master: make mitogen minimally functional under gevent

It seems gevent automatically sets blocking behaviour on fds produced by
the socket module, which causes the Python process we fork to fail
horribly. So in the child, always reset the blocking flag.

mitogen/core.py

@@ -162,6 +162,11 @@ def set_nonblock(fd):
     fcntl.fcntl(fd, fcntl.F_SETFL, flags | os.O_NONBLOCK)
 
 
+def set_block(fd):
+    flags = fcntl.fcntl(fd, fcntl.F_GETFL)
+    fcntl.fcntl(fd, fcntl.F_SETFL, flags & ~os.O_NONBLOCK)
+
+
 def io_op(func, *args):
     try:
         return func(*args), False

mitogen/master.py

@@ -102,6 +102,7 @@ def create_child(*args):
     parentfp, childfp = socket.socketpair()
     pid = os.fork()
     if not pid:
+        mitogen.core.set_block(childfp.fileno())
         os.dup2(childfp.fileno(), 0)
         os.dup2(childfp.fileno(), 1)
         childfp.close()