fix(gateway): output raw H264+Opus (not NUT) so Demuxer parses NAL keyframes correctly

Root cause: prepareStream muxed video+audio into a NUT container on pipe:1.
The Demuxer scans pipe:1 for AnnexB start codes (00 00 01) to split NAL
units into access units and classify keyframes (nal_type 5). NUT container
framing bytes sat in the stream and were scanned as NALs — NAL type 0
(NUT header) instead of 5 (IDR) → every frame classified key=false →
Discord decoder never got a decodable frame → static/black GoLive tile.

Fix: output raw H264 AnnexB on pipe:1 (demuxer target) and Ogg Opus on
fd3/pipe:3 for audio. NUT is only needed for *input* parsing (single
pipe carries both streams); output is demuxed into separate raw streams.

services/discord-gateway/src/goLive/prepareStream.ts

@@ -28,8 +28,8 @@ export interface PrepareStreamResult {
   height: number;
   frameRate?: number;
   includeAudio: boolean;
-  /** Container the encoder muxes to: "nut" (audio-capable) or "h264" (raw). */
-  format: "nut" | "h264";
+  /** Output container — always raw H264 AnnexB (audio demuxed to pipe:3). */
+  format: "h264";
 }
 
 function isFiniteNonZero(n: unknown): n is number {
@@ -183,10 +183,22 @@ export function prepareStream(
     );
   }
 
-  // Audio
+  // Audio (transcode to libopus for encoding). Video output is ALWAYS raw
+  // H264 AnnexB on pipe:1 — NOT NUT. NUT container framing on stdout corrupts
+  // the AnnexB start-code scanner in Demuxer (NUT headers misread as NAL type 0
+  // → every frame classified non-keyframe → static tile). NUT is only needed
+  // for the *input* (one pipe carries both streams); output demuxes each stream
+  // to its own raw format.
+  // Video output: raw H264 AnnexB on pipe:1 — demux() scans start codes.
+  args.push("-f", "h264", "pipe:1");
+  // Audio output: Ogg Opus on fd3 (pipe:3), separate from video stdout so it
+  // never pollutes the AnnexB start-code stream on pipe:1. Map the audio
+  // stream here (after the video -map 0:v:0 above) so it targets only this
+  // output.
   if (mergedOptions.includeAudio) {
-    args.push("-map", "0:a:0?");
     args.push(
+      "-map",
+      "0:a:0?",
       "-c:a",
       "libopus",
       "-b:a",
@@ -195,18 +207,14 @@ export function prepareStream(
       "48000",
       "-ac",
       "2",
+      "-f",
+      "opus",
+      "pipe:3",
     );
   } else {
     args.push("-an");
   }
 
-  args.push(...mergedOptions.customFfmpegFlags);
-  // NUT muxer carries video+audio; the raw h264 muxer cannot ("h264 muxer
-  // does not support any stream of type audio" → header write fails →
-  // empty stdout → black tile). Audio delivery requires NUT.
-  const outFormat = mergedOptions.includeAudio ? "nut" : "h264";
-  args.push("-f", outFormat, "pipe:1");
-
   const isUrl = typeof input === "string";
   const proc: ChildProcess = isUrl
     ? spawn(FFMPEG_BIN, args, { stdio: ["ignore", "pipe", "pipe"] })
@@ -255,7 +263,7 @@ export function prepareStream(
     height: mergedOptions.height,
     frameRate: mergedOptions.frameRate,
     includeAudio: !!mergedOptions.includeAudio,
-    format: outFormat,
+    format: "h264",
   };
 }