Fix deadlock in multithreaded excerpt serialisation

[ajuncosa]

The goal is to avoid side effects on the score during write/serialisation, so excerpt serialisation can safely run in separate threads.

There was a deadlock when saving scores (with "Save as...") that contained multi measure rests and some hidden parts. The save called layout in that case, which frees Systems/Pages, and the EngravingItem destructor calls Score::onElementDestruction which ends with score->elementDestroyed().send(e). This send was cross-thread, and that channel has a main-thread receiver for every score, so the send must register a port on the main thread's slot (QueuePool::regPort), and this locks the same mutex that the main thread is holding while it's processing queued callbacks (in QueuePool::processMessages), and the save itself is one of the queued commands.

Apart from the freezing doLayout() call, the write was also modifying the score in other ways (start/endCmd(), undoChangeProperty(VISIBLE, true)), and all this routes to the master score's single shared TransactionManager, whose endTransaction calls m_masterScore->update(), relaying out the master from a worker thread.

I have moved the unhide and relayout out of the writer.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

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Run ID: 78b2b6bb-a791-465e-be1b-b3e08d7423c2

📥 Commits

Reviewing files that changed from the base of the PR and between 2963eb8 and bc57855.

📒 Files selected for processing (3)

• src/engraving/rw/mscsaver.cpp
• src/engraving/rw/mscsaver.h
• src/engraving/rw/write/writer.cpp

💤 Files with no reviewable changes (1)

• src/engraving/rw/write/writer.cpp

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📝 Walkthrough

Walkthrough

writeMscz and exportPart now use UnhidePartsForWrite during serialization. The guard temporarily makes hidden parts visible, recalculates layout data, and rolls back the visibility changes through the undo stack. Writer::write no longer performs its own temporary unhide and restore sequence.

Merge Risk: 🔵 Low · up to bc578

The change moves temporary visibility and layout work before parallel serialization, but overlapping saves or active score transactions could leave unrelated score changes rolled back or temporarily inconsistent. The PR is mergeable with explicit owner awareness and follow-up to ensure rollback is limited to transactions opened by serialization.

🚥 Pre-merge checks | ✅ 3 | ❌ 2

❌ Failed checks (2 warnings)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 11.11% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 9 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.
Description check	⚠️ Warning	The description clearly explains the deadlock, its cause, and the implementation approach. However, it omits the required issue reference and all checklist items from the repository template.	Add a Resolves: #NNNNN`` issue reference, complete every checklist item, and document any applicable testing or prior attempts.

✅ Passed checks (3 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly identifies the primary change: fixing a deadlock during multithreaded excerpt serialization.

• Fix all pre-merge checks with AI

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

Warning

Linked repositories: Public OSS repositories can only analyze public repositories installed in this organization. No linked repositories were analyzed; skipped musescore/muse_framework.git.

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