Implement current_fiber_for()

This will be used to access the OTEL context after
open-telemetry/opentelemetry-ruby#1807 .

Author: ivoanjo

ext/datadog_profiling_native_extension/extconf.rb

@@ -164,6 +164,10 @@ def skip_building_extension!(reason)
 # On older Rubies, "pop" was not a primitive operation
 $defs << "-DNO_PRIMITIVE_POP" if RUBY_VERSION < "3.2"
 
+# We could support this for older Rubies, but since this only gets used by the OTEL context extraction, and that
+# use-case is only for 3.1+, we didn't bother supporting it farther back yet.
+$defs << "-DNO_CURRENT_FIBER_FOR" if RUBY_VERSION < "3.1"
+
 # On older Rubies, there was no tid member in the internal thread structure
 $defs << "-DNO_THREAD_TID" if RUBY_VERSION < "3.1"
 

ext/datadog_profiling_native_extension/private_vm_api_access.c

@@ -803,3 +803,50 @@ static inline int ddtrace_imemo_type(VALUE imemo) {
 
 // Is the VM smack in the middle of raising an exception?
 bool is_raised_flag_set(VALUE thread) { return thread_struct_from_object(thread)->ec->raised_flag > 0; }
+
+#ifndef NO_CURRENT_FIBER_FOR
+  // The following three declarations are all
+  // taken from upstream cont.c at commit d97884a58be32e829fd03a80cd521f4733d65c79 (February 2025, master branch)
+  // (See the Ruby project copyright and license above)
+  // to enable building `current_fiber_for`.
+  //
+  // We needed to copy them because they aren't otherwise exposed in any VM APIs or headers.
+  // @ivoanjo: I manually checked the Ruby 3.1, 3.2, 3.3 and 3.4 branches + master, and the parts we care about in these
+  // structures have not changed in many years (in fact, last change I spotted was for 2.7).
+  enum context_type {
+      CONTINUATION_CONTEXT = 0,
+      FIBER_CONTEXT = 1
+  };
+
+  typedef struct rb_context_struct { // This declaration is incomplete -- only contains up to `self` which is the part we care about
+      enum context_type type;
+      int argc;
+      int kw_splat;
+      VALUE self;
+  } rb_context_t;
+
+  struct rb_fiber_struct { // This declaration is incomplete -- only contains the first entry which is the part we care about
+    rb_context_t cont;
+  };
+
+  VALUE current_fiber_for(VALUE thread) {
+    VALUE self = thread_struct_from_object(thread)->ec->fiber_ptr->cont.self;
+    return self == 0 ? Qnil : self;
+  }
+
+  void self_test_current_fiber_for(void) {
+    VALUE expected_current_fiber = current_fiber_for(rb_thread_current());
+    VALUE actual_current_fiber = rb_fiber_current();
+
+    if (expected_current_fiber == Qnil) {
+      // On purpose above we tried reading before calling `rb_fiber_current()` so the fiber may have not existed yet.
+      // But now it should be there.
+      expected_current_fiber = current_fiber_for(rb_thread_current());
+    }
+
+    if (expected_current_fiber != actual_current_fiber) rb_raise(rb_eRuntimeError, "current_fiber_for() self-test failed");
+  }
+#else
+  VALUE current_fiber_for(VALUE thread) { rb_raise(rb_eRuntimeError, "Not implemented for Ruby < 3.1"); }
+  void self_test_current_fiber_for(void) { } // Nothing to do
+#endif

ext/datadog_profiling_native_extension/private_vm_api_access.h

@@ -44,6 +44,7 @@ bool is_thread_alive(VALUE thread);
 VALUE thread_name_for(VALUE thread);
 
 int ddtrace_rb_profile_frames(VALUE thread, int start, int limit, frame_info *stack_buffer);
+
 // Returns true if the current thread belongs to the main Ractor or if Ruby has no Ractor support
 bool ddtrace_rb_ractor_main_p(void);
 
@@ -70,3 +71,9 @@ const char *imemo_kind(VALUE imemo);
   { if (RB_UNLIKELY(!rb_typeddata_is_kind_of(value, RTYPEDDATA_TYPE(rb_thread_current())))) raise_unexpected_type(value, ADD_QUOTES(value), "Thread", __FILE__, __LINE__, __func__); }
 
 bool is_raised_flag_set(VALUE thread);
+
+// Can be nil if `rb_fiber_current()` or similar has not been called (gets allocated lazily)
+// Only implemented for Ruby 3.1+
+VALUE current_fiber_for(VALUE thread);
+
+void self_test_current_fiber_for(void);

ext/datadog_profiling_native_extension/profiling.c

@@ -86,6 +86,7 @@ void DDTRACE_EXPORT Init_datadog_profiling_native_extension(void) {
 
 static VALUE native_working_p(DDTRACE_UNUSED VALUE _self) {
   self_test_clock_id();
+  self_test_current_fiber_for();
   self_test_mn_enabled();
 
   return Qtrue;