Fix musl TLS init crash with signal blocking

[jbachorik]

What does this PR do?:
Fixes a crash on musl libc systems where profiling signal handlers interrupt thread-local storage (TLS) initialization, causing reentrancy into musl's non-reentrant TLS setup code.

Motivation:
On musl-based systems (Alpine Linux, embedded systems), the profiler crashes with this stack trace:

#0 Recording::recordExecutionSample
#1 FlightRecorder::recordEvent
#2 Profiler::recordSample
#3 CTimer::signalHandler         ← Signal handler runs
#4 ld-musl-x86_64.so.1           ← INSIDE musl's TLS initialization
#5 Java_...initializeContextTls0 ← JNI call accessing context_tls_v1

The issue occurs because:

1. Thread calls initializeContextTls0() which accesses context_tls_v1 (thread_local variable)
2. First access to thread_local triggers musl's TLS initialization
3. Profiling signal (SIGPROF/SIGVTALRM/CTimer) arrives during musl's TLS setup
4. Signal handler tries to access ProfiledThread TLS via pthread_getspecific()
5. This reenters musl's non-reentrant TLS code → crash/deadlock

Additional Notes:

• The fix uses RAII-based signal blocking during the critical TLS initialization window
• Consolidates CriticalSection and new SignalBlocker into unified guards.h/cpp module
• Uses proper ACQUIRE/RELEASE memory ordering for thread safety
• Blocks all profiling signals: SIGPROF, SIGVTALRM, and RT signals (SIGRTMIN to SIGRTMIN+5)
• All 128 tests pass after the fix

How to test the change?:

1. Automated tests: All existing tests pass (128/128), including TLS-related tests:

   • gtestDebug_context_sanity_ut - Context TLS tests
   • gtestDebug_test_tlsPriming - TLS priming tests
   • gtestDebug_stress_callTraceStorage - Uses CriticalSection

2. Manual testing on musl: Deploy on Alpine Linux or musl-based system with high thread creation rate during active profiling to reproduce the race condition.

3. Code review: Focus areas:

   • Thread safety: ACQUIRE/RELEASE memory ordering in guards.cpp
   • Async-signal safety: Signal blocking covers all profiling signals
   • Exception safety: RAII pattern guarantees signal mask restoration

For Datadog employees:

• This PR doesn't touch any of that.
• JIRA: PROF-13683

🤖 Generated with Claude Code

[dd-octo-sts]

CI Test Results

Run: #21720936948 | Commit: 15cbf27 | Duration: 0s (longest job)

✅ All 0 test jobs passed

Summary

Metric	Value
Total jobs	0
Passed	0
Failed	0

---

Updated: 2026-02-05 17:19:56 UTC

[dd-octo-sts]

Scan-Build Report

User:	runner@runnervmkj6or
Working Directory:	/home/runner/work/java-profiler/java-profiler/ddprof-lib/src/test/make
Command Line:	make -j4 all
Clang Version:	Ubuntu clang version 18.1.3 (1ubuntu1)
Date:	Thu Feb 5 17:06:48 2026

Bug Summary

Bug Type	Quantity	Display?
All Bugs	1
Unused code
Dead assignment	1

Reports

Bug Group	Bug Type ▾	File	Function/Method	Line	Path Length
Unused code	Dead assignment	libraryPatcher_linux.cpp	patch_library_unlocked	94	1