issue #362: cap max open files in children.

ansible_mitogen/target.py

@@ -43,6 +43,7 @@ import operator
 import os
 import pwd
 import re
+import resource
 import signal
 import stat
 import subprocess
@@ -90,6 +91,15 @@ _fork_parent = None
 good_temp_dir = None
 
 
+# subprocess.Popen(close_fds=True) aka. AnsibleModule.run_command() loops the
+# entire SC_OPEN_MAX space. CentOS>5 ships with 1,048,576 FDs by default,
+# resulting in huge (>500ms) runtime waste running many commands. Therefore if
+# we are a child context, cap the insane FD count to something reasonable.
+if subprocess.MAXFD > 512 and not mitogen.is_master:
+    subprocess.MAXFD = 512
+    resource.setrlimit(resource.RLIMIT_NOFILE, (512, 512))
+
+
 def get_small_file(context, path):
     """
     Basic in-memory caching module fetcher. This generates an one roundtrip for