issue #549: increase open file limit automatically if possible

While catching every possible case where "open file limit exceeded" is
not possible, we can at least increase the soft limit to the available
hard limit without any user effort.

Do this in Ansible top-level process, even though we probably only need
it in the MuxProcess. It seems there is no reason this could hurt

ansible_mitogen/process.py

@@ -32,6 +32,7 @@ import errno
 import logging
 import multiprocessing
 import os
+import resource
 import signal
 import socket
 import sys
@@ -237,9 +238,27 @@ def _setup_responder(responder):
     )
 
 
+def increase_open_file_limit():
+    """
+    #549: in order to reduce the possibility of hitting an open files limit,
+    increase :data:`resource.RLIMIT_NOFILE` from its soft limit to its hard
+    limit, if they differ.
+
+    It is common that a low soft limit is configured by default, where the hard
+    limit is much higher.
+    """
+    soft, hard = resource.getrlimit(resource.RLIMIT_NOFILE)
+    if soft < hard:
+        LOG.debug('raising soft open file limit from %d to %d', soft, hard)
+        resource.setrlimit(resource.RLIMIT_NOFILE, (hard, hard))
+    else:
+        LOG.debug('cannot increase open file limit; existing limit is %d', hard)
+
+
 def common_setup(enable_affinity=True, _init_logging=True):
     save_pid('controller')
     ansible_mitogen.logging.set_process_name('top')
+
     if enable_affinity:
         ansible_mitogen.affinity.policy.assign_controller()
 
@@ -255,6 +274,7 @@ def common_setup(enable_affinity=True, _init_logging=True):
         mitogen.core.enable_profiling()
 
     MuxProcess.cls_original_env = dict(os.environ)
+    increase_open_file_limit()
 
 
 def get_cpu_count(default=None):