Fix bug 544473 - "Queue module can deadlock".

Use try/finally to ensure all Queue locks remain stable.
Includes test case.  Bugfix candidate.

Lib/Queue.py

@@ -55,13 +55,24 @@ class Queue:
         elif not self.fsema.acquire(0):
             raise Full
         self.mutex.acquire()
-        was_empty = self._empty()
-        self._put(item)
-        if was_empty:
-            self.esema.release()
-        if not self._full():
-            self.fsema.release()
-        self.mutex.release()
+        release_fsema = True
+        try:
+            was_empty = self._empty()
+            self._put(item)
+            # If we fail before here, the empty state has
+            # not changed, so we can skip the release of esema
+            if was_empty:
+                self.esema.release()
+            # If we fail before here, the queue can not be full, so
+            # release_full_sema remains True
+            release_fsema = not self._full()
+        finally:
+            # Catching system level exceptions here (RecursionDepth,
+            # OutOfMemory, etc) - so do as little as possible in terms
+            # of Python calls.
+            if release_fsema:
+                self.fsema.release()
+            self.mutex.release()
 
     def put_nowait(self, item):
         """Put an item into the queue without blocking.
@@ -84,13 +95,21 @@ class Queue:
         elif not self.esema.acquire(0):
             raise Empty
         self.mutex.acquire()
-        was_full = self._full()
-        item = self._get()
-        if was_full:
-            self.fsema.release()
-        if not self._empty():
-            self.esema.release()
-        self.mutex.release()
+        release_esema = True
+        try:
+            was_full = self._full()
+            item = self._get()
+            # If we fail before here, the full state has
+            # not changed, so we can skip the release of fsema
+            if was_full:
+                self.fsema.release()
+            # Failure means empty state also unchanged - release_esema
+            # remains True.
+            release_esema = not self._empty()
+        finally:
+            if release_esema:
+                self.esema.release()
+            self.mutex.release()
         return item
 
     def get_nowait(self):

Lib/test/test_queue.py

+# Some simple Queue module tests, plus some failure conditions
+# to ensure the Queue locks remain stable
+import Queue
+import sys
+import threading
+import time
+
+from test_support import verify, TestFailed, verbose
+
+queue_size = 5
+
+# Execute a function that blocks, and in a seperate thread, a function that
+# triggers the release.  Returns the result of the blocking function.
+class _TriggerThread(threading.Thread):
+    def __init__(self, fn, args):
+        self.fn = fn
+        self.args = args
+        self.startedEvent = threading.Event()
+        threading.Thread.__init__(self)
+    def run(self):
+        time.sleep(.1)
+        self.startedEvent.set()
+        self.fn(*self.args)
+
+def _doBlockingTest( block_func, block_args, trigger_func, trigger_args):
+    t = _TriggerThread(trigger_func, trigger_args)
+    t.start()
+    try:
+        return block_func(*block_args)
+    finally:
+        # If we unblocked before our thread made the call, we failed!
+        if not t.startedEvent.isSet():
+            raise TestFailed("blocking function '%r' appeared not to block" % (block_func,))
+        t.join(1) # make sure the thread terminates
+        if t.isAlive():
+            raise TestFailed("trigger function '%r' appeared to not return" % (trigger_func,))
+
+# A Queue subclass that can provoke failure at a moment's notice :)
+class FailingQueueException(Exception):
+    pass
+
+class FailingQueue(Queue.Queue):
+    def __init__(self, *args):
+        self.fail_next_put = False
+        self.fail_next_get = False
+        Queue.Queue.__init__(self, *args)
+    def _put(self, item):
+        if self.fail_next_put:
+            self.fail_next_put = False
+            raise FailingQueueException, "You Lose"
+        return Queue.Queue._put(self, item)
+    def _get(self):
+        if self.fail_next_get:
+            self.fail_next_get = False
+            raise FailingQueueException, "You Lose"
+        return Queue.Queue._get(self)
+
+def FailingQueueTest(q):
+    if not q.empty():
+        raise RuntimeError, "Call this function with an empty queue"
+    for i in range(queue_size-1):
+        q.put(i)
+    q.fail_next_put = True
+    # Test a failing non-blocking put.
+    try:
+        q.put("oops", block=0)
+        raise TestFailed("The queue didn't fail when it should have")
+    except FailingQueueException:
+        pass
+    q.put("last")
+    verify(q.full(), "Queue should be full")
+    q.fail_next_put = True
+    # Test a failing blocking put
+    try:
+        _doBlockingTest( q.put, ("full",), q.get, ())
+        raise TestFailed("The queue didn't fail when it should have")
+    except FailingQueueException:
+        pass
+    # Check the Queue isn't damaged.
+    # put failed, but get succeeded - re-add
+    q.put("last")
+    verify(q.full(), "Queue should be full")
+    q.get()
+    verify(not q.full(), "Queue should not be full")
+    q.put("last")
+    verify(q.full(), "Queue should be full")
+    # Test a blocking put
+    _doBlockingTest( q.put, ("full",), q.get, ())
+    # Empty it
+    for i in range(queue_size):
+        q.get()
+    verify(q.empty(), "Queue should be empty")
+    q.put("first")
+    q.fail_next_get = True
+    try:
+        q.get()
+        raise TestFailed("The queue didn't fail when it should have")
+    except FailingQueueException:
+        pass
+    verify(not q.empty(), "Queue should not be empty")
+    q.get()
+    verify(q.empty(), "Queue should be empty")
+    q.fail_next_get = True
+    try:
+        _doBlockingTest( q.get, (), q.put, ('empty',))
+        raise TestFailed("The queue didn't fail when it should have")
+    except FailingQueueException:
+        pass
+    # put succeeded, but get failed.
+    verify(not q.empty(), "Queue should not be empty")
+    q.get()
+    verify(q.empty(), "Queue should be empty")
+
+def SimpleQueueTest(q):
+    if not q.empty():
+        raise RuntimeError, "Call this function with an empty queue"
+    # I guess we better check things actually queue correctly a little :)
+    q.put(111)
+    q.put(222)
+    verify(q.get()==111 and q.get()==222, "Didn't seem to queue the correct data!")
+    for i in range(queue_size-1):
+        q.put(i)
+    verify(not q.full(), "Queue should not be full")
+    q.put("last")
+    verify(q.full(), "Queue should be full")
+    try:
+        q.put("full", block=0)
+        raise TestFailed("Didn't appear to block with a full queue")
+    except Queue.Full:
+        pass
+    # Test a blocking put
+    _doBlockingTest( q.put, ("full",), q.get, ())
+    # Empty it
+    for i in range(queue_size):
+        q.get()
+    verify(q.empty(), "Queue should be empty")
+    try:
+        q.get(block=0)
+        raise TestFailed("Didn't appear to block with an empty queue")
+    except Queue.Empty:
+        pass
+    # Test a blocking get
+    _doBlockingTest( q.get, (), q.put, ('empty',))
+
+def test():
+    q=Queue.Queue(queue_size)
+    # Do it a couple of times on the same queue
+    SimpleQueueTest(q)
+    SimpleQueueTest(q)
+    if verbose:
+        print "Simple Queue tests seemed to work"
+    q = FailingQueue(queue_size)
+    FailingQueueTest(q)
+    FailingQueueTest(q)
+    if verbose:
+        print "Failing Queue tests seemed to work"
+
+test()