Musepack: don't hang on an SV8 packet smaller than its header - #736

Fixes #433

Problem

MusepackInfo.__parse_sv8 gets the payload size of each SV8 packet with data_size = frame_size - key_size - slen, because the packet size counts the 2 byte key and the size field itself. A packet that claims fewer bytes than its own header makes data_size negative, and packets other than SH and RG are skipped with fileobj.seek(data_size, 1). With a size of 0 that seek lands exactly on the same packet key again, so the loop never ends:

import io, mutagen
mutagen.File(io.BytesIO(b"MPCK" + b"XX\x00"))  # never returns

The same happens when the size 0 is written in two bytes (b"XX\x80\x00"). Sizes 1 and 2 also seek backwards into the header, but the position still moves forward overall, so they don't hang.

Fix

When data_size is negative, raise MusepackHeaderError("Invalid packet size."), the same error already raised when the size field can't be read. ffmpeg's mpc8 demuxer rejects the same case ("Invalid chunk length" in mpc8_read_header). Every accepted packet then moves the file position forward by at least its header, so the loop always makes progress. A packet with an empty payload (data_size == 0) is still accepted, since it moves forward too.

SH and RG packets with a negative size already raised an error, but could first call fileobj.read() with a negative count, which reads the rest of the file. They are now rejected before that.

Tests

Added test_packet_size_smaller_than_header to tests/test_musepack.py, with the size 0 written in one and in two bytes. On main it never finishes (I stopped it after 30 seconds); with the change it passes. test_packet_with_empty_payload checks that a packet whose size only covers its header still parses. uv run pytest passes (4057 tests), and uv run flake8 and uv run mypy . report nothing.