Fixes issue #15756: subprocess.poll() now properly handles errno.ECHILD

to return a returncode of 0 when the child has already exited or cannot
be waited on.

Lib/test/subprocessdata/sigchild_ignore.py

@@ -1,6 +1,15 @@
-import signal, subprocess, sys
+import signal, subprocess, sys, time
 # On Linux this causes os.waitpid to fail with OSError as the OS has already
 # reaped our child process.  The wait() passing the OSError on to the caller
 # and causing us to exit with an error is what we are testing against.
 signal.signal(signal.SIGCHLD, signal.SIG_IGN)
 subprocess.Popen([sys.executable, '-c', 'print("albatross")']).wait()
+# Also ensure poll() handles an errno.ECHILD appropriately.
+p = subprocess.Popen([sys.executable, '-c', 'print("albatross")'])
+num_polls = 0
+while p.poll() is None:
+    # Waiting for the process to finish.
+    time.sleep(0.01)  # Avoid being a CPU busy loop.
+    num_polls += 1
+    if num_polls > 3000:
+        raise RuntimeError('poll should have returned 0 within 30 seconds')