Allow for providing a customized semaphore container

It can be undesirable to have a globally shared sempahore
container, especially since oslo.concurrency can now be shared
among many disjoint applications and libraries.

When a single container is used it is now possible to have those
disjoint applications/libraries collide on the same sempahore names.
This is not a good pattern to continue with, so in order to move away
from it allow a custom container to be provided (which defaults to the
existing global one) so that users of oslo.concurrency may provide there
own container if they so desire.

Change-Id: I9aab42e21ba0f52997de3e7c9b0fea51db5c7289

oslo_concurrency/lockutils.py

@@ -185,8 +185,42 @@ else:
     import fcntl
     InterProcessLock = _FcntlLock
 
-_semaphores = weakref.WeakValueDictionary()
-_semaphores_lock = threading.Lock()
+
+class Semaphores(object):
+    """A garbage collected container of semaphores.
+
+    This collection internally uses a weak value dictionary so that when a
+    semaphore is no longer in use (by any threads) it will automatically be
+    removed from this container by the garbage collector.
+    """
+
+    def __init__(self):
+        self._semaphores = weakref.WeakValueDictionary()
+        self._lock = threading.Lock()
+
+    def get(self, name):
+        """Gets (or creates) a semaphore with a given name.
+
+        :param name: The semaphore name to get/create (used to associate
+                     previously created names with the same semaphore).
+
+        Returns an newly constructed semaphore (or an existing one if it was
+        already created for the given name).
+        """
+        with self._lock:
+            try:
+                return self._semaphores[name]
+            except KeyError:
+                sem = threading.Semaphore()
+                self._semaphores[name] = sem
+                return sem
+
+    def __len__(self):
+        """Returns how many semaphores exist at the current time."""
+        return len(self._semaphores)
+
+
+_semaphores = Semaphores()
 
 
 def _get_lock_path(name, lock_file_prefix, lock_path=None):
@@ -211,11 +245,12 @@ def external_lock(name, lock_file_prefix=None, lock_path=None):
     return InterProcessLock(lock_file_path)
 
 
-def remove_external_lock_file(name, lock_file_prefix=None, lock_path=None):
+def remove_external_lock_file(name, lock_file_prefix=None, lock_path=None,
+                              semaphores=None):
     """Remove an external lock file when it's not used anymore
     This will be helpful when we have a lot of lock files
     """
-    with internal_lock(name):
+    with internal_lock(name, semaphores=semaphores):
         lock_file_path = _get_lock_path(name, lock_file_prefix, lock_path)
         try:
             os.remove(lock_file_path)
@@ -224,20 +259,15 @@ def remove_external_lock_file(name, lock_file_prefix=None, lock_path=None):
                      {'file': lock_file_path})
 
 
-def internal_lock(name):
-    with _semaphores_lock:
-        try:
-            sem = _semaphores[name]
-        except KeyError:
-            sem = threading.Semaphore()
-            _semaphores[name] = sem
-
-    return sem
+def internal_lock(name, semaphores=None):
+    if semaphores is None:
+        semaphores = _semaphores
+    return semaphores.get(name)
 
 
 @contextlib.contextmanager
 def lock(name, lock_file_prefix=None, external=False, lock_path=None,
-         do_log=True):
+         do_log=True, semaphores=None):
     """Context based lock
 
     This function yields a `threading.Semaphore` instance (if we don't use
@@ -259,8 +289,13 @@ def lock(name, lock_file_prefix=None, external=False, lock_path=None,
     :param do_log: Whether to log acquire/release messages.  This is primarily
       intended to reduce log message duplication when `lock` is used from the
       `synchronized` decorator.
+
+    :param semaphores: Container that provides semaphores to use when locking.
+        This ensures that threads inside the same application can not collide,
+        due to the fact that external process locks are unaware of a processes
+        active threads.
     """
-    int_lock = internal_lock(name)
+    int_lock = internal_lock(name, semaphores=semaphores)
     with int_lock:
         if do_log:
             LOG.debug('Acquired semaphore "%(lock)s"', {'lock': name})
@@ -276,7 +311,8 @@ def lock(name, lock_file_prefix=None, external=False, lock_path=None,
                 LOG.debug('Releasing semaphore "%(lock)s"', {'lock': name})
 
 
-def synchronized(name, lock_file_prefix=None, external=False, lock_path=None):
+def synchronized(name, lock_file_prefix=None, external=False, lock_path=None,
+                 semaphores=None):
     """Synchronization decorator.
 
     Decorating a method like so::
@@ -307,7 +343,7 @@ def synchronized(name, lock_file_prefix=None, external=False, lock_path=None):
             t2 = None
             try:
                 with lock(name, lock_file_prefix, external, lock_path,
-                          do_log=False):
+                          do_log=False, semaphores=semaphores):
                     t2 = time.time()
                     LOG.debug('Lock "%(name)s" acquired by "%(function)s" :: '
                               'waited %(wait_secs)0.3fs',

oslo_concurrency/tests/unit/test_lockutils.py

@@ -12,6 +12,7 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+import collections
 import errno
 import fcntl
 import multiprocessing
@@ -93,6 +94,30 @@ class LockTestCase(test_base.BaseTestCase):
             except IOError:
                 pass
 
+    def test_lock_internally_different_collections(self):
+        s1 = lockutils.Semaphores()
+        s2 = lockutils.Semaphores()
+        trigger = threading.Event()
+        who_ran = collections.deque()
+
+        def f(name, semaphores, pull_trigger):
+            with lockutils.internal_lock('testing', semaphores=semaphores):
+                if pull_trigger:
+                    trigger.set()
+                else:
+                    trigger.wait()
+                who_ran.append(name)
+
+        threads = [
+            threading.Thread(target=f, args=(1, s1, True)),
+            threading.Thread(target=f, args=(2, s2, False)),
+        ]
+        for thread in threads:
+            thread.start()
+        for thread in threads:
+            thread.join()
+        self.assertEqual([1, 2], sorted(who_ran))
+
     def test_lock_internally(self):
         """We can lock across multiple threads."""
         saved_sem_num = len(lockutils._semaphores)