From 649bfbe00d8032fa2c0536e596a284f69926e87f Mon Sep 17 00:00:00 2001
From: Ivo Anjo <ivo.anjo@datadoghq.com>
Date: Mon, 11 Jul 2022 14:51:44 +0100
Subject: [PATCH] Fix `rb_profile_frames` output includes dummy main thread
 frame

The `rb_profile_frames` API did not skip the two dummy frames that
each thread has at its beginning. This was unlike `backtrace_each` and
`rb_ec_parcial_backtrace_object`, which do skip them.

This does not seem to be a problem for non-main thread frames,
because both `VM_FRAME_RUBYFRAME_P(cfp)` and
`rb_vm_frame_method_entry(cfp)` are NULL for them.

BUT, on the main thread `VM_FRAME_RUBYFRAME_P(cfp)` was true
and thus the dummy thread was still included in the output of
`rb_profile_frames`.

I've now made `rb_profile_frames` skip this extra frame (like
`backtrace_each` and friends), as well as add a test that asserts
the size and contents of `rb_profile_frames`.

Fixes [Bug #18907] (<https://bugs.ruby-lang.org/issues/18907>)
---
 ext/-test-/debug/profile_frames.c       |  1 +
 test/-ext-/debug/test_profile_frames.rb | 24 ++++++++++++++++++++++++
 vm_backtrace.c                          |  3 +++
 3 files changed, 28 insertions(+)

diff --git a/ext/-test-/debug/profile_frames.c b/ext/-test-/debug/profile_frames.c
index 5f14755ce7..d2bba7d183 100644
--- a/ext/-test-/debug/profile_frames.c
+++ b/ext/-test-/debug/profile_frames.c
@@ -29,6 +29,7 @@ profile_frames(VALUE self, VALUE start_v, VALUE num_v)
         rb_ary_push(ary, rb_profile_frame_singleton_method_p(buff[i]));
         rb_ary_push(ary, rb_profile_frame_method_name(buff[i]));
         rb_ary_push(ary, rb_profile_frame_qualified_method_name(buff[i]));
+        rb_ary_push(ary, INT2NUM(lines[i]));
 
         rb_ary_push(result, ary);
     }
diff --git a/test/-ext-/debug/test_profile_frames.rb b/test/-ext-/debug/test_profile_frames.rb
index e0152247e7..d6ae953dd2 100644
--- a/test/-ext-/debug/test_profile_frames.rb
+++ b/test/-ext-/debug/test_profile_frames.rb
@@ -137,6 +137,30 @@ class TestProfileFrames < Test::Unit::TestCase
     }
   end
 
+  def test_matches_backtrace_locations_main_thread
+    assert_equal(Thread.current, Thread.main)
+
+    # Keep these in the same line, so the backtraces match exactly
+    backtrace_locations, profile_frames = [Thread.current.backtrace_locations, Bug::Debug.profile_frames(0, 100)]
+
+    assert_equal(backtrace_locations.size, profile_frames.size)
+
+    # The first entries are not going to match, since one is #backtrace_locations and the other #profile_frames
+    backtrace_locations.shift
+    profile_frames.shift
+
+    # The rest of the stack is expected to look the same...
+    backtrace_locations.zip(profile_frames).each.with_index do |(location, (path, absolute_path, _, base_label, _, _, _, _, _, _, lineno)), i|
+      next if absolute_path == "<cfunc>" # ...except for cfunc frames
+
+      err_msg = "#{i}th frame"
+      assert_equal(location.absolute_path, absolute_path, err_msg)
+      assert_equal(location.base_label, base_label, err_msg)
+      assert_equal(location.lineno, lineno, err_msg)
+      assert_equal(location.path, path, err_msg)
+    end
+  end
+
   def test_ifunc_frame
     bug11851 = '[ruby-core:72409] [Bug #11851]'
     assert_ruby_status([], <<~'end;', bug11851) # do
diff --git a/vm_backtrace.c b/vm_backtrace.c
index ee749deb14..5bd588df12 100644
--- a/vm_backtrace.c
+++ b/vm_backtrace.c
@@ -1551,6 +1551,9 @@ rb_profile_frames(int start, int limit, VALUE *buff, int *lines)
     const rb_control_frame_t *cfp = ec->cfp, *end_cfp = RUBY_VM_END_CONTROL_FRAME(ec);
     const rb_callable_method_entry_t *cme;
 
+    // Skip dummy frame; see `rb_ec_partial_backtrace_object` for details
+    end_cfp = RUBY_VM_NEXT_CONTROL_FRAME(end_cfp);
+
     for (i=0; i<limit && cfp != end_cfp;) {
         if (VM_FRAME_RUBYFRAME_P(cfp)) {
             if (start > 0) {