Stop locking in our SysLogHandler

It shouldn't be necessary since we only allow it to use UDS or UDP.

Leave an escape hatch in case operators suspect an issue with it; they
can set the environment variable

    SWIFT_NOOP_LOGGING_MUTEX=false

to return to prior behavior. Future releases of Swift may remove this
option based on operator feedback.

Change-Id: I33007868e6d8a6a19eebdf14d5cfe18028424759
Related-Change: https://review.opendev.org/c/openstack/swift/+/493636

swift/common/utils.py

@@ -6202,9 +6202,44 @@ def __del__(self):
         self.close()
 
 
+class NoopMutex(object):
+    """
+    "Mutex" that doesn't lock anything.
+
+    We only allow our syslog logging to be configured via UDS or UDP, neither
+    of which have the message-interleaving trouble you'd expect from TCP or
+    file handlers.
+    """
+    def __init__(self):
+        # Usually, it's an error to have multiple greenthreads all waiting
+        # to write to the same file descriptor. It's often a sign of inadequate
+        # concurrency control; for example, if you have two greenthreads
+        # trying to use the same memcache connection, they'll end up writing
+        # interleaved garbage to the socket or stealing part of each others'
+        # responses.
+        #
+        # In this case, we have multiple greenthreads waiting on the same
+        # (logging) file descriptor by design. So, similar to the PipeMutex,
+        # we must turn off eventlet's multiple-waiter detection.
+        #
+        # It would be better to turn off multiple-reader detection for only
+        # the logging socket fd, but eventlet does not support that.
+        eventlet.debug.hub_prevent_multiple_readers(False)
+
+    def acquire(self, blocking=True):
+        pass
+
+    def release(self):
+        pass
+
+
 class ThreadSafeSysLogHandler(SysLogHandler):
     def createLock(self):
-        self.lock = PipeMutex()
+        if config_true_value(os.environ.get(
+                'SWIFT_NOOP_LOGGING_MUTEX') or 'true'):
+            self.lock = NoopMutex()
+        else:
+            self.lock = PipeMutex()
 
 
 def round_robin_iter(its):