Issue #19310: asyncio: fix child processes reaping logic.

Lib/asyncio/unix_events.py

@@ -167,20 +167,22 @@ class SelectorEventLoop(selector_events.BaseSelectorEventLoop):
 
     def _sig_chld(self):
         try:
+            # because of signal coalescing, we must keep calling waitpid() as
+            # long as we're able to reap a child
+            while True:
                 try:
                     pid, status = os.waitpid(-1, os.WNOHANG)
                 except ChildProcessError:
-                return
+                    break
                 if pid == 0:
-                self.call_soon(self._sig_chld)
-                return
+                    break
                 elif os.WIFSIGNALED(status):
                     returncode = -os.WTERMSIG(status)
                 elif os.WIFEXITED(status):
                     returncode = os.WEXITSTATUS(status)
                 else:
-                self.call_soon(self._sig_chld)
-                return
+                    # shouldn't happen
+                    continue
                 transp = self._subprocesses.get(pid)
                 if transp is not None:
                     transp._process_exited(returncode)