fix(profiling): fix race condition on fork in child

[KowalskiThomas]

Description

Fixes a segmentation fault (SIGSEGV) that occurs when a profiled application calls os.fork().

Root cause: The Sampler and ProfilerState each register pthread_atfork child handlers. Due to LIFO ordering, the Sampler's handler runs first: it cleans up stale state and then restarts the sampling thread via restart_after_fork(). On a multi-core system, the new thread can begin executing immediately — acquiring profile_mtx and calling ddog_prof_Profile_add2 — before ProfilerState's handler runs. When ProfilerState's handler subsequently reinitializes profile_mtx (placement-new) and drops/recreates the profile, it races with the already-running sampling thread, causing concurrent mutation and a SIGSEGV in free() during ddog_prof_Profile_drop.

Fix: Move the sampling thread restart from the Sampler's atfork child handler to the end of ProfilerState::postfork_child(), after profile and dictionary are fully re-initialized. The Sampler's atfork handler now only does cleanup (safe to run first). A new free function stack_restart_sampler_after_fork() in sampler.cpp is called via extern from profiler_state.cpp.

Changes

• Remove restart_after_fork() call from stack_atfork_child() in sampler.cpp
• Add stack_restart_sampler_after_fork() free function in sampler.cpp
• Call it at the end of ProfilerState::postfork_child() via extern declaration in profiler_state.cpp

Testing

• Ran cppcheck and clang-format on all modified C++ files (passed)
• Full native build could not be run due to sandboxed environment network restrictions

Risks

Low — the sampling thread restart still happens during the same __libc_fork call, just later in the atfork handler sequence. No change to steady-state profiler behavior.

Additional Notes

Crash stack trace: free → drop_in_place<IndexSet<StackTrace>> → drop_in_place<Profile> → ddog_prof_Profile_drop → Profile::postfork_child → __libc_fork → os_fork

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Bits Dev status: ✅ Done

CI Auto-fix: Disabled | Enable

Comment @DataDog to request changes

[datadog-prod-us1-5]

I can only run on private repositories.

[cit-pr-commenter-54b7da]

Codeowners resolved as

ddtrace/internal/datadog/profiling/dd_wrapper/include/ddup_interface.hpp  @DataDog/profiling-python
ddtrace/internal/datadog/profiling/dd_wrapper/include/profiler_state.hpp  @DataDog/profiling-python
ddtrace/internal/datadog/profiling/dd_wrapper/src/ddup_interface.cpp    @DataDog/profiling-python
ddtrace/internal/datadog/profiling/dd_wrapper/src/profiler_state.cpp    @DataDog/profiling-python
ddtrace/internal/datadog/profiling/stack/src/sampler.cpp                @DataDog/profiling-python

[datadog-datadog-prod-us1-2]

✅ Tests

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

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