fix(codecs): skip RIFF pad byte after odd-sized WAV chunks

[chuenchen309]

What

RIFF chunks are word-aligned: a chunk whose size is odd is followed by a single pad byte that is not counted in the chunk size. The inline WAV decoder (_WavInlineDecoder) consumed exactly chunk_size bytes for skipped ancillary chunks (LIST/bext/cue /...) and for the fmt chunk, then read the next chunk header immediately — one byte too early whenever the preceding chunk was odd. The misaligned header parses garbage, so the real data chunk is never found and every frame is silently dropped (0 frames, no error).

This hits any spec-compliant WAV that carries an odd-length metadata chunk before data, which tools like ffmpeg/sox routinely emit.

Repro (before fix):

import asyncio, struct
from livekit.agents.utils.codecs import AudioStreamDecoder

def make_wav(anc_len):          # LIST chunk before data; odd len => 1 RIFF pad byte
    sr, ch, bps, n = 24000, 1, 16, 2400
    fmt = struct.pack("<HHIIHH", 1, ch, sr, sr*ch*bps//8, ch*bps//8, bps)
    anc = b"\x00"*anc_len + (b"\x00" if anc_len % 2 else b"")
    body = (b"fmt " + struct.pack("<I", len(fmt)) + fmt
            + b"LIST" + struct.pack("<I", anc_len) + anc
            + b"data" + struct.pack("<I", n*2) + b"\x00"*(n*2))
    return b"RIFF" + struct.pack("<I", 4+len(body)) + b"WAVE" + body, n

async def total(anc_len):
    wav, n = make_wav(anc_len)
    dec = AudioStreamDecoder(sample_rate=24000, num_channels=1, format="audio/wav")
    dec.push(wav); dec.end_input()
    return n, sum(f.samples_per_channel async for f in dec)

async def main():
    print("even LIST (no pad):", await total(4))   # (2400, 2400) ok
    print("odd  LIST (+pad):  ", await total(3))   # (2400, 0)    all audio lost
asyncio.run(main())

Fix

Fold the pad byte into the skip length for ancillary chunks (chunk_size + (chunk_size & 1)), and skip the trailing pad byte for an odd-sized fmt chunk, so the following chunk header stays word-aligned.

Testing

• New parametrized regression test test_wav_inline_decoder_odd_ancillary_chunk (even control + odd case, fed byte-by-byte to exercise the incremental state machine across the pad byte) — red on the odd case before the fix, green after.
• Full tests/test_audio_decoder.py unit suite: 15 passed, 1 skipped (cloud-cred test), no regressions.
• make check (ruff format-check + ruff lint + strict mypy) clean.

AI disclosure

Investigated and implemented with Claude Code (Claude Opus 4.8); I reviewed the diff, wrote and ran the repro, ran the full test suite and make check myself before opening this PR.

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.