From 0117a6d389c5b607eaec1b7f917132efa356f665 Mon Sep 17 00:00:00 2001
From: KJ Tsanaktsidis <kj@kjtsanaktsidis.id.au>
Date: Fri, 15 Sep 2023 11:33:32 +1000
Subject: [PATCH] Fix Thread#native_thread_id being cached across fork (#8418)

The native thread ID can and does change on some operating systems (e.g.
Linux) after forking, so it needs to be re-queried.

[Bug #19873]
---
 test/ruby/test_thread.rb | 30 ++++++++++++++++++++++++++++++
 thread_pthread.c         | 27 +++++++++++++++------------
 2 files changed, 45 insertions(+), 12 deletions(-)

diff --git a/test/ruby/test_thread.rb b/test/ruby/test_thread.rb
index 703373b11e..bcd6590903 100644
--- a/test/ruby/test_thread.rb
+++ b/test/ruby/test_thread.rb
@@ -3,6 +3,7 @@
 require 'test/unit'
 require "rbconfig/sizeof"
 require "timeout"
+require "fiddle"
 
 class TestThread < Test::Unit::TestCase
   class Thread < ::Thread
@@ -1443,6 +1444,35 @@ q.pop
     assert_nil th1.native_thread_id
   end
 
+  def test_thread_native_thread_id_across_fork_on_linux
+    rtld_default = Fiddle.dlopen(nil)
+    omit "this test is only for Linux" unless rtld_default.sym_defined?('gettid')
+
+    gettid = Fiddle::Function.new(rtld_default['gettid'], [], Fiddle::TYPE_INT)
+
+    parent_thread_id = Thread.main.native_thread_id
+    real_parent_thread_id = gettid.call
+
+    assert_equal real_parent_thread_id, parent_thread_id
+
+    child_lines = nil
+    IO.popen('-') do |pipe|
+      if pipe
+        # parent
+        child_lines = pipe.read.lines
+      else
+        # child
+        puts Thread.main.native_thread_id
+        puts gettid.call
+      end
+    end
+    child_thread_id = child_lines[0].chomp.to_i
+    real_child_thread_id = child_lines[1].chomp.to_i
+
+    assert_equal real_child_thread_id, child_thread_id
+    refute_equal parent_thread_id, child_thread_id
+  end
+
   def test_thread_interrupt_for_killed_thread
     opts = { timeout: 5, timeout_error: nil }
 
diff --git a/thread_pthread.c b/thread_pthread.c
index 057c50c124..7c53325240 100644
--- a/thread_pthread.c
+++ b/thread_pthread.c
@@ -637,6 +637,18 @@ rb_thread_sched_destroy(struct rb_thread_sched *sched)
     clear_thread_cache_altstack();
 }
 
+#ifdef RB_THREAD_T_HAS_NATIVE_ID
+static int
+get_native_thread_id(void)
+{
+#ifdef __linux__
+    return (int)syscall(SYS_gettid);
+#elif defined(__FreeBSD__)
+    return pthread_getthreadid_np();
+#endif
+}
+#endif
+
 #if defined(HAVE_WORKING_FORK)
 static void thread_cache_reset(void);
 static void
@@ -646,6 +658,9 @@ thread_sched_atfork(struct rb_thread_sched *sched)
     thread_cache_reset();
     rb_thread_sched_init(sched);
     thread_sched_to_running(sched, GET_THREAD());
+#ifdef RB_THREAD_T_HAS_NATIVE_ID
+    GET_THREAD()->nt->tid = get_native_thread_id();
+#endif
 }
 #endif
 
@@ -693,18 +708,6 @@ ruby_thread_set_native(rb_thread_t *th)
 #endif
 }
 
-#ifdef RB_THREAD_T_HAS_NATIVE_ID
-static int
-get_native_thread_id(void)
-{
-#ifdef __linux__
-    return (int)syscall(SYS_gettid);
-#elif defined(__FreeBSD__)
-    return pthread_getthreadid_np();
-#endif
-}
-#endif
-
 static void
 native_thread_init(struct rb_native_thread *nt)
 {