GVL Instrumentation: pass thread->self as part of event data

Context: https://github.com/ivoanjo/gvl-tracing/pull/4

Some hooks may want to collect data on a per thread basis.
Right now the only way to identify the concerned thread is to
use `rb_nativethread_self()` or similar, but even then because
of the thread cache or MaNy, two distinct Ruby threads may report
the same native thread id.

By passing `thread->self`, hooks can use it as a key to store
the metadata.

NB: Most hooks are executed outside the GVL, so such data collection
need to use a thread-safe data-structure, and shouldn't use the
reference in other ways from inside the hook.

They must also either pin that value or handle compaction.

1 files changed, 19 insertions, 0 deletions

diff --git a/ext/-test-/thread/instrumentation/instrumentation.c b/ext/-test-/thread/instrumentation/instrumentation.c
index edb8738a29..4d76f4cbb3 100644
--- a/ext/-test-/thread/instrumentation/instrumentation.c
+++ b/ext/-test-/thread/instrumentation/instrumentation.c
@@ -16,11 +16,14 @@ static RB_THREAD_LOCAL_SPECIFIER unsigned int local_ready_count = 0;
 static RB_THREAD_LOCAL_SPECIFIER unsigned int local_resumed_count = 0;
 static RB_THREAD_LOCAL_SPECIFIER unsigned int local_suspended_count = 0;
 
+static VALUE last_thread = Qnil;
+
 static void
 ex_callback(rb_event_flag_t event, const rb_internal_thread_event_data_t *event_data, void *user_data)
 {
     switch (event) {
       case RUBY_INTERNAL_THREAD_EVENT_STARTED:
+        last_thread = event_data->thread;
         RUBY_ATOMIC_INC(started_count);
         break;
       case RUBY_INTERNAL_THREAD_EVENT_READY:
@@ -122,15 +125,31 @@ thread_register_and_unregister_callback(VALUE thread)
     return Qtrue;
 }
 
+static VALUE
+thread_last_spawned(VALUE mod)
+{
+    return last_thread;
+}
+
+static VALUE
+thread_set_last_spawned(VALUE mod, VALUE value)
+{
+    return last_thread = value;
+}
+
 void
 Init_instrumentation(void)
 {
     VALUE mBug = rb_define_module("Bug");
     VALUE klass = rb_define_module_under(mBug, "ThreadInstrumentation");
+    rb_global_variable(&last_thread);
     rb_define_singleton_method(klass, "counters", thread_counters, 0);
     rb_define_singleton_method(klass, "local_counters", thread_local_counters, 0);
     rb_define_singleton_method(klass, "reset_counters", thread_reset_counters, 0);
     rb_define_singleton_method(klass, "register_callback", thread_register_callback, 0);
     rb_define_singleton_method(klass, "unregister_callback", thread_unregister_callback, 0);
     rb_define_singleton_method(klass, "register_and_unregister_callbacks", thread_register_and_unregister_callback, 0);
+
+    rb_define_singleton_method(klass, "last_spawned_thread", thread_last_spawned, 0);
+    rb_define_singleton_method(klass, "last_spawned_thread=", thread_set_last_spawned, 1);
 }