feat(music): transcode WMA/Musepack to AAC so they actually play

Tagging and covers for these two formats landed already, but neither one
decodes in any mainstream browser's <audio> element at all — a real
library scan turned up 273 such files that would show up correctly in the
Music app and then simply fail on click. This closes that gap: the node
transcodes to AAC/M4A on request (a one-shot whole-file conversion, not
live-piped like video's fMP4 segments — an audio file is small enough that
streaming it buys nothing), caches the result under its own content hash
the same way a TMDB poster or a MusicBrainz cover is cached, and serves it
back through the ordinary file_req/chunk path. That path used to assume
anything in the media cache was thumbnail-sized (single chunk, always);
generalized it to slice a cached blob the same way a real file on disk
gets sliced, since a transcoded track can be several MB.

New MNP pair (`audio_transcode_req`/`_resp`, version bump to 0.9), shares
its concurrency cap with video's transcode pool rather than getting its
own — both are real ffmpeg processes on the same node. Every other audio
format is untouched: this only fires for .wma/.mpc, the two extensions
that need it.

1 files changed, 129 insertions, 7 deletions

diff --git a/packages/meshbay-node/src/meshbay_node/transport/webrtc_server.py b/packages/meshbay-node/src/meshbay_node/transport/webrtc_server.py
index 5da1b7e..1629801 100644
--- a/packages/meshbay-node/src/meshbay_node/transport/webrtc_server.py
+++ b/packages/meshbay-node/src/meshbay_node/transport/webrtc_server.py
@@ -29,6 +29,7 @@ import hmac
 import logging
 import os
 import struct
+import tempfile
 import time
 from pathlib import Path
 from typing import Any
@@ -137,6 +138,18 @@ PRE_HANDSHAKE_MAX_MSG = 64 * 1024
 # so the operator sets `max_concurrent_streams` under [node] in node.toml. This
 # value applies when they have said nothing.
 MAX_CONCURRENT_TRANSCODES = 8
+# Extensions no mainstream browser's <audio> element decodes natively, no
+# matter how well-tagged (enrich_audio.py's problem) — the Music app's own
+# analogue of BROWSER_INCOMPATIBLE_VIDEO_CODECS above, keyed by extension
+# rather than a probed codec name since these two are a red flag on their
+# own, not something that varies by how the file happens to be encoded
+# inside.
+BROWSER_INCOMPATIBLE_AUDIO_EXTS = frozenset({".wma", ".mpc"})
+# A whole audio file is small enough to transcode in one shot rather than
+# live-piped like video's fMP4 segments — a few seconds of ffmpeg at most,
+# bounded generously so one slow/huge outlier can't pin a transcode slot
+# (shared with video, MAX_CONCURRENT_TRANSCODES above) indefinitely.
+AUDIO_TRANSCODE_TIMEOUT_SECS = 120
 # Bundle fetches are served in the pre-proof window (C4). Bounded and audited
 # until the native client removes remote keypair bundles entirely.
 MAX_PRE_PROOF_FETCHES = 4
@@ -399,6 +412,8 @@ class WebRTCPeerSession:
                 self._do_musicbrainz_enabled(msg)
             elif mtype == MNP.MUSIC_META_REQ:
                 self._spawn(self._do_music_meta_request(msg))
+            elif mtype == MNP.AUDIO_TRANSCODE_REQ:
+                self._spawn(self._do_audio_transcode_request(msg))
             elif mtype == MNP.MEMBER_UNPIN:
                 self._do_member_unpin(msg)
             elif mtype == MNP.GEK_ROTATE:
@@ -2439,18 +2454,26 @@ class WebRTCPeerSession:
         """
         docs/mediacenter.md §5.3: a thumbnail is served through the same
         chunked file_req path as a real file, resolved against the media
-        cache instead of the index when the id doesn't match a file. Always
-        a single chunk in practice (a thumbnail-sized JPEG never approaches
-        CHUNK_SIZE) — a request for any chunk beyond 0 is just a miss.
+        cache instead of the index when the id doesn't match a file.
+        Sliced by `chunk_index` like a real file's chunks, not just handed
+        back whole: a thumbnail/poster/cover never approached CHUNK_SIZE so
+        this used to be equivalent to "only chunk 0 exists", but an audio
+        transcode result (docs/musicbay.md, the WMA/Musepack exception) is
+        cached in the same media_cache blob store and can be several MB —
+        genuinely multi-chunk, same as a file read straight off disk.
         """
         media_cache = self._ctx.get("media_cache")
-        if media_cache is None or chunk_index != 0:
+        if media_cache is None:
             return None
-        jpeg = await media_cache.get_thumb(thumb_hash)
-        if jpeg is None:
+        blob = await media_cache.get_thumb(thumb_hash)
+        if blob is None:
             return None
+        start = chunk_index * CHUNK_SIZE
+        if start > len(blob) or (start == len(blob) and chunk_index != 0):
+            return None
+        piece = blob[start:start + CHUNK_SIZE]
         return _encrypt_chunk_bytes(
-            self._ctx["sk_node"], gek, jpeg, 0,
+            self._ctx["sk_node"], gek, piece, chunk_index,
             bytes.fromhex(thumb_hash), thumb_hash,
         )
 
@@ -2556,6 +2579,64 @@ class WebRTCPeerSession:
         await media_cache.put_thumb(thumb_hash, synthetic_id, content)
         return thumb_hash
 
+    async def _do_audio_transcode_request(self, msg: dict) -> None:
+        """
+        docs/musicbay.md's one exception to "no node-side transcode pool":
+        WMA and Musepack tag/cover fine (enrich_audio.py) but decode in no
+        mainstream browser's <audio> element at all. Transcoded to AAC/M4A
+        once and cached under its own content hash — same "computed once,
+        reused forever" shape as `_fetch_and_cache_poster`/`_cover` above,
+        served back to the client through the ordinary file_req/chunk path
+        (`_try_serve_thumbnail`, generalized to multi-chunk for this) rather
+        than a new download mechanism.
+        """
+        ctx = self._group_ctx()
+        file_id = msg.get("file_id", "")
+        entry = ctx["index"].get_entry(file_id)
+        if not entry:
+            self._send({"type": "error", "detail": "File not found"})
+            return
+        file_path = entry_abs_path(ctx["roots"], entry)
+        if not file_path.exists():
+            self._send({"type": "error", "detail": "File not on disk"})
+            return
+
+        media_cache = self._ctx.get("media_cache")
+        if media_cache is None:
+            self._send({"type": "error", "detail": "Transcoding unavailable"})
+            return
+
+        synthetic_id = f"audio_transcode:{entry.id}"
+        cached_hash = await media_cache.get_thumb_hash_by_file_id(synthetic_id)
+        if cached_hash is not None:
+            blob = await media_cache.get_thumb(cached_hash)
+            if blob is not None:
+                self._send({"type": MNP.AUDIO_TRANSCODE_RESP, "v": MNP_VERSION,
+                           "file_id": file_id, "hash": cached_hash,
+                           "size": len(blob), "mime": "audio/mp4"})
+                return
+            # Cached hash but the blob itself was pruned: fall through and
+            # transcode again below, same as a cold cache.
+
+        sem = self._transcode_semaphore()
+        if sem.locked() and sem._value <= 0:
+            self._send({"type": "error", "detail": "Server busy, retry shortly"})
+            return
+        async with sem:
+            try:
+                blob = await _transcode_audio_to_aac(file_path)
+            except Exception as e:
+                log.warning("Audio transcode failed for %s: %s", entry.id[:12], e)
+                self._send({"type": "error", "detail": f"Transcode failed: {e}"})
+                return
+
+        transcode_hash = blake3.blake3(blob).hexdigest()
+        await media_cache.put_thumb(transcode_hash, synthetic_id, blob)
+        self._audit("audio_transcode", entry.name)
+        self._send({"type": MNP.AUDIO_TRANSCODE_RESP, "v": MNP_VERSION,
+                   "file_id": file_id, "hash": transcode_hash,
+                   "size": len(blob), "mime": "audio/mp4"})
+
     async def _do_music_meta_request(self, msg: dict) -> None:
         """
         docs/musicbay.md §4.3: MusicBrainz metadata for one path, resolved
@@ -4018,6 +4099,47 @@ def _read_and_encrypt(
     return _encrypt_chunk_bytes(sk_node, gek, plaintext, chunk_index, file_hash, file_id)
 
 
+async def _transcode_audio_to_aac(file_path: Path) -> bytes:
+    """
+    One-shot, whole-file transcode to AAC in an M4A container — no live
+    piping, no seeking, unlike `_stream_video_inner`'s fMP4 segments: a
+    WMA/Musepack source here is a few MB at most, so there is nothing to
+    gain from streaming it and a real cost to the added complexity
+    (fragmented output needs `-movflags empty_moov` and its own
+    client-side reassembly). A plain temp file lets ffmpeg write a normal,
+    fully-seekable M4A container instead. `-vn` drops any attached-picture
+    "video" stream some taggers embed as cover art — without it, ffmpeg's
+    mp4 muxer has been seen treating that picture as a video track to
+    encode, which is not what this is for; cover art still comes from the
+    ordinary embedded/sibling-file path (enrich_audio.py), never from here.
+    """
+    fd, tmp_name = tempfile.mkstemp(suffix=".m4a")
+    os.close(fd)
+    tmp_path = Path(tmp_name)
+    try:
+        proc = await asyncio.create_subprocess_exec(
+            "ffmpeg", "-hide_banner", "-loglevel", "error", "-y",
+            "-i", str(file_path),
+            "-vn", "-c:a", "aac", "-ac", "2", "-b:a", "192k",
+            "-f", "ipod", str(tmp_path),
+            stdout=asyncio.subprocess.DEVNULL,
+            stderr=asyncio.subprocess.PIPE,
+        )
+        try:
+            _, stderr = await asyncio.wait_for(
+                proc.communicate(), timeout=AUDIO_TRANSCODE_TIMEOUT_SECS)
+        except asyncio.TimeoutError:
+            proc.kill()
+            await proc.wait()
+            raise RuntimeError(f"ffmpeg timed out after {AUDIO_TRANSCODE_TIMEOUT_SECS}s")
+        if proc.returncode != 0:
+            raise RuntimeError(
+                f"ffmpeg exited {proc.returncode}: {stderr.decode(errors='replace')[:300]}")
+        return tmp_path.read_bytes()
+    finally:
+        tmp_path.unlink(missing_ok=True)
+
+
 class WebRTCTransport:
     """
     Manages WebRTC peer connections for browser clients.