Allow resetting and ignoring SIGCHLD. Fixes #696.

changelog.rst

@@ -16,7 +16,10 @@
   ``sendall``.
 - gevent's SSL socket's ``sendall`` method should no longer raise ``SSL3_WRITE_PENDING``
   in rare cases when sending large buffers. Reported in :issue:`317`.
-
+- ``gevent.signal`` now allows resetting (SIG_DFL) and ignoring (SIG_IGN) the
+  SIGCHLD signal at the process level (although this may allow race
+  conditions with libev child watchers). Reported in :issue:`696` by
+  Adam Ning.
 
 1.1rc1 (Nov 14, 2015)
 =====================

gevent/signal.py

@@ -61,6 +61,11 @@ def signal(signalnum, handler):
        the builtin :func:`signal.signal` would be triggered either;
        libev typically overwrites such a handler at the C level. At
        the very least, it's full of race conditions.)
+
+    .. note::
+
+        Use of ``SIG_IGN`` and ``SIG_DFL`` may also have race conditions
+        with libev child watchers and the :mod:`gevent.subprocess` module.
     """
     if signalnum != _signal.SIGCHLD:
         return _signal_signal(signalnum, handler)
@@ -75,6 +80,11 @@ def signal(signalnum, handler):
     old_handler = getsignal(signalnum)
     global _child_handler
     _child_handler = handler
+    if handler == _signal.SIG_IGN or handler == _signal.SIG_DFL:
+        # Allow resetting/ignoring this signal at the process level.
+        # Note that this conflicts with gevent.subprocess and other users
+        # of child watchers.
+        _signal_signal(signalnum, handler)
     return old_handler
 
 

greentest/test__monkey_sigchld_2.py

+# Mimics what gunicorn workers do: monkey patch in the child process
+# and try to reset signal handlers to SIG_DFL.
+# NOTE: This breaks again when gevent.subprocess is used, or any child
+# watcher.
+import os
+import sys
+
+import signal
+
+
+def handle(*args):
+    if not pid:
+        # We only do this is the child so our
+        # parent's waitpid can get the status.
+        # This is the opposite of gunicorn.
+        os.waitpid(-1, os.WNOHANG)
+# The signal watcher must be installed *before* monkey patching
+signal.signal(signal.SIGCHLD, handle)
+
+pid = os.fork()
+
+if pid: # parent
+    try:
+        _, stat = os.waitpid(pid, 0)
+    except OSError:
+        # Interrupted system call
+        _, stat = os.waitpid(pid, 0)
+    assert stat == 0, stat
+else:
+    import gevent.monkey
+    gevent.monkey.patch_all()
+    signal.signal(signal.SIGCHLD, signal.SIG_DFL)
+    # Under Python 2, os.popen() directly uses the popen call, and
+    # popen's file uses the pclose() system call to
+    # wait for the child. If it's already waited on,
+    # it raises the same exception.
+    # Python 3 uses the subprocess module directly which doesn't
+    # have this problem.
+    f = os.popen('true')
+    f.close()
+
+    sys.exit(0)