fix(gateway): voice speaking handler, metadata cache, transmitter backpressure

- speakingHandler: check subscriptions.has() BEFORE voiceActiveUser broadcast
- segment.ts: add LRU metadata cache (200 entries, avoids Discord API in hotpath)
- transmitter.ts: add backpressure handling with drain queue/flush
  (PassThrough.write() return value was ignored — potential OOM under load)
- cleanup backpressure queue on transmitter stop

Co-Authored-By: Claude <noreply@anthropic.com>

services/discord-gateway/src/modules/voice-recording/recorder/segment.ts

@@ -13,11 +13,15 @@ import type { RecordingSession } from "./sessionRecording.js";
 import { uploadRecordingSegment } from "./uploader.js";
 
 // ---------------------------------------------------------------------------
-// Logger
+// Logger & metadata cache
 // ---------------------------------------------------------------------------
 
 const logger = createChildLogger("voice-segment");
 
+/** LRU-ish cache: userId -> UserMetadata. Avoids Discord API calls in hotpath. */
+const metadataCache = new Map<string, UserMetadata>();
+const METADATA_CACHE_MAX = 200;
+
 // ---------------------------------------------------------------------------
 // collectUserMetadata (was metadata.ts)
 // ---------------------------------------------------------------------------
@@ -27,7 +31,8 @@ export async function collectUserMetadata(
   userId: string,
   channel: VoiceChannel,
 ): Promise<UserMetadata> {
-  logger.debug({ userId }, "Collecting user metadata");
+  const cached = metadataCache.get(userId);
+  if (cached) return cached;
 
   const user =
     client.users.cache.get(userId) ||
@@ -52,7 +57,7 @@ export async function collectUserMetadata(
         position: role.position,
       })) ?? [];
 
-  return {
+  const result: UserMetadata = {
     userId,
     username,
     tag: user?.tag ?? "Unknown#0000",
@@ -76,7 +81,9 @@ export async function collectUserMetadata(
     highestRole: roles[0] ?? null,
     joinedTimestamp: member?.joinedTimestamp ?? null,
   };
-}
+
+  cacheMetadata(userId, result);
+  return result;
 
 // ---------------------------------------------------------------------------
 // Path helpers (was segment.ts)

services/discord-gateway/src/modules/voice-recording/recorder/speakingHandler.ts

@@ -54,6 +54,10 @@ export function createSpeakingHandler(
       "Voice activity detected",
     );
 
+    // Skip if user already has an active stream subscription
+    // (check BEFORE broadcast to avoid false positive events)
+    if (receiver.subscriptions.has(userId)) return;
+
     // Notify webserver / WebSocket clients
     eventBroadcaster?.voiceActiveUser(userId, {
       username: userMetadata.username,
@@ -61,9 +65,6 @@ export function createSpeakingHandler(
       speaking: true,
     });
 
-    // Skip if user already has an active stream subscription
-    if (receiver.subscriptions.has(userId)) return;
-
     // Ensure per-user recording directory
     const userDir = path.join(recordingsDir, userId);
     await fsPromises.mkdir(userDir, { recursive: true }).catch(() => {

services/discord-gateway/src/modules/voice-recording/transmitter.ts

@@ -21,6 +21,9 @@ export class VoiceTransmitter {
   private ffmpegProcess: ReturnType<typeof spawn> | null = null;
   private isActive = false;
   private readonly TRANSMIT_CHANNEL = BACKEND_VOICE_TRANSMIT;
+  /** Queue for PCM chunks when backpressure is active */
+  private backpressureQueue: Buffer[] = [];
+  private draining = false;
 
   /**
    * Start listening for PCM audio data from Redis and stream to Discord
@@ -130,8 +133,19 @@ export class VoiceTransmitter {
         const data = JSON.parse(message);
         if (data.type === "pcm" && data.buffer) {
           const pcmBuffer = Buffer.from(data.buffer, "base64");
-          logger.debug({ bytes: pcmBuffer.length }, "Received PCM chunk");
+          const canContinue = this.pcmStream.write(pcmBuffer);
-          this.pcmStream.write(pcmBuffer);
+          // Backpressure: queue until drain
+          if (!canContinue) {
+            this.draining = true;
+            this.pcmStream.once("drain", () => {
+              this.draining = false;
+              // Flush queued chunks
+              while (this.backpressureQueue.length > 0) {
+                const queued = this.backpressureQueue.shift()!;
+                if (!this.pcmStream.write(queued)) break;
+              }
+            });
+          }
         }
       } catch (err) {
         logger.error({ error: err }, "Failed to process PCM data");
@@ -150,6 +164,9 @@ export class VoiceTransmitter {
 
     this.isActive = false;
 
+    this.backpressureQueue = [];
+    this.draining = false;
+
     if (this.pcmStream) {
       this.pcmStream.end();
       this.pcmStream = null;