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

Manual backport of #11768 to 2.17

---------

Co-authored-by: Taegyun Kim <[email protected]>
(cherry picked from commit d855c4a)

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

@@ -201,6 +201,6 @@ 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/uploader.cpp

@@ -176,5 +176,6 @@ 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.