Warn user if needed when the process is forked

This change warns the library consumer when the process if forked and
we can't be sure that the library work as expected.

This also add some documentation about forking oslo.messaging Transport
object.

Change-Id: I2938421775aa72866adac198d70214856d45e165
Related-bug: #1330199

doc/source/transport.rst

@@ -14,3 +14,17 @@ Transport
 .. autoclass:: TransportHost
 
 .. autofunction:: set_transport_defaults
+
+
+About fork oslo.messaging transport object
+------------------------------------------
+
+oslo.messaging can't ensure that forking a process that shares the same
+transport object is safe for the library consumer, because it relies on
+different 3rd party libraries that don't ensure that too, but in certain
+case/driver it works:
+
+* rabbit: works only if no connection have already been established.
+* qpid: doesn't work (qpid library have a global state that use fd
+  that can't be resetted)
+* amqp1: works

oslo/messaging/_drivers/impl_qpid.py

@@ -16,6 +16,7 @@
 import functools
 import itertools
 import logging
+import os
 import random
 import time
 
@@ -490,6 +491,7 @@ class Connection(object):
         random.shuffle(self.brokers_params)
         self.brokers = itertools.cycle(self.brokers_params)
 
+        self._initial_pid = os.getpid()
         self.reconnect()
 
     def _connect(self, broker):
@@ -578,6 +580,21 @@ class Connection(object):
             LOG.debug("Re-established AMQP queues")
 
     def ensure(self, error_callback, method, retry=None):
+
+        current_pid = os.getpid()
+        if self._initial_pid != current_pid:
+            # NOTE(sileht):
+            # to get the same level of fork support that rabbit driver have
+            # (ie: allow fork before the first connection established)
+            # we could use the kombu workaround:
+            # https://github.com/celery/kombu/blob/master/kombu/transport/
+            # qpid_patches.py#L67
+            LOG.warn("Process forked! "
+                     "This can results to unpredictable behavior. "
+                     "See: http://docs.openstack.org/developer/"
+                     "oslo.messaging/transport.html")
+            self._initial_pid = current_pid
+
         while True:
             try:
                 return method()

oslo/messaging/_drivers/impl_rabbit.py

@@ -15,6 +15,7 @@
 import functools
 import itertools
 import logging
+import os
 import socket
 import ssl
 import time
@@ -472,6 +473,8 @@ class Connection(object):
                     hostname, port,
                     virtual_host)
 
+        self._initial_pid = os.getpid()
+
         self.do_consume = True
 
         self.channel = None
@@ -553,6 +556,14 @@ class Connection(object):
         retry = N means N retries
         """
 
+        current_pid = os.getpid()
+        if self._initial_pid != current_pid:
+            LOG.warn("Process forked after connection established! "
+                     "This can results to unpredictable behavior. "
+                     "See: http://docs.openstack.org/developer/"
+                     "oslo.messaging/transport.html")
+            self._initial_pid = current_pid
+
         if retry is None:
             retry = self.max_retries
         if retry is None or retry < 0: