Introduce a connection locking infrastructure

The lock file is (a temporary file) opened in the parent process, whose
open fd is inherited by the workers after fork, and passed down through
the PlayContext. Connection grows lock/unlock methods which can be used
by individual connection plugins.

lib/ansible/executor/task_queue_manager.py

@@ -23,6 +23,7 @@ import multiprocessing
 import os
 import socket
 import sys
+import tempfile
 
 from ansible import constants as C
 from ansible.errors import AnsibleError
@@ -78,6 +79,10 @@ class TaskQueueManager:
         self._failed_hosts      = dict()
         self._unreachable_hosts = dict()
 
+        # A temporary file (opened pre-fork) used by connection plugins for
+        # inter-process locking.
+        self._options.connection_lockfile = tempfile.TemporaryFile()
+
         self._final_q = multiprocessing.Queue()
 
         # create the pool of worker threads, based on the number of forks specified

lib/ansible/playbook/play_context.py

@@ -161,6 +161,7 @@ class PlayContext(Base):
     _private_key_file = FieldAttribute(isa='string', default=C.DEFAULT_PRIVATE_KEY_FILE)
     _timeout          = FieldAttribute(isa='int', default=C.DEFAULT_TIMEOUT)
     _shell            = FieldAttribute(isa='string')
+    _connection_lockfd= FieldAttribute(isa='int', default=None)
 
     # privilege escalation fields
     _become           = FieldAttribute(isa='bool')
@@ -244,6 +245,11 @@ class PlayContext(Base):
         if options.connection:
             self.connection = options.connection
 
+        # The lock file is opened in the parent process, and the workers will
+        # inherit the open file, so we just need to help them find it.
+        if options.connection_lockfile:
+            self.connection_lockfd = options.connection_lockfile.fileno()
+
         self.remote_user = options.remote_user
         self.private_key_file = options.private_key_file
 

lib/ansible/plugins/connections/__init__.py

@@ -155,3 +155,13 @@ class ConnectionBase(with_metaclass(ABCMeta, object)):
         if incorrect_password in output:
             raise AnsibleError('Incorrect %s password' % self._play_context.become_method)
 
+    def lock_connection(self):
+        f = self._play_context.connection_lockfd
+        self._display.vvvv('CONNECTION: pid %d waiting for lock on %d' % (os.getpid(), f))
+        fcntl.lockf(f, fcntl.LOCK_EX)
+        self._display.vvvv('CONNECTION: pid %d acquired lock on %d' % (os.getpid(), f))
+
+    def unlock_connection(self):
+        f = self._play_context.connection_lockfd
+        fcntl.lockf(f, fcntl.LOCK_UN)
+        self._display.vvvv('CONNECTION: pid %d released lock on %d' % (os.getpid(), f))