bpo-14976: Reentrant simple queue (#3346)

Add a queue.SimpleQueue class, an unbounded FIFO queue with a reentrant C implementation of put().

Lib/queue.py

@@ -4,17 +4,26 @@ import threading
 from collections import deque
 from heapq import heappush, heappop
 from time import monotonic as time
+try:
+    from _queue import SimpleQueue
+except ImportError:
+    SimpleQueue = None
 
-__all__ = ['Empty', 'Full', 'Queue', 'PriorityQueue', 'LifoQueue']
+__all__ = ['Empty', 'Full', 'Queue', 'PriorityQueue', 'LifoQueue', 'SimpleQueue']
 
-class Empty(Exception):
-    'Exception raised by Queue.get(block=0)/get_nowait().'
-    pass
+
+try:
+    from _queue import Empty
+except AttributeError:
+    class Empty(Exception):
+        'Exception raised by Queue.get(block=0)/get_nowait().'
+        pass
 
 class Full(Exception):
     'Exception raised by Queue.put(block=0)/put_nowait().'
     pass
 
+
 class Queue:
     '''Create a queue object with a given maximum size.
 
@@ -241,3 +250,72 @@ class LifoQueue(Queue):
 
     def _get(self):
         return self.queue.pop()
+
+
+class _PySimpleQueue:
+    '''Simple, unbounded FIFO queue.
+
+    This pure Python implementation is not reentrant.
+    '''
+    # Note: while this pure Python version provides fairness
+    # (by using a threading.Semaphore which is itself fair, being based
+    #  on threading.Condition), fairness is not part of the API contract.
+    # This allows the C version to use a different implementation.
+
+    def __init__(self):
+        self._queue = deque()
+        self._count = threading.Semaphore(0)
+
+    def put(self, item, block=True, timeout=None):
+        '''Put the item on the queue.
+
+        The optional 'block' and 'timeout' arguments are ignored, as this method
+        never blocks.  They are provided for compatibility with the Queue class.
+        '''
+        self._queue.append(item)
+        self._count.release()
+
+    def get(self, block=True, timeout=None):
+        '''Remove and return an item from the queue.
+
+        If optional args 'block' is true and 'timeout' is None (the default),
+        block if necessary until an item is available. If 'timeout' is
+        a non-negative number, it blocks at most 'timeout' seconds and raises
+        the Empty exception if no item was available within that time.
+        Otherwise ('block' is false), return an item if one is immediately
+        available, else raise the Empty exception ('timeout' is ignored
+        in that case).
+        '''
+        if timeout is not None and timeout < 0:
+            raise ValueError("'timeout' must be a non-negative number")
+        if not self._count.acquire(block, timeout):
+            raise Empty
+        return self._queue.popleft()
+
+    def put_nowait(self, item):
+        '''Put an item into the queue without blocking.
+
+        This is exactly equivalent to `put(item)` and is only provided
+        for compatibility with the Queue class.
+        '''
+        return self.put(item, block=False)
+
+    def get_nowait(self):
+        '''Remove and return an item from the queue without blocking.
+
+        Only get an item if one is immediately available. Otherwise
+        raise the Empty exception.
+        '''
+        return self.get(block=False)
+
+    def empty(self):
+        '''Return True if the queue is empty, False otherwise (not reliable!).'''
+        return len(self._queue) == 0
+
+    def qsize(self):
+        '''Return the approximate size of the queue (not reliable!).'''
+        return len(self._queue)
+
+
+if SimpleQueue is None:
+    SimpleQueue = _PySimpleQueue