FROM invite_codes WHERE code = $1")
+        .bind(code)
+        .fetch_one(db)
+        .await
+        .expect("read used_count")
+}
+
+async fn use_rows(db: &sqlx::PgPool, code: &str) -> Vec<(String, String)> {
+    sqlx::query_as::<_, (String, String)>(
+        "SELECT did, handle FROM invite_code_uses WHERE code = $1 ORDER BY used_at",
+    )
+    .bind(code)
+    .fetch_all(db)
+    .await
+    .expect("read invite_code_uses")
+}
+
+/// A unique throwaway handle.
+///
+/// `createAccount` caps handles at 64 characters, and
+/// `<prefix>_<32 hex>.maarcadetweet.local` overshoots that for anything
+/// but the shortest prefix — a limit that shows up as a confusing
+/// `InvalidHandle` in a test that is about invite codes. Half the UUID
+/// is 64 bits of uniqueness, which is plenty for a test fixture and
+/// leaves room for a readable prefix.
+fn handle(prefix: &str) -> String {
+    let uniq = uuid::Uuid::new_v4().simple().to_string();
+    format!("{}_{}.maarcadetweet.local", prefix, &uniq[..16])
+}
+
+/// Every invite rejection must look the same to a client: `400` with the
+/// module's usual `{error, message}` body and the name
+/// `InvalidInviteCode`.
+fn assert_invalid_invite(status: u16, body: &Value, what: &str) {
+    assert_eq!(status, 400, "{what}: expected 400, body = {body}");
+    assert_eq!(
+        body["error"], "InvalidInviteCode",
+        "{what}: wrong error name, body = {body}"
+    );
+    assert!(
+        body["message"].is_string(),
+        "{what}: error body must carry a message, body = {body}"
+    );
+    // No DID may have been minted on a rejected request.
+    assert!(
+        body["did"].is_null(),
+        "{what}: rejected request returned a did, body = {body}"
+    );
+}
+
+// -- enforcement ------------------------------------------------------------
+
+/// The happy path, and the property that makes a single-use code
+/// single-use: after the account exists, the same code is dead.
+///
+/// Also checks the audit trail, which is the reason
+/// `invite_code_uses` exists at all — "which account did this code
+/// create" has to be answerable after the fact.
+#[tokio::test]
+async fn valid_code_admits_one_account_then_is_spent() {
+    let Some(db) = db_pool().await else {
+        eprintln!("no pds database, skipping");
+        return;
+    };
+    let Some(pds) = start_pds(true).await else {
+        eprintln!("could not start a pds with PDS_INVITE_REQUIRED=true, skipping");
+        return;
+    };
+
+    // The switch must be advertised, not just enforced — a client reads
+    // this before it asks the user for anything.
+    assert_eq!(
+        pds.describe().await["invite_code_required"],
+        json!(true),
+        "describeServer must report the actual PDS_INVITE_REQUIRED value"
+    );
+
+    let code = seed_code(&db, 1, 0, false).await;
+    let h = handle("inv_ok");
+    let (status, body) = pds
+        .create_account(json!({
+            "handle": h,
+            "password": "hunter2hunter2",
+            "invite_code": code,
+        }))
+        .await;
+    assert_eq!(status, 200, "valid code must create an account: {body}");
+    let did = body["did"].as_str().expect("did").to_string();
+
+    assert_eq!(used_count(&db, &code).await, 1, "code must be counted as used");
+    let uses = use_rows(&db, &code).await;
+    assert_eq!(uses.len(), 1);
+    assert_eq!(uses[0].0, did, "audit row must name the account it created");
+    assert_eq!(uses[0].1, h, "audit row must snapshot the handle");
+
+    // Second attempt on the now-spent code.
+    let (status2, body2) = pds
+        .create_account(json!({
+            "handle": handle("inv_second"),
+            "password": "hunter2hunter2",
+            "invite_code": code,
+        }))
+        .await;
+    assert_invalid_invite(status2, &body2, "spent code");
+    assert_eq!(
+        used_count(&db, &code).await,
+        1,
+        "a rejected attempt must not move the counter"
+    );
+}
+
+/// Every way a code can fail, and the missing-code case, all land on the
+/// same `400 InvalidInviteCode`.
+#[tokio::test]
+async fn unknown_disabled_spent_and_missing_codes_are_rejected() {
+    let Some(db) = db_pool().await else {
+        eprintln!("no pds database, skipping");
+        return;
+    };
+    let Some(pds) = start_pds(true).await else {
+        eprintln!("could not start a pds with PDS_INVITE_REQUIRED=true, skipping");
+        return;
+    };
+
+    // Unknown.
+    let (s, b) = pds
+        .create_account(json!({
+            "handle": handle("inv_unknown"),
+            "password": "hunter2hunter2",
+            "invite_code": "mt-zzzzz-zzzzz",
+        }))
+        .await;
+    assert_invalid_invite(s, &b, "unknown code");
+
+    // Disabled, with uses left — proves `disabled` is checked and not
+    // just the counter.
+    let disabled = seed_code(&db, 5, 0, true).await;
+    let (s, b) = pds
+        .create_account(json!({
+            "handle": handle("inv_disabled"),
+            "password": "hunter2hunter2",
+            "invite_code": disabled,
+        }))
+        .await;
+    assert_invalid_invite(s, &b, "disabled code");
+    assert_eq!(used_count(&db, &disabled).await, 0);
+
+    // Already at its limit.
+    let spent = seed_code(&db, 2, 2, false).await;
+    let (s, b) = pds
+        .create_account(json!({
+            "handle": handle("inv_spent"),
+            "password": "hunter2hunter2",
+            "invite_code": spent,
+        }))
+        .await;
+    assert_invalid_invite(s, &b, "exhausted code");
+
+    // No field at all.
+    let (s, b) = pds
+        .create_account(json!({
+            "handle": handle("inv_none"),
+            "password": "hunter2hunter2",
+        }))
+        .await;
+    assert_invalid_invite(s, &b, "missing code");
+
+    // Present but blank / whitespace — must be indistinguishable from
+    // absent, not an attempt to look up the empty string.
+    for blank in ["", "   "] {
+        let (s, b) = pds
+            .create_account(json!({
+                "handle": handle("inv_blank"),
+                "password": "hunter2hunter2",
+                "invite_code": blank,
+            }))
+            .await;
+        assert_invalid_invite(s, &b, "blank code");
+    }
+}
+
+/// The camelCase spelling from the atproto lexicon, and the multi-use
+/// case the schema exists for.
+#[tokio::test]
+async fn camel_case_spelling_works_and_multi_use_codes_stop_at_the_limit() {
+    let Some(db) = db_pool().await else {
+        eprintln!("no pds database, skipping");
+        return;
+    };
+    let Some(pds) = start_pds(true).await else {
+        eprintln!("could not start a pds with PDS_INVITE_REQUIRED=true, skipping");
+        return;
+    };
+
+    // `inviteCode` is what an off-the-shelf atproto client sends.
+    let code = seed_code(&db, 3, 0, false).await;
+    let (s, b) = pds
+        .create_account(json!({
+            "handle": handle("inv_camel"),
+            "password": "hunter2hunter2",
+            "inviteCode": code,
+        }))
+        .await;
+    assert_eq!(s, 200, "inviteCode spelling must be accepted: {b}");
+
+    // Case and stray whitespace are normalised, so a code shouted or
+    // pasted out of a chat window still works.
+    let (s, b) = pds
+        .create_account(json!({
+            "handle": handle("inv_case"),
+            "password": "hunter2hunter2",
+            "invite_code": format!("  {}  ", code.to_uppercase()),
+        }))
+        .await;
+    assert_eq!(s, 200, "normalised code must be accepted: {b}");
+
+    // Third and last use.
+    let (s, _) = pds
+        .create_account(json!({
+            "handle": handle("inv_third"),
+            "password": "hunter2hunter2",
+            "invite_code": code,
+        }))
+        .await;
+    assert_eq!(s, 200);
+
+    // Fourth is one too many.
+    let (s, b) = pds
+        .create_account(json!({
+            "handle": handle("inv_fourth"),
+            "password": "hunter2hunter2",
+            "invite_code": code,
+        }))
+        .await;
+    assert_invalid_invite(s, &b, "one past max_uses");
+
+    assert_eq!(used_count(&db, &code).await, 3);
+    assert_eq!(use_rows(&db, &code).await.len(), 3);
+}
+
+/// A failed account creation must not consume the code.
+///
+/// The cheapest way to make the account creation fail *after* the
+/// redemption has already run is a handle that is already taken: the
+/// redeem happens first inside the transaction, the `users` insert then
+/// trips the unique index, and the whole transaction rolls back. If the
+/// redemption had been done outside the transaction (or committed
+/// separately) the user would have lost their code to someone else's
+/// handle.
+#[tokio::test]
+async fn a_failed_registration_does_not_burn_the_code() {
+    let Some(db) = db_pool().await else {
+        eprintln!("no pds database, skipping");
+        return;
+    };
+    let Some(pds) = start_pds(true).await else {
+        eprintln!("could not start a pds with PDS_INVITE_REQUIRED=true, skipping");
+        return;
+    };
+
+    let taken = handle("inv_taken");
+    let first = seed_code(&db, 1, 0, false).await;
+    let (s, b) = pds
+        .create_account(json!({
+            "handle": taken, "password": "hunter2hunter2", "invite_code": first,
+        }))
+        .await;
+    assert_eq!(s, 200, "{b}");
+
+    // Now a *different* code, used on a handle that cannot be created.
+    let code = seed_code(&db, 1, 0, false).await;
+    let (s, _b) = pds
+        .create_account(json!({
+            "handle": taken, "password": "hunter2hunter2", "invite_code": code,
+        }))
+        .await;
+    assert_eq!(s, 409, "duplicate handle is still a 409");
+    assert_eq!(
+        used_count(&db, &code).await,
+        0,
+        "the code must survive a registration that rolled back"
+    );
+    assert!(use_rows(&db, &code).await.is_empty());
+
+    // And it still works afterwards.
+    let (s, b) = pds
+        .create_account(json!({
+            "handle": handle("inv_retry"), "password": "hunter2hunter2", "invite_code": code,
+        }))
+        .await;
+    assert_eq!(s, 200, "unburned code must still be redeemable: {b}");
+}
+
+// -- the race ---------------------------------------------------------------
+
+/// Two (here: eight) registrations arriving at the same instant on the
+/// last remaining use of a code. Exactly one may get in.
+///
+/// This is the test the whole design is built around. A
+/// `SELECT`-then-`UPDATE` implementation passes every other test in this
+/// file and fails this one: all eight requests read `used_count = 0`,
+/// all eight decide they are allowed, and the server hands out eight
+/// accounts for a one-use code while the row afterwards claims a single
+/// redemption. The fix is that `invite::redeem` never reads before it
+/// writes — the `WHERE used_count < max_uses` is part of the `UPDATE`,
+/// so Postgres re-evaluates it against the committed row after the
+/// row lock is released and the losers match zero rows.
+///
+/// Every request uses a distinct handle, so nothing but the invite code
+/// can be what serialises them.
+#[tokio::test]
+async fn concurrent_registrations_cannot_share_one_use() {
+    let Some(db) = db_pool().await else {
+        eprintln!("no pds database, skipping");
+        return;
+    };
+    let Some(pds) = start_pds(true).await else {
+        eprintln!("could not start a pds with PDS_INVITE_REQUIRED=true, skipping");
+        return;
+    };
+
+    const N: usize = 8;
+    let code = seed_code(&db, 1, 0, false).await;
+
+    let mut tasks = Vec::with_capacity(N);
+    for i in 0..N {
+        let http = pds.http.clone();
+        let url = pds.url("/xrpc/com.atproto.server.createAccount");
+        let code = code.clone();
+        let h = handle(&format!("inv_race{i}"));
+        tasks.push(tokio::spawn(async move {
+            let resp = http
+                .post(url)
+                .json(&json!({
+                    "handle": h,
+                    "password": "hunter2hunter2",
+                    "invite_code": code,
+                }))
+                .send()
+                .await
+                .expect("concurrent createAccount");
+            let status = resp.status().as_u16();
+            let body: Value = resp.json().await.unwrap_or(Value::Null);
+            (status, body)
+        }));
+    }
+
+    let mut ok = Vec::new();
+    let mut rejected = 0usize;
+    for t in tasks {
+        let (status, body) = t.await.unwrap();
+        match status {
+            200 => ok.push(body),
+            400 => {
+                assert_eq!(body["error"], "InvalidInviteCode", "body = {body}");
+                rejected += 1;
+            }
+            other => panic!("unexpected status {other}: {body}"),
+        }
+    }
+
+    assert_eq!(
+        ok.len(),
+        1,
+        "a one-use code let {} concurrent registrations through — the redeem is racy",
+        ok.len()
+    );
+    assert_eq!(rejected, N - 1);
+    assert_eq!(used_count(&db, &code).await, 1);
+    let uses = use_rows(&db, &code).await;
+    assert_eq!(uses.len(), 1, "counter and audit rows disagree: {uses:?}");
+    assert_eq!(uses[0].0, ok[0]["did"].as_str().unwrap());
+}
+
+/// The same race with room for more than one winner: a three-use code
+/// hit by eight simultaneous registrations must admit exactly three.
+///
+/// Worth having next to the one-use case because an implementation can
+/// be "safe" by accident for a single use (e.g. by serialising every
+/// registration globally) and still lose count when several are
+/// genuinely allowed to proceed.
+#[tokio::test]
+async fn concurrent_registrations_respect_a_multi_use_limit() {
+    let Some(db) = db_pool().await else {
+        eprintln!("no pds database, skipping");
+        return;
+    };
+    let Some(pds) = start_pds(true).await else {
+        eprintln!("could not start a pds with PDS_INVITE_REQUIRED=true, skipping");
+        return;
+    };
+
+    const N: usize = 8;
+    const USES: i32 = 3;
+    let code = seed_code(&db, USES, 0, false).await;
+
+    let mut tasks = Vec::with_capacity(N);
+    for i in 0..N {
+        let http = pds.http.clone();
+        let url = pds.url("/xrpc/com.atproto.server.createAccount");
+        let code = code.clone();
+        let h = handle(&format!("inv_mrace{i}"));
+        tasks.push(tokio::spawn(async move {
+            let resp = http
+                .post(url)
+                .json(&json!({
+                    "handle": h,
+                    "password": "hunter2hunter2",
+                    "invite_code": code,
+                }))
+                .send()
+                .await
+                .expect("concurrent createAccount");
+            let status = resp.status().as_u16();
+            let body: Value = resp.json().await.unwrap_or(Value::Null);
+            (status, body)
+        }));
+    }
+
+    let mut ok = 0usize;
+    for t in tasks {
+        let (status, body) = t.await.unwrap();
+        match status {
+            200 => ok += 1,
+            400 => assert_eq!(body["error"], "InvalidInviteCode", "body = {body}"),
+            other => panic!("unexpected status {other}: {body}"),
+        }
+    }
+    assert_eq!(ok, USES as usize, "a {USES}-use code admitted {ok} accounts");
+    assert_eq!(used_count(&db, &code).await, USES);
+    assert_eq!(use_rows(&db, &code).await.len(), USES as usize);
+}
+
+// -- the switch off ---------------------------------------------------------
+
+/// With `PDS_INVITE_REQUIRED=false` — the default, and what every other
+/// test suite in this workspace relies on — nothing about `createAccount`
+/// changes.
+///
+/// This is the regression test for the whole feature's blast radius: the
+/// switch is off by default precisely so that the existing suites keep
+/// creating accounts with no code, and if that ever stopped being true
+/// the failure would show up as dozens of unrelated tests breaking. It
+/// shows up here instead.
+#[tokio::test]
+async fn switch_off_leaves_create_account_untouched() {
+    let Some(_db) = db_pool().await else {
+        eprintln!("no pds database, skipping");
+        return;
+    };
+    let Some(pds) = start_pds(false).await else {
+        eprintln!("could not start a pds with PDS_INVITE_REQUIRED=false, skipping");
+        return;
+    };
+
+    assert_eq!(
+        pds.describe().await["invite_code_required"],
+        json!(false),
+        "describeServer must report the actual PDS_INVITE_REQUIRED value"
+    );
+
+    // No code at all: the historical behaviour.
+    let (s, b) = pds
+        .create_account(json!({
+            "handle": handle("inv_off"),
+            "password": "hunter2hunter2",
+        }))
+        .await;
+    assert_eq!(s, 200, "no-code registration must still work: {b}");
+    assert!(b["did"].as_str().unwrap().starts_with("did:"));
+    assert!(b["access_jwt"].is_string());
+
+    // A code that does not exist is simply ignored rather than becoming
+    // a new way to fail — a client that was talking to an invite-only
+    // PDS yesterday must not break when the operator opens the server up.
+    let (s, b) = pds
+        .create_account(json!({
+            "handle": handle("inv_off_bogus"),
+            "password": "hunter2hunter2",
+            "invite_code": "mt-does-notexist",
+        }))
+        .await;
+    assert_eq!(s, 200, "an ignored code must not fail the request: {b}");
+
+    // The other validations are untouched.
+    let (s, b) = pds
+        .create_account(json!({
+            "handle": handle("inv_off_short"),
+            "password": "short",
+        }))
+        .await;
+    assert_eq!(s, 400);
+    assert_eq!(b["error"], "InvalidPassword");
+}
+
+// -- the CLI ----------------------------------------------------------------
+
+/// `pds-server invite create` / `list` / `disable`, run as the operator
+/// would run them, against the real database.
+///
+/// The point is not that the SQL works (the tests above cover that) but
+/// that the *binary* exposes it: that `invite` short-circuits before the
+/// server starts, that `create` prints bare codes one per line so they
+/// can be pasted, and that a code it minted is actually redeemable.
+#[tokio::test]
+async fn invite_cli_mints_listable_redeemable_codes() {
+    let Some(db) = db_pool().await else {
+        eprintln!("no pds database, skipping");
+        return;
+    };
+
+    let out = Command::new(env!("CARGO_BIN_EXE_pds-server"))
+        .args(["invite", "create", "--count", "3", "--uses", "2", "--note", "cli test"])
+        .env("RUST_LOG", "warn")
+        .output()
+        .expect("run invite create");
+    if !out.status.success() {
+        eprintln!(
+            "invite create failed (no env/db?), skipping: {}",
+            String::from_utf8_lossy(&out.stderr)
+        );
+        return;
+    }
+
+    let stdout = String::from_utf8_lossy(&out.stdout);
+    let codes: Vec<&str> = stdout.lines().filter(|l| !l.trim().is_empty()).collect();
+    assert_eq!(codes.len(), 3, "one code per line, nothing else: {stdout:?}");
+    for c in &codes {
+        // Bare, paste-ready: no labels, no quotes, no indentation.
+        assert_eq!(*c, c.trim(), "code line has surrounding whitespace: {c:?}");
+        assert!(c.starts_with("mt-"), "unexpected code shape: {c}");
+        assert_eq!(
+            sqlx::query_scalar::<_, i32>("SELECT max_uses FROM invite_codes WHERE code = $1")
+                .bind(c)
+                .fetch_one(&db)
+                .await
+                .expect("minted code must be in the table"),
+            2,
+            "--uses must reach the row"
+        );
+    }
+    // All three distinct — a generator that returned a constant would
+    // otherwise only show up as a primary-key error.
+    let unique: std::collections::HashSet<&&str> = codes.iter().collect();
+    assert_eq!(unique.len(), 3);
+
+    // `list` must show what `redeem` would accept.
+    let listed = Command::new(env!("CARGO_BIN_EXE_pds-server"))
+        .args(["invite", "list"])
+        .env("RUST_LOG", "warn")
+        .output()
+        .expect("run invite list");
+    assert!(listed.status.success());
+    let listed = String::from_utf8_lossy(&listed.stdout);
+    for c in &codes {
+        assert!(listed.contains(*c), "invite list omitted {c}");
+    }
+
+    // `disable` takes a code out without deleting it.
+    let disabled = Command::new(env!("CARGO_BIN_EXE_pds-server"))
+        .args(["invite", "disable", codes[0]])
+        .env("RUST_LOG", "warn")
+        .output()
+        .expect("run invite disable");
+    assert!(disabled.status.success());
+    assert!(
+        sqlx::query_scalar::<_, bool>("SELECT disabled FROM invite_codes WHERE code = $1")
+            .bind(codes[0])
+            .fetch_one(&db)
+            .await
+            .unwrap()
+    );
+    let listed = Command::new(env!("CARGO_BIN_EXE_pds-server"))
+        .args(["invite", "list"])
+        .env("RUST_LOG", "warn")
+        .output()
+        .expect("run invite list");
+    let listed = String::from_utf8_lossy(&listed.stdout);
+    assert!(
+        !listed.contains(codes[0]),
+        "a disabled code must not show in the default listing"
+    );
+
+    // And a minted code really lets an account through.
+    let Some(pds) = start_pds(true).await else {
+        eprintln!("could not start a pds with PDS_INVITE_REQUIRED=true, skipping redeem check");
+        return;
+    };
+    let (s, b) = pds
+        .create_account(json!({
+            "handle": handle("inv_cli"),
+            "password": "hunter2hunter2",
+            "invite_code": codes[1],
+        }))
+        .await;
+    assert_eq!(s, 200, "CLI-minted code must be redeemable: {b}");
+}
+
+/// An unknown subcommand must not silently boot a server, and `help`
+/// must not need a database.
+#[tokio::test]
+async fn unknown_subcommand_fails_instead_of_starting_a_server() {
+    let out = Command::new(env!("CARGO_BIN_EXE_pds-server"))
+        .args(["invit"])
+        .env("RUST_LOG", "warn")
+        .output()
+        .expect("run bad subcommand");
+    assert!(!out.status.success(), "a typo'd subcommand must not exit 0");
+    let stderr = String::from_utf8_lossy(&out.stderr);
+    assert!(stderr.contains("unknown command"), "stderr = {stderr}");
+
+    let out = Command::new(env!("CARGO_BIN_EXE_pds-server"))
+        .args(["invite", "help"])
+        .env("RUST_LOG", "warn")
+        .output()
+        .expect("run invite help");
+    assert!(out.status.success());
+    assert!(String::from_utf8_lossy(&out.stdout).contains("pds-server invite"));
+}

crates/pds-server/tests/pds_integration.rs

@@ -44,7 +44,32 @@ async fn describe_server() {
     let did = r["did"].as_str().expect("describeServer must return a did");
     assert!(did.starts_with("did:web:"), "did = {did}");
     assert!(r["available_user_domains"].is_array());
-    assert_eq!(r["invite_code_required"], json!(false));
+    // `invite_code_required` used to be a hardcoded `false` here. It is
+    // now whatever `PDS_INVITE_REQUIRED` says, so this suite — which
+    // talks to whatever PDS the developer happens to be running — can
+    // only assert the type. That the value actually tracks the switch is
+    // pinned in `invite_integration.rs`, which starts a PDS with the
+    // flag set both ways and checks both answers.
+    assert!(
+        r["invite_code_required"].is_boolean(),
+        "invite_code_required = {}",
+        r["invite_code_required"]
+    );
+    // If this test process shares the server's environment (the
+    // documented way to run the suite is
+    // `set -a; . ./.env; set +a; cargo test`), hold it to the exact
+    // value too.
+    if let Ok(raw) = std::env::var("PDS_INVITE_REQUIRED") {
+        let expected = matches!(
+            raw.trim().to_ascii_lowercase().as_str(),
+            "1" | "true" | "yes" | "on"
+        );
+        assert_eq!(
+            r["invite_code_required"],
+            json!(expected),
+            "describeServer disagrees with PDS_INVITE_REQUIRED={raw}"
+        );
+    }
 }
 
 /// `GET /.well-known/did.json` — the document the AppView fetches to

migrations/pds/0004_invite_codes.sql

@@ -0,0 +1,186 @@
+-- PDS database schema 0004: invite codes for `com.atproto.server.createAccount`.
+--
+-- Why
+--
+-- Until now `createAccount` had no gate of any kind: no invite code, no rate
+-- limit, and `describeServer` advertised `invite_code_required: false`. That
+-- was survivable while the only reachable instance was `127.0.0.1:2583`. It
+-- stops being survivable the moment the PDS answers on a public name, because
+-- every accepted account is not just a row in `users` — it creates a `repos`
+-- head, a key pair the server has to keep, an MST that grows with every write,
+-- and firehose events that every subscribed AppView is obliged to index. A
+-- single script could mint accounts until the disk filled up, and nothing in
+-- the write path would consider that abnormal.
+--
+-- This migration adds the smallest gate that actually closes that hole: an
+-- account may only be created by presenting a code the operator handed out.
+-- The gate is opt-in via `PDS_INVITE_REQUIRED` (default `false`, so the dozens
+-- of integration tests that create throwaway accounts keep working); the
+-- tables below exist unconditionally so that switching the flag on is a
+-- restart, not a migration.
+--
+-- Two tables, not one
+-- -------------------
+-- A code can be worth more than one account (`--uses 5` for a group of
+-- friends, a conference badge, a family). That means "who redeemed this code"
+-- is a *set*, not a single column, so it cannot live on the code row. Putting
+-- a `redeemed_by TEXT` column on `invite_codes` would have forced either
+-- one-code-one-account (losing the multi-use case the operator actually wants)
+-- or an array column that no foreign key, index or `COUNT(*)` can reason
+-- about. `invite_code_uses` is that set, one row per redemption.
+--
+-- The redemption *counter* still lives on the code row even though it is
+-- derivable from `COUNT(*)` over `invite_code_uses`. That duplication is
+-- deliberate and is the entire concurrency story — see below.
+--
+-- invite_codes
+-- ------------
+--
+--   code        TEXT PRIMARY KEY — the code itself, and the natural key. No
+--               surrogate `id`: the code is what the user types, what the
+--               operator pastes into a chat window, and what the redeem query
+--               looks up, so a second identifier would only add a join.
+--               Codes are generated lowercase from a 32-character
+--               Crockford-style alphabet (`crates/pds-server/src/invite.rs`),
+--               and the server lowercases and trims what the client sends
+--               before looking it up. Because every stored code is already
+--               lowercase ASCII, that normalisation happens in Rust rather
+--               than as `WHERE lower(code) = …`, which would throw away this
+--               primary-key index on the hottest lookup this table has.
+--
+--   created_at  when the operator minted it. Purely for the `invite list`
+--               output and for answering "where did this wave of signups come
+--               from" after the fact.
+--
+--   note        free-text label the operator can attach at creation time
+--               (`--note "meetup 2026-09"`). Nullable, never interpreted.
+--               It exists because a bare list of random strings is unusable
+--               a month later.
+--
+--   max_uses    how many accounts this code may create. `CHECK (max_uses > 0)`
+--               because a zero-use code is not a thing you would ever mean to
+--               create — it is a typo that would silently hand out a code that
+--               can never work.
+--
+--   used_count  how many it has already created. Kept in sync with
+--               `invite_code_uses` inside the same transaction that writes
+--               both.
+--
+--   disabled    a code the operator wants to stop honouring *without* losing
+--               the audit trail. Deleting the row would work for the future
+--               but would take the `invite_code_uses` rows with it (see the
+--               FK below) and with them the record of which accounts came
+--               from that code — which is the one question you ask when a
+--               code leaks. A boolean keeps the history and is checked in the
+--               same `WHERE` clause as the counter, so disabling costs nothing
+--               at redeem time.
+--
+--   CHECK (used_count <= max_uses) — the belt to the redeem query's braces.
+--               The application never over-redeems (the conditional UPDATE
+--               below makes that impossible), but this constraint means that
+--               *no* future query — a hand-written `UPDATE` during an
+--               incident, a bug in a later refactor — can hand out more
+--               accounts than the operator authorised. The database refuses.
+--
+-- How the redeem race is closed
+-- -----------------------------
+-- The obvious implementation is "SELECT the code, check `used_count <
+-- max_uses` in Rust, then UPDATE". That is a check-then-act, and two
+-- registrations arriving at the same instant with the same last remaining use
+-- both read `used_count = 0`, both decide they are allowed, and both write
+-- `used_count = 1` — two accounts from a one-use code, with the row still
+-- claiming a single redemption.
+--
+-- So the check and the act are one statement, and the database performs both:
+--
+--     UPDATE invite_codes
+--        SET used_count = used_count + 1
+--      WHERE code = $1
+--        AND NOT disabled
+--        AND used_count < max_uses
+--     RETURNING used_count, max_uses;
+--
+-- Under Postgres's READ COMMITTED isolation the second transaction to reach
+-- this row blocks on the row lock the first one took. When the first commits,
+-- the second does not proceed with its stale snapshot: it re-reads the updated
+-- row and re-evaluates the `WHERE` clause against it (EvalPlanQual). The
+-- counter is now `1`, `used_count < max_uses` is false, the row no longer
+-- matches, and the statement returns zero rows. Zero rows returned *is* the
+-- rejection — the route turns it into `400 InvalidInviteCode` without ever
+-- having formed an opinion of its own about whether the code was still valid.
+--
+-- If the first transaction instead rolls back — the handle turned out to be
+-- taken, key generation failed, anything — the lock is released with the
+-- counter back at `0` and the waiting transaction's re-check succeeds. The
+-- code is only consumed by a registration that actually completed, which is
+-- why the redemption is issued inside `create_account`'s existing
+-- transaction rather than before it.
+--
+-- This is also why `used_count` is stored rather than computed. A
+-- `COUNT(*) FROM invite_code_uses` has no row to lock — concurrent counters
+-- both see the same pre-insert count and both pass. The counter column gives
+-- the conditional UPDATE a single row to serialise on.
+CREATE TABLE IF NOT EXISTS invite_codes (
+    code           TEXT PRIMARY KEY,
+    created_at     TIMESTAMPTZ NOT NULL DEFAULT now(),
+    note           TEXT,
+    max_uses       INTEGER NOT NULL DEFAULT 1,
+    used_count     INTEGER NOT NULL DEFAULT 0,
+    disabled       BOOLEAN NOT NULL DEFAULT FALSE,
+    CONSTRAINT invite_codes_max_uses_positive CHECK (max_uses > 0),
+    CONSTRAINT invite_codes_used_count_sane   CHECK (used_count >= 0 AND used_count <= max_uses)
+);
+
+-- `invite list` shows the newest codes first, and that is the only listing
+-- this table has. Small table, but the operator runs it interactively and an
+-- ordered index keeps the output instant even after a few thousand codes.
+CREATE INDEX IF NOT EXISTS invite_codes_created_at_idx
+    ON invite_codes (created_at DESC);
+
+-- =====================================================
+-- invite_code_uses — which account came from which code
+-- =====================================================
+--
+--   code       FK to `invite_codes(code)` ON DELETE CASCADE. Cascading is the
+--              right call *here* (unlike `firehose_events.did` in 0003, which
+--              deliberately has no FK) because these rows are meaningless
+--              without the code they describe: they exist to answer "which
+--              accounts did code X create", and a use-row whose code has been
+--              deleted answers nothing. The operator who wants to stop a code
+--              but keep the trail sets `disabled` instead of deleting — which
+--              is precisely why that column exists.
+--
+--   did        the account that was created. Intentionally NOT a foreign key
+--              to `users(did)`: this is an audit record of something that
+--              happened, and it has to survive the account being deleted. If
+--              it cascaded from `users`, deleting a spam account would erase
+--              the evidence linking it to the code that let it in — the exact
+--              moment the link matters most. The trade-off is that a `did`
+--              here may point at a user that no longer exists; that is
+--              accepted and is what an audit log looks like.
+--
+--   handle     the handle as it was at creation time, denormalised on
+--              purpose. Handles can change, and `users` may be gone entirely
+--              (see above); this column is a snapshot so the listing stays
+--              readable without a join that may find nothing.
+--
+--   used_at    when the redemption happened.
+--
+--   PRIMARY KEY (code, did) — one account can only consume a given code once.
+--              This is not the mechanism that enforces the use limit (the
+--              conditional UPDATE is), it is a guard against a redemption
+--              being recorded twice for one account, which would make
+--              `used_count` and this table disagree.
+CREATE TABLE IF NOT EXISTS invite_code_uses (
+    code           TEXT NOT NULL REFERENCES invite_codes(code) ON DELETE CASCADE,
+    did            TEXT NOT NULL,
+    handle         TEXT NOT NULL,
+    used_at        TIMESTAMPTZ NOT NULL DEFAULT now(),
+    PRIMARY KEY (code, did)
+);
+
+-- The reverse lookup: "which code let this account in?". Asked per-account
+-- during abuse triage, so it needs its own index — the (code, did) primary
+-- key cannot serve a query whose only predicate is `did`.
+CREATE INDEX IF NOT EXISTS invite_code_uses_did_idx
+    ON invite_code_uses (did);