fix: add_track(str) no longer reuses the source file's mkvmerge info

[GitBib]

Fixes #123.

The issue reports wrong metadata. It is worse than that — the added track is silently dropped from the muxed output.

mkv = MKVFile("tests/file_2.mkv")
mkv.add_track("s.srt")
mkv.mux("out.mkv")     # returns 0, no warning

generated command, trimmed to the added file:

--no-audio --video-tracks 0 --no-subtitles s.srt

The track is built with existing_info=self._info_json, which is the mkvmerge -J output of the file MKVFile was constructed from, not of s.srt. MKVTrack reads metadata from that JSON instead of probing the file, and track_id defaults to 0, so the SRT reports itself as track 0 of the source — video. TrackOptions then asks the SRT for a video track and explicitly suppresses subtitles. An SRT has no video track, so nothing is taken from it.

	tracks in output
before	video, audio
after	video, audio, subtitles

The same line also suppressed verification: MKVTrack.file_path skips verify_supported when _info_json is set, so any path was accepted through this branch.

Nothing is lost by dropping it. MKVFile.__init__ builds its tracks with existing_info=info_struct directly and passes MKVTrack objects to add_track, so the str branch is never used while loading a file — the reuse only ever applied to caller-supplied paths, where it was wrong by construction.

Behaviour change worth noting: add_track(str) on an MKVFile built from a file now raises ValueError for an unsupported path instead of accepting it silently. Code that relied on the old silence was already producing a broken mux, but it will now fail loudly.

Cost: add_track(str) now runs mkvmerge -J on the added file, about 33 ms here. MKVTrack(path) already pays exactly this, so add_track(str) just costs the same as the equivalent explicit construction.

Both new tests fail on master and pass with the fix — the second one covers the verification bypass, which is DID NOT RAISE before the change.

---

Second commit: probe each track file once instead of twice.

Profiling the fix showed MKVTrack running mkvmerge -J twice per construction — verify_supported() in the file_path setter fetches the info and returns only a bool, then the track_id setter fetches the same JSON again. Same root cause as above: _info_json doubles as both the probe cache and the "caller vouched, skip verification" flag.

Split by _info_path, the path the cached info describes. The setter now probes once and keeps the result, existing_info is trusted for the first assignment only, and pointing the track elsewhere drops the cache.

This also fixes stale metadata on reassignment, which was a second latent bug of the same family:

t = MKVTrack("sub.srt")      # subtitles
t.file_path = "video.mkv"
t.track_type                 # 'subtitles' before, 'video' now

Verification of the new path was skipped too, for the same reason.

20 iterations of MKVFile + add_track + command: 1.56s → 1.10s, add_track itself halved from 1.01s to 0.53s.

verify_supported is replaced by info.container.supported, which is literally what that function returns; checking_file_path had already validated the path, so only the mkvmerge lookup remains and that one is cached.

Test churn worth flagging: 20 patches of verify_supported in the MKVTrack namespace are gone. They targeted a function that is no longer called, so leaving them would mean tests passing even with verification broken. test_track_file_path_setter_verification_failure now uses a real empty file, which mkvmerge reports as unsupported, instead of mocking the check away.

Both new reassignment tests fail without this commit.

[codecov]

Codecov Report

❌ Patch coverage is 86.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.92%. Comparing base (3165152) to head (d8deb2d).

Files with missing lines	Patch %	Lines
pymkv/MKVTrack.py	85.71%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #129      +/-   ##
==========================================
- Coverage   99.11%   98.92%   -0.20%     
==========================================
  Files          21       21              
  Lines        1477     1487      +10     
==========================================
+ Hits         1464     1471       +7     
- Misses         13       16       +3     

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