Revert 4.0.11 threads normal-mutex debug implementation

[slowriot]

Resolves #26619.

For motivation, please see description and discussion at #26622.

This PR simply reverts the changes between 4.0.10 and 4.0.11 around debug behaviour for PTHREAD_MUTEX_NORMAL.

This also disables the test_pthread_mutex_deadlock test which was testing this functionality, but preserves the code and test setup so it can be re-enabled in future if needed.

Finally, this PR adds a test to detect the regression, as in #26622. The test passes with this fix.

[Copilot]

Pull request overview

This PR reverts Emscripten’s 4.0.11-era debug behavior changes for PTHREAD_MUTEX_NORMAL in musl, restoring the pre-4.0.11 fast-path semantics to address the reported allocator regression under -pthread + -sWASM_WORKERS with assertions/debug-enabled system libraries.

Changes:

• Revert musl pthread normal-mutex debug behavior to always use the historical fast path (and remove the debug deadlock assertion path).
• Disable test_pthread_mutex_deadlock while preserving the underlying test source for potential future re-enable.
• Add a browser regression test for the allocator crash and update pthreads codesize baselines.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
system/lib/libc/musl/src/thread/pthread_mutex_trylock.c	Restores fast-path behavior for normal mutex trylock in debug builds.
system/lib/libc/musl/src/thread/pthread_mutex_lock.c	Restores fast-path behavior for normal mutex lock in debug builds.
system/lib/libc/musl/src/thread/pthread_mutex_timedlock.c	Removes Emscripten debug deadlock assertion and restores normal fast-path.
test/wasm_worker/pthread_mutex_debug_allocator_regression.cpp	Adds a wasm-worker allocator churn repro/regression test.
test/test_browser.py	Wires the new browser regression test into the wasm worker test suite.
test/test_other.py	Disables the deadlock test while documenting the rollback intent.
test/other/test_pthread_mutex_deadlock.c	Updates comments to reflect that deadlock-assert behavior is reverted/disabled.
test/codesize/test_codesize_minimal_pthreads.json	Updates expected code size baselines after the revert.
test/codesize/test_codesize_minimal_pthreads_memgrowth.json	Updates expected code size baselines (memgrowth variant).
AUTHORS	Adds a new author entry.

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[sbc100]

The the bug still reproduce if you convert this from .cpp to .c and use malloc and free?

(You might need to add --no-builtin maybe ?)

[slowriot]

Yes, this test is basically equivalent to the CPP and C repros I provided in the issue description at #26619. Only the C++ path is tested here because I understand them to be identical. --no-builtin shouldn't be necessary as the tests are built with -O0 by default which should prevent those calls being optimised away. Let me know if you'd like me to add a pure C test as well just to be belt-and-braces about it, but I wouldn't normally bother because from previous stack traces (as per the issue report) we can see that malloc and free are always the culprit calls behind the scenes when using new and delete.

[sbc100]

I would rather have just the C version of the test. We tend to prefer C tests for various reasons.