sidecar/src/main.rs

@@ -72,7 +72,26 @@ use tracing_subscriber::EnvFilter;
 /// index only the columns they already had, so the new fields ride inside the stored JSON and the
 /// new kind lands in `events` like any other. That is the dumb-forwarder property doing its job:
 /// the sidecar defines no schema for a frame's contents and so needs no change when they grow.
-pub const PROTOCOL_VERSION: u32 = 5;
+///
+/// v6 (Protocol 6): the first bump that is about a GUARANTEE rather than about data, and the first
+/// the sidecar mostly gets for free. Two things:
+///
+///   * **`idempotencyKey` on inbound commands.** A command that carries one is executed by the shard
+///     at most once; a repeat is answered with the original reply rather than re-run. That is what
+///     makes a world-writing verb retryable at all — until now a lost acknowledgement was
+///     indistinguishable from a command that never applied, so the website had to declare every
+///     write un-retryable and accept losing one rather than risk doubling it. The sidecar's part is
+///     to CARRY the key (it rides in the command body, which every write endpoint already passes
+///     through verbatim) and to understand the one new answer the shard can now give: `bridge.busy`,
+///     meaning a command under that key is still in flight. See `web::respond`.
+///   * **`champ.boss.killed` is a new kind**: a champion's defeat, with the damage table only the
+///     shard ever sees. It was previously inferable from `champ.update` going `bossUp` true then
+///     false alongside a nearby `mob.killed`, which is fragile and says nothing about who did the
+///     work. It lands in `events` and on the feed like any other kind, with no code here at all —
+///     the dumb-forwarder property again.
+///
+/// **No store migration.** Nothing gains a column; the new kind is persisted whole like every other.
+pub const PROTOCOL_VERSION: u32 = 6;
 
 // Not `#[tokio::main]`: on Windows the SCM dispatcher takes over this thread and starts the runtime
 // itself, on its own thread, once the service actually begins. The runtime is built by whichever

sidecar/src/web.rs

@@ -246,13 +246,31 @@ fn constant_time_eq(a: &[u8], b: &[u8]) -> bool {
 
 // ---- shared reply handling ----
 
+/// Protocol 6. `bridge.busy` says a command carrying this `idempotencyKey` is already in flight on
+/// the shard: nothing was run, and the caller should come back.
+///
+/// It maps to **425 Too Early**, which is what that status is for — a server unwilling to risk
+/// processing a request that might be a replay. The obvious alternative, 409, is already the
+/// protocol-version gate's answer, and those two want opposite dispositions from a client: a version
+/// mismatch is a deployment fault nobody should retry, and a busy shard is a retry that should
+/// succeed on its own. Sharing a status would have made the difference readable only by inspecting
+/// the body, which is exactly how a retry loop ends up hiding a mismatched deployment.
+///
+/// It is checked BEFORE the `.error` suffix test in each responder below, and it is deliberately not
+/// spelled `bridge.busy.error`: nothing is wrong. The work is happening.
+const BUSY_KIND: &str = "bridge.busy";
+const BUSY_STATUS: StatusCode = StatusCode::TOO_EARLY;
+
 /// Turns an RPC result into an HTTP response. A `bridge.error` reply from the shard becomes a 4xx;
-/// a real reply is returned as-is; transport failures map to 503/504.
+/// a `bridge.busy` reply becomes a 425; a real reply is returned as-is; transport failures map to
+/// 503/504.
 fn respond(result: Result<Value, RpcError>) -> (StatusCode, Json<Value>) {
     match result {
         Ok(value) => {
             let kind = value.get("kind").and_then(|k| k.as_str()).unwrap_or("");
-            if kind == "bridge.error" || kind.ends_with(".error") {
+            if kind == BUSY_KIND {
+                (BUSY_STATUS, Json(value))
+            } else if kind == "bridge.error" || kind.ends_with(".error") {
                 let reason = value
                     .get("reason")
                     .and_then(|r| r.as_str())
@@ -286,7 +304,9 @@ fn respond_admin(result: Result<Value, RpcError>) -> (StatusCode, Json<Value>) {
     match result {
         Ok(value) => {
             let kind = value.get("kind").and_then(|k| k.as_str()).unwrap_or("");
-            if kind == "admin.error" {
+            if kind == BUSY_KIND {
+                (BUSY_STATUS, Json(value))
+            } else if kind == "admin.error" {
                 let reason = value
                     .get("reason")
                     .and_then(|r| r.as_str())
@@ -324,7 +344,9 @@ fn respond_account(result: Result<Value, RpcError>) -> (StatusCode, Json<Value>)
     match result {
         Ok(value) => {
             let kind = value.get("kind").and_then(|k| k.as_str()).unwrap_or("");
-            if kind == "account.error" {
+            if kind == BUSY_KIND {
+                (BUSY_STATUS, Json(value))
+            } else if kind == "account.error" {
                 let reason = value
                     .get("reason")
                     .and_then(|r| r.as_str())
@@ -454,6 +476,16 @@ async fn link_delete(
 /// Forwards a staff moderation command to the shard, correlated on a fresh reqId. Injects `kind`
 /// and `reqId`, requiring the caller-supplied `actor` up front (the shard enforces it too). The
 /// body's remaining fields (account/serial/durationSec/reason/text/hue) pass straight through.
+///
+/// **Protocol 6: `idempotencyKey` is one of those remaining fields**, and passing it through is the
+/// whole of the sidecar's part in the guarantee. It is worth stating rather than leaving to the
+/// word "remaining", because a later refactor that narrowed this to a known field list would quietly
+/// turn every retried world write back into a possible duplicate, and nothing here would fail.
+///
+/// The key belongs to the CALLER's unit of work — the website's event step — so the sidecar neither
+/// generates one nor validates it. Note also that `reqId` is regenerated on every call: a retry
+/// carries the same idempotency key under a NEW correlation id, which is exactly why the shard
+/// re-stamps a replayed reply rather than echoing the id the first attempt used.
 async fn admin_call(st: &AppState, kind: &str, body: Value) -> (StatusCode, Json<Value>) {
     let mut obj = match body {
         Value::Object(m) => m,
@@ -978,3 +1010,67 @@ async fn ws_client(mut socket: WebSocket, state: AppState) {
 
     info!("ws client disconnected");
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn reply(kind: &str) -> Result<Value, RpcError> {
+        Ok(json!({"t": 1, "kind": kind, "reqId": "r-9"}))
+    }
+
+    /// Protocol 6. Every responder must recognise `bridge.busy`, because every write plane can be
+    /// retried: the staff plane, the account plane and the plain command plane all reach handlers
+    /// that a keyed retry can arrive at. A responder that missed it would return 200 with a body
+    /// saying nothing happened, which is the worst of the three possible answers.
+    #[test]
+    fn busy_maps_to_425_on_every_plane() {
+        assert_eq!(respond(reply("bridge.busy")).0, StatusCode::TOO_EARLY);
+        assert_eq!(respond_admin(reply("bridge.busy")).0, StatusCode::TOO_EARLY);
+        assert_eq!(
+            respond_account(reply("bridge.busy")).0,
+            StatusCode::TOO_EARLY
+        );
+    }
+
+    /// 425 must not collide with the protocol-version gate's 409: a mismatch is a deployment fault
+    /// nobody should retry, a busy shard is a retry that will succeed. Same-status would make the
+    /// two readable only by inspecting the body.
+    #[test]
+    fn busy_is_not_the_version_gates_status() {
+        assert_ne!(BUSY_STATUS, StatusCode::CONFLICT);
+    }
+
+    /// A replayed reply is an ordinary success. The shard marks it `replayed: true` for the log, and
+    /// the caller must be able to treat it exactly as it would have treated the answer it lost.
+    #[test]
+    fn a_replayed_reply_is_still_a_200() {
+        let value = json!({"t": 1, "kind": "admin.ok", "reqId": "r-9", "replayed": true});
+        let (status, body) = respond_admin(Ok(value));
+        assert_eq!(status, StatusCode::OK);
+        assert_eq!(body.0.get("replayed").and_then(|v| v.as_bool()), Some(true));
+    }
+
+    /// The error mapping the busy arm is threaded in front of must be untouched by it.
+    #[test]
+    fn errors_still_map_as_before() {
+        assert_eq!(
+            respond(Ok(
+                json!({"kind": "bridge.error", "reason": "unknown account"})
+            ))
+            .0,
+            StatusCode::NOT_FOUND
+        );
+        assert_eq!(
+            respond_admin(Ok(json!({"kind": "admin.error", "reason": "protected"}))).0,
+            StatusCode::FORBIDDEN
+        );
+        assert_eq!(
+            respond_account(Ok(
+                json!({"kind": "account.error", "reason": "already exists"})
+            ))
+            .0,
+            StatusCode::CONFLICT
+        );
+    }
+}