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MP4: Remove orphaned mdat when removing QT chapter track

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write() appends a new mdat at EOF to hold chapter text samples but the
removal code (both remove() and the replace-existing path in write())
only deleted the chapter trak and tref atoms from inside moov.  Each
add/remove cycle left the previous chapter mdat behind, causing orphaned
mdat atoms to accumulate.

Fix: extract a removeQTChapterTrack() helper that performs all three
removals atomically.  Before deleting the chapter trak, the helper reads
the first stco chunk offset (which points 8 bytes past the chapter mdat
header) to locate the mdat.  After removing the trak and tref (both
inside moov, which precedes the mdat at EOF), it adjusts the mdat offset
by -(chapterLen + trefLen) and removes the atom, leaving no orphaned data.
This commit is contained in:
Ryan Francesconi
2026-04-22 05:44:23 -07:00
parent 78c7208bc9
commit ae171ee237
1 changed files with 62 additions and 33 deletions
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95
taglib/mp4/mp4qtchapterlist.cpp
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@@ -935,6 +935,66 @@ namespace
}
}
// Removes the QT chapter trak, the tref from the audio track, and the orphaned
// mdat atom that holds the chapter text samples.
//
// The chapter mdat was appended at EOF by write(). Its location is derived
// from the chapter track's stco entry before the track is deleted. Both the
// chapter trak and tref live inside moov, which precedes the mdat, so removing
// them shifts the mdat by -(chapterLen + trefLen).
void removeQTChapterTrack(TagLib::File *file, const MP4::Atoms *atoms,
MP4::Atom *moov, MP4::Atom *chapterTrak,
const MP4::Atom *audioTrak)
{
// Read the first stco chunk offset BEFORE deleting the trak: the chapter
// mdat data starts at stco[0], so the mdat atom header is 8 bytes earlier.
offset_t chapterMdatOffset = -1;
{
const std::vector<unsigned int> stco = readStco(file, chapterTrak);
if(!stco.empty() && stco[0] >= 8)
chapterMdatOffset = static_cast<offset_t>(stco[0]) - 8;
}
// Record tref length BEFORE removeAudioTref so we can adjust the mdat offset.
offset_t trefLen = 0;
for(const auto &child : audioTrak->children()) {
if(child->name() == "tref") {
trefLen = child->length();
break;
}
}
// Remove chapter trak FIRST (higher offset in file).
const offset_t chapterOff = chapterTrak->offset();
const offset_t chapterLen = chapterTrak->length();
// Remove from in-memory tree so updateChunkOffsets skips its stco.
moov->removeChild(chapterTrak);
delete chapterTrak;
file->removeBlock(chapterOff, chapterLen);
const MP4::AtomList moovPath = atoms->path("moov");
updateParentSizes(file, moovPath, -chapterLen);
updateChunkOffsets(file, atoms, -chapterLen, chapterOff);
// Remove tref from audio trak (lower offset, still valid after chapter trak removal).
removeAudioTref(file, atoms, audioTrak);
// Remove the orphaned chapter mdat. Both removals above are inside moov,
// which precedes the mdat at EOF, so adjust the stored offset accordingly.
if(chapterMdatOffset >= 0) {
const offset_t adjustedOffset = chapterMdatOffset - chapterLen - trefLen;
file->seek(adjustedOffset);
const ByteVector header = file->readBlock(8);
if(header.size() == 8 && header.mid(4, 4) == "mdat") {
const offset_t mdatSize = header.toUInt();
if(mdatSize >= 8)
file->removeBlock(adjustedOffset, mdatSize);
}
}
}
} // namespace
////////////////////////////////////////////////////////////////////////////////
@@ -1040,22 +1100,7 @@ bool MP4::QtChapterList::write(TagLib::File *file)
std::unique_ptr<Atoms> cleanAtoms;
if(Atom *existingChapter = findChapterTrak(file, &atoms, audio.trak)) {
// Remove chapter trak FIRST (higher offset in file).
const offset_t chapterOff = existingChapter->offset();
const offset_t chapterLen = existingChapter->length();
// Remove from in-memory tree so updateChunkOffsets skips its stco.
moov->removeChild(existingChapter);
delete existingChapter;
file->removeBlock(chapterOff, chapterLen);
const AtomList moovPath = atoms.path("moov");
updateParentSizes(file, moovPath, -chapterLen);
updateChunkOffsets(file, &atoms, -chapterLen, chapterOff);
// Remove tref from audio trak (lower offset, still valid).
removeAudioTref(file, &atoms, audio.trak);
removeQTChapterTrack(file, &atoms, moov, existingChapter, audio.trak);
// Re-parse to get clean state after removals.
cleanAtoms = std::make_unique<Atoms>(file);
@@ -1171,22 +1216,6 @@ bool MP4::QtChapterList::remove(TagLib::File *file)
if(!moov)
return false;
// Remove chapter trak FIRST (higher offset in file).
const offset_t chapterOff = chapterTrak->offset();
const offset_t chapterLen = chapterTrak->length();
// Remove from in-memory tree so updateChunkOffsets skips its stco.
moov->removeChild(chapterTrak);
delete chapterTrak;
file->removeBlock(chapterOff, chapterLen);
const AtomList moovPath = atoms.path("moov");
updateParentSizes(file, moovPath, -chapterLen);
updateChunkOffsets(file, &atoms, -chapterLen, chapterOff);
// Remove tref from audio trak (lower offset, still valid after chapter trak removal).
removeAudioTref(file, &atoms, audio.trak);
removeQTChapterTrack(file, &atoms, moov, chapterTrak, audio.trak);
return true;
}