fix: bind queue idempotency to compatibility

server/api/service_test.go

@@ -221,6 +221,51 @@ func TestQueueCreateRejectsCandidateMetadataMismatch(t *testing.T) {
 	}
 }
 
+func TestQueueCreateAPIRetriesIdenticallyAndRejectsKeyReuseWithChangedPayload(t *testing.T) {
+	now := time.Unix(1000, 0).UTC()
+	sessions := domain.NewSessionStore()
+	session, token, err := sessions.Issue("player-1", time.Hour, now)
+	if err != nil {
+		t.Fatal(err)
+	}
+	service := &Service{Sessions: sessions, Queue: domain.NewQueue(), Now: func() time.Time { return now }, Candidate: func(playerID, ticketID string) (domain.Candidate, error) {
+		return domain.Candidate{PlayerID: playerID, TicketID: ticketID, EnqueuedAt: now}, nil
+	}}
+	server := httptest.NewServer(service.Handler())
+	defer server.Close()
+	request := func(body, key string) (int, queueResponse) {
+		req, _ := http.NewRequest(http.MethodPost, server.URL+"/v1/queue", strings.NewReader(body))
+		req.Header.Set("Authorization", "Bearer "+session.SessionID+":"+token)
+		req.Header.Set("Idempotency-Key", key)
+		response, requestErr := http.DefaultClient.Do(req)
+		if requestErr != nil {
+			t.Fatal(requestErr)
+		}
+		defer response.Body.Close()
+		var decoded queueResponse
+		if response.StatusCode == http.StatusCreated {
+			if err := json.NewDecoder(response.Body).Decode(&decoded); err != nil {
+				t.Fatal(err)
+			}
+		}
+		return response.StatusCode, decoded
+	}
+	body := `{"ticket_id":"ticket-idempotent","playlist":"casual","client_build":"build-1","protocol_version":1}`
+	status, first := request(body, "idempotency-key-123456")
+	if status != http.StatusCreated {
+		t.Fatalf("first create status=%d", status)
+	}
+	status, replay := request(body, "idempotency-key-123456")
+	if status != http.StatusCreated || replay != first {
+		t.Fatalf("identical replay status=%d first=%+v replay=%+v", status, first, replay)
+	}
+	changed := `{"ticket_id":"ticket-idempotent","playlist":"casual","client_build":"build-2","protocol_version":1}`
+	status, _ = request(changed, "idempotency-key-123456")
+	if status != http.StatusConflict {
+		t.Fatalf("changed-payload replay status=%d, want conflict", status)
+	}
+}
+
 func TestQueueAPIUsesInjectedPersistentBackendWithoutCandidateProvider(t *testing.T) {
 	now := time.Unix(1000, 0).UTC()
 	sessions := domain.NewSessionStore()

server/domain/queue.go

@@ -218,5 +218,5 @@ func createPayload(playerID, ticketID string, candidate Candidate) string {
 	for _, region := range regions {
 		rtts = append(rtts, fmt.Sprintf("%s=%.9f", region, candidate.PredictedRTT[region]))
 	}
-	return strings.Join([]string{playerID, ticketID, candidate.PlayerID, candidate.TicketID, fmt.Sprintf("%.9f", candidate.Rating), candidate.EnqueuedAt.UTC().Format(time.RFC3339Nano), strings.Join(rtts, ",")}, "\x00")
+	return strings.Join([]string{playerID, ticketID, candidate.PlayerID, candidate.TicketID, string(candidate.Playlist), candidate.ClientBuild, fmt.Sprintf("%d", candidate.ProtocolVersion), fmt.Sprintf("%.9f", candidate.Rating), candidate.EnqueuedAt.UTC().Format(time.RFC3339Nano), strings.Join(rtts, ",")}, "\x00")
 }

server/domain/queue_test.go

@@ -77,6 +77,21 @@ func TestQueueCreateIdempotencyIncludesCandidatePayload(t *testing.T) {
 	if _, err := q.Create("player-a", "ticket-a", "create-key-123456", changed, now); !errors.Is(err, ErrConflict) {
 		t.Fatalf("changed create payload error = %v", err)
 	}
+	changed = base
+	changed.Playlist = Ranked
+	if _, err := q.Create("player-a", "ticket-a", "create-key-123456", changed, now); !errors.Is(err, ErrConflict) {
+		t.Fatalf("changed playlist payload error = %v", err)
+	}
+	changed = base
+	changed.ClientBuild = "build-2"
+	if _, err := q.Create("player-a", "ticket-a", "create-key-123456", changed, now); !errors.Is(err, ErrConflict) {
+		t.Fatalf("changed build payload error = %v", err)
+	}
+	changed = base
+	changed.ProtocolVersion = 2
+	if _, err := q.Create("player-a", "ticket-a", "create-key-123456", changed, now); !errors.Is(err, ErrConflict) {
+		t.Fatalf("changed protocol payload error = %v", err)
+	}
 }
 
 func TestQueueCreateRejectsCandidateOwnedByAnotherPlayer(t *testing.T) {