Allow waitpid(-1, Process::WNOHANG) to be woken if a waitpid(pid) is

...pending

If two threads are running, with one calling waitpid(-1, Process::WNOHANG),
and another calling waitpid($some_pid), and then
$some_other_pid exits, we would expect the waitpid(-1,
Process::WNOHANG) call to retrieve that exit status. However, it
cannot actually do so until $some_pid _also_ exits.

This patch fixes the issue by calling do_waitpid unconditionally in
waitpid_wait; this will ensure that a waitpid -1 actually reaps
something (after first checking that no PID-directed call wants the
process).

[Bug #20490]

1 files changed, 27 insertions, 3 deletions

diff --git a/test/ruby/test_process.rb b/test/ruby/test_process.rb
index 1228f2c0b1..8e1c3a0d47 100644
--- a/test/ruby/test_process.rb
+++ b/test/ruby/test_process.rb
@@ -2659,7 +2659,7 @@ EOS
     end;
   end if Process.respond_to?(:_fork)
 
-  def test_concurrent_group_and_pid_wait
+  def _test_concurrent_group_and_pid_wait(nohang)
     # Use a pair of pipes that will make long_pid exit when this test exits, to avoid
     # leaking temp processes.
     long_rpipe, long_wpipe = IO.pipe
@@ -2681,10 +2681,26 @@ EOS
       end
       # Wait for us to be blocking in a call to waitpid2
       Thread.pass until t1.stop?
+      assert_nil Process.waitpid(-1, Process::WNOHANG)
       short_wpipe.close # Make short_pid exit
 
-      # The short pid has exited, so -1 should pick that up.
-      assert_equal short_pid, Process.waitpid(-1)
+      # The short pid has exited, so waitpid(-1) should pick that up.
+      exited_pid =
+        unless nohang
+          Process.waitpid(-1)
+        else
+          EnvUtil.timeout(5) do
+            loop do
+              pid = Process.waitpid(-1, Process::WNOHANG)
+
+              break pid if pid
+
+              sleep 0.1
+            end
+          end
+        end
+
+      assert_equal short_pid, exited_pid
 
       # Terminate t1 for the next phase of the test.
       t1.kill
@@ -2714,6 +2730,14 @@ EOS
     [long_rpipe, long_wpipe, short_rpipe, short_wpipe].each { _1&.close rescue nil }
   end if defined?(fork)
 
+  def test_concurrent_group_and_pid_wait
+    _test_concurrent_group_and_pid_wait(false)
+  end if defined?(fork)
+
+  def test_concurrent_group_and_pid_wait_nohang
+    _test_concurrent_group_and_pid_wait(true)
+  end if defined?(fork)
+
   def test_handle_interrupt_with_fork
     Thread.handle_interrupt(RuntimeError => :never) do
       Thread.current.raise(RuntimeError, "Queued error")