From 526b0a514b323ac8eae597ff4416dafd8765a293 Mon Sep 17 00:00:00 2001
From: David Wilson <dw@botanicus.net>
Date: Tue, 20 Mar 2018 13:14:51 +0545
Subject: [PATCH] issue #156: prevent Latch.close() triggering spurious wakeups

---
 docs/howitworks.rst | 17 ++++++++++++++---
 mitogen/core.py     | 11 ++++++-----
 2 files changed, 20 insertions(+), 8 deletions(-)

diff --git a/docs/howitworks.rst b/docs/howitworks.rst
index c49828e5..ac496a67 100644
--- a/docs/howitworks.rst
+++ b/docs/howitworks.rst
@@ -889,11 +889,17 @@ Latch.close()
 ~~~~~~~~~~~
 
 :py:meth:`mitogen.core.Latch.close` acquires `lock`, sets `closed` to
-:py:data:`True`, then writes a byte to every socket in `sleeping`. Per above,
-on waking from sleep, after removing itself from `sleeping`, each sleeping
-thread tests if `closed` is :py:data:`True`, and if so throws
+:py:data:`True`, then writes a byte to every `sleeping[waking]` socket, while
+incrementing `waking`, until no more unwoken sockets exist. Per above, on
+waking from sleep, after removing itself from `sleeping`, each sleeping thread
+tests if `closed` is :py:data:`True`, and if so throws
 :py:class:`mitogen.core.LatchError`.
 
+It is necessary to ensure at most one byte is delivered on each socket, even if
+the latch is being torn down, as the sockets outlive the scope of a single
+latch, and must never have extraneous data buffered on them, as this will cause
+unexpected wakeups if future latches sleep on the same thread.
+
 
 Latch.get()
 ~~~~~~~~~~~
@@ -930,6 +936,11 @@ item.
     `waking`,  then pops and returns the first item in `queue` that is
     guaranteed to exist.
 
+    It is paramount that in every case, if :py:func:`select.select` indicates a
+    byte was written to the socket, that the byte is read away. The socket is
+    reused by subsequent latches sleeping on the same thread, and unexpected
+    wakeups are triggered if extraneous data remains buffered on the socket.
+
 
 .. rubric:: Footnotes
 
diff --git a/mitogen/core.py b/mitogen/core.py
index 7617be26..fbe0d6eb 100644
--- a/mitogen/core.py
+++ b/mitogen/core.py
@@ -918,8 +918,9 @@ class Latch(object):
         self._lock.acquire()
         try:
             self.closed = True
-            for wsock in self._sleeping:
-                self._wake(wsock)
+            while self._waking < len(self._sleeping):
+                self._wake(self._sleeping[self._waking])
+                self._waking += 1
         finally:
             self._lock.release()
 
@@ -957,13 +958,13 @@ class Latch(object):
         self._lock.acquire()
         try:
             self._sleeping.remove(_tls.wsock)
-            if self.closed:
-                raise LatchError()
             if not rfds:
                 raise TimeoutError()
-            self._waking -= 1
             if _tls.rsock.recv(2) != '\x7f':
                 raise LatchError('internal error: received >1 wakeups')
+            self._waking -= 1
+            if self.closed:
+                raise LatchError()
             try:
                 _vv and IOLOG.debug('%r.get() wake -> %r', self, self._queue[0])
                 return self._queue.pop(0)