issue #156: handle multiple _put() before wake of first sleeper

- If latch.get() is called and the queue is empty, a thread is put to
  sleep.

- If Latch.put() from another thread then appends an item to the queue and
  wakes the sleeping thread, and

- If a subsequent Latch.put() from the same or another thread manages to
  acquire `lock` before the sleeping thread is scheduled,

- The sleeping thread's wake socket would have multiple bytes written to
  it.

Therefore create a new _pending variable to track the only item assigned
to each thread (keyed by its write socket), and remove the socket from
`sleeping` from within put.

mitogen/core.py

@@ -911,13 +911,14 @@ class Latch(object):
     def __init__(self):
         self._lock = threading.Lock()
         self._queue = []
-        self._waiters = []
+        self._sleeping = []
+        self._pending = {}
 
     def close(self):
         self._lock.acquire()
         try:
             self.closed = True
-            for wsock in self._waiters:
+            for wsock in self._sleeping:
                 self._wake(wsock)
         finally:
             self._lock.release()
@@ -939,13 +940,13 @@ class Latch(object):
         try:
             if self.closed:
                 raise LatchError()
-            if self._queue and not self._waiters:
+            if self._queue:
                 _vv and IOLOG.debug('%r.get() -> %r', self, self._queue[0])
                 return self._queue.pop(0)
             if not block:
                 raise TimeoutError()
             self._tls_init()
-            self._waiters.append(_tls.wsock)
+            self._sleeping.append(_tls.wsock)
         finally:
             self._lock.release()
 
@@ -955,15 +956,16 @@ class Latch(object):
 
         self._lock.acquire()
         try:
-            self._waiters.remove(_tls.wsock)
             if self.closed:
                 raise LatchError()
             if not rfds:
                 raise TimeoutError()
-            assert _tls.rsock.recv(1) == '\x7f'
+            if _tls.rsock.recv(2) != '\x7f':
+                raise LatchError('internal error: received >1 wakeups')
             try:
-                _vv and IOLOG.debug('%r.get() wake -> %r', self, self._queue[0])
-                return self._queue.pop(0)
+                obj = self._pending.pop(_tls.wsock)
+                _vv and IOLOG.debug('%r.get() wake -> %r', self, obj)
+                return obj
             except IndexError:
                 IOLOG.exception('%r.get() INDEX ERROR', self)
                 raise
@@ -976,11 +978,14 @@ class Latch(object):
         try:
             if self.closed:
                 raise LatchError()
-            self._queue.append(obj)
-            if self._waiters:
+            if self._sleeping:
+                sock = self._sleeping.pop(0)
+                self._pending[sock] = obj
                 _vv and IOLOG.debug('%r.put() -> waking wfd=%r',
-                                    self, self._waiters[0].fileno())
-                self._wake(self._waiters[0])
+                                    self, sock.fileno())
+                self._wake(sock)
+            else:
+                self._queue.append(obj)
         finally:
             self._lock.release()