Add profiler helper for detecting if we're on the main Ractor

ext/ddtrace_profiling_native_extension/extconf.rb

@@ -125,6 +125,9 @@ def add_compiler_flag(flag)
 # On older Rubies, we need to use a backported version of this function. See private_vm_api_access.h for details.
 $defs << '-DUSE_BACKPORTED_RB_PROFILE_FRAME_METHOD_NAME' if RUBY_VERSION < '3'
 
+# On older Rubies, there are no Ractors
+$defs << '-DNO_RACTORS' if RUBY_VERSION < '3'
+
 # On older Rubies, we need to use rb_thread_t instead of rb_execution_context_t
 $defs << '-DUSE_THREAD_INSTEAD_OF_EXECUTION_CONTEXT' if RUBY_VERSION < '2.5'
 

ext/ddtrace_profiling_native_extension/private_vm_api_access.c

@@ -690,3 +690,28 @@ int ruby_thread_has_gvl_p(void) {
   return 0;
 }
 #endif // NO_THREAD_HAS_GVL
+
+#ifndef NO_RACTORS
+  // This API and definition are exported as a public symbol by the VM BUT the function header is not defined in any public header, so we
+  // repeat it here to be able to use in our code.
+  bool rb_ractor_main_p_(void);
+  extern struct rb_ractor_struct *ruby_single_main_ractor;
+
+  // Taken from upstream ractor_core.h at commit d9cf0388599a3234b9f3c06ddd006cd59a58ab8b (November 2022, Ruby 3.2 trunk)
+  // to allow us to ensure that we're always operating on the main ractor (if Ruby has ractors)
+  // Modifications:
+  // * None
+  bool ddtrace_rb_ractor_main_p(void)
+  {
+      if (ruby_single_main_ractor) {
+          return true;
+      }
+      else {
+          return rb_ractor_main_p_();
+      }
+  }
+#else
+  // Simplify callers on older Rubies, instead of having them probe if the VM supports Ractors we just tell them that yes
+  // they're always on the main Ractor
+  bool ddtrace_rb_ractor_main_p(void) { return true; }
+#endif // NO_RACTORS

ext/ddtrace_profiling_native_extension/private_vm_api_access.h

@@ -18,6 +18,8 @@ bool is_thread_alive(VALUE thread);
 VALUE thread_name_for(VALUE thread);
 
 int ddtrace_rb_profile_frames(VALUE thread, int start, int limit, VALUE *buff, int *lines, bool* is_ruby_frame);
+// 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);
 
 // Ruby 3.0 finally added support for showing CFUNC frames (frames for methods written using native code)
 // in stack traces gathered via `rb_profile_frames` (https://github.com/ruby/ruby/pull/3299).

ext/ddtrace_profiling_native_extension/profiling.c

@@ -2,6 +2,7 @@
 
 #include "clock_id.h"
 #include "helpers.h"
+#include "private_vm_api_access.h"
 
 // Each class/module here is implemented in their separate file
 void collectors_cpu_and_wall_time_init(VALUE profiling_module);
@@ -11,6 +12,7 @@ void http_transport_init(VALUE profiling_module);
 void stack_recorder_init(VALUE profiling_module);
 
 static VALUE native_working_p(VALUE self);
+static VALUE _native_ddtrace_rb_ractor_main_p(DDTRACE_UNUSED VALUE _self);
 
 void DDTRACE_EXPORT Init_ddtrace_profiling_native_extension(void) {
   VALUE datadog_module = rb_define_module("Datadog");
@@ -27,10 +29,18 @@ void DDTRACE_EXPORT Init_ddtrace_profiling_native_extension(void) {
   collectors_stack_init(profiling_module);
   http_transport_init(profiling_module);
   stack_recorder_init(profiling_module);
+
+  // Hosts methods used for testing the native code using RSpec
+  VALUE testing_module = rb_define_module_under(native_extension_module, "Testing");
+  rb_define_singleton_method(testing_module, "_native_ddtrace_rb_ractor_main_p", _native_ddtrace_rb_ractor_main_p, 0);
 }
 
 static VALUE native_working_p(DDTRACE_UNUSED VALUE _self) {
   self_test_clock_id();
 
   return Qtrue;
 }
+
+static VALUE _native_ddtrace_rb_ractor_main_p(DDTRACE_UNUSED VALUE _self) {
+  return ddtrace_rb_ractor_main_p() ? Qtrue : Qfalse;
+}

spec/datadog/profiling/native_extension_spec.rb

@@ -1,4 +1,4 @@
-# typed: false
+# typed: ignore
 
 require 'datadog/profiling/spec_helper'
 
@@ -160,4 +160,41 @@ def wait_for_thread_to_die
       end
     end
   end
+
+  describe 'ddtrace_rb_ractor_main_p' do
+    subject(:ddtrace_rb_ractor_main_p) { Datadog::Profiling::NativeExtension::Testing._native_ddtrace_rb_ractor_main_p }
+
+    context 'when Ruby has no support for Ractors' do
+      before { skip 'Behavior does not apply to current Ruby version' if RUBY_VERSION >= '3' }
+
+      it { is_expected.to be true }
+    end
+
+    context 'when Ruby has support for Ractors' do
+      before { skip 'Behavior does not apply to current Ruby version' if RUBY_VERSION < '3' }
+
+      context 'on the main Ractor' do
+        it { is_expected.to be true }
+      end
+
+      context 'on a background Ractor' do
+        # @ivoanjo: When we initially added this test, our test suite kept deadlocking in CI in a later test (not on
+        # this one).
+        #
+        # It turns out that Ruby 3.0 Ractors seem to have some bug that even running `Ractor.new { 'hello' }.take` will
+        # cause a later spec to fail, usually with a (native C) stack with `gc_finalize_deferred`.
+        #
+        # I was able to see this even on both Linux with 3.0.3 and macOS with 3.0.4. Thus, I decided to skip this
+        # spec on Ruby 3.0. We can always run it manually if we change something around this helper; and we have
+        # coverage on 3.1+ anyway.
+        before { skip 'Ruby 3.0 Ractors are too buggy to run this spec' if RUBY_VERSION.start_with?('3.0.') }
+
+        subject(:ddtrace_rb_ractor_main_p) do
+          Ractor.new { Datadog::Profiling::NativeExtension::Testing._native_ddtrace_rb_ractor_main_p }.take
+        end
+
+        it { is_expected.to be false }
+      end
+    end
+  end
 end