fix(profiling): reset all profiling c++ mutexes on fork (#11768)

I'm not sure why it took so long to surface this defect, but it turns
out that stack v2 can deadlock applications because not all mutices are
reset.

The repro in #11762 appears to be pretty durable. I need to investigate
it a bit more in order to distill it down into a native stress test we
can use moving forward. In practice, this patch suppresses the noted
behavior in the repro.

## Checklist
- [X] PR author has checked that all the criteria below are met
- The PR description includes an overview of the change
- The PR description articulates the motivation for the change
- The change includes tests OR the PR description describes a testing
strategy
- The PR description notes risks associated with the change, if any
- Newly-added code is easy to change
- The change follows the [library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
- The change includes or references documentation updates if necessary
- Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist
- [x] Reviewer has checked that all the criteria below are met 
- Title is accurate
- All changes are related to the pull request's stated goal
- Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes
- Testing strategy adequately addresses listed risks
- Newly-added code is easy to change
- Release note makes sense to a user of the library
- If necessary, author has acknowledged and discussed the performance
implications of this PR as reported in the benchmarks PR comment
- Backport labels are set in a manner that is consistent with the
[release branch maintenance
policy](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting)

---------

Co-authored-by: Taegyun Kim <[email protected]>

ddtrace/internal/datadog/profiling/dd_wrapper/include/uploader_builder.hpp

@@ -43,6 +43,8 @@ class UploaderBuilder
     static void set_output_filename(std::string_view _output_filename);
 
     static std::variant<Uploader, std::string> build();
+
+    static void postfork_child();
 };
 
 } // namespace Datadog

ddtrace/internal/datadog/profiling/dd_wrapper/src/code_provenance.cpp

@@ -14,9 +14,8 @@ namespace Datadog {
 void
 Datadog::CodeProvenance::postfork_child()
 {
-    get_instance().mtx.~mutex();            // Destroy the mutex
+    // NB placement-new to re-init and leak the mutex because doing anything else is UB
     new (&get_instance().mtx) std::mutex(); // Recreate the mutex
-    get_instance().reset();                 // Reset the state
 }
 
 void

ddtrace/internal/datadog/profiling/dd_wrapper/src/ddup_interface.cpp

@@ -24,6 +24,7 @@ ddup_postfork_child()
     Datadog::Uploader::postfork_child();
     Datadog::SampleManager::postfork_child();
     Datadog::CodeProvenance::postfork_child();
+    Datadog::UploaderBuilder::postfork_child();
 }
 
 void

ddtrace/internal/datadog/profiling/dd_wrapper/src/profile.cpp

@@ -203,6 +203,6 @@ Datadog::Profile::collect(const ddog_prof_Sample& sample, int64_t endtime_ns)
 void
 Datadog::Profile::postfork_child()
 {
-    profile_mtx.unlock();
+    new (&profile_mtx) std::mutex();
     cycle_buffers();
 }

ddtrace/internal/datadog/profiling/dd_wrapper/src/sample.cpp

@@ -408,7 +408,6 @@ Datadog::Sample::push_absolute_ns(int64_t _timestamp_ns)
     return true;
 }
 
-
 bool
 Datadog::Sample::push_monotonic_ns(int64_t _monotonic_ns)
 {

ddtrace/internal/datadog/profiling/dd_wrapper/src/uploader.cpp

@@ -175,5 +175,6 @@ Datadog::Uploader::postfork_parent()
 void
 Datadog::Uploader::postfork_child()
 {
-    unlock();
+    // NB placement-new to re-init and leak the mutex because doing anything else is UB
+    new (&upload_lock) std::mutex();
 }

ddtrace/internal/datadog/profiling/dd_wrapper/src/uploader_builder.cpp

@@ -186,3 +186,10 @@ Datadog::UploaderBuilder::build()
 
     return Datadog::Uploader{ output_filename, ddog_exporter };
 }
+
+void
+Datadog::UploaderBuilder::postfork_child()
+{
+    // NB placement-new to re-init and leak the mutex because doing anything else is UB
+    new (&tag_mutex) std::mutex();
+}

ddtrace/internal/datadog/profiling/stack_v2/src/sampler.cpp

@@ -67,6 +67,11 @@ _stack_v2_atfork_child()
     // so we don't even reveal this function to the user
     _set_pid(getpid());
     ThreadSpanLinks::postfork_child();
+
+    // `thread_info_map_lock` and `task_link_map_lock` are global locks held in echion
+    // NB placement-new to re-init and leak the mutex because doing anything else is UB
+    new (&thread_info_map_lock) std::mutex;
+    new (&task_link_map_lock) std::mutex;
 }
 
 __attribute__((constructor)) void

ddtrace/internal/datadog/profiling/stack_v2/src/thread_span_links.cpp

@@ -53,10 +53,8 @@ ThreadSpanLinks::reset()
 void
 ThreadSpanLinks::postfork_child()
 {
-    // Explicitly destroy and reconstruct the mutex to avoid undefined behavior
-    get_instance().mtx.~mutex();
+    // NB placement-new to re-init and leak the mutex because doing anything else is UB
     new (&get_instance().mtx) std::mutex();
-
     get_instance().reset();
 }
 

releasenotes/notes/fix-profiling-native-mutices-62440b5a3d9d6c4b.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    profiling: Fixes a bug where profiling mutexes were not cleared on fork in the child process.  This could
+    cause deadlocks in certain configurations.