[PATCH v2 stable] sshpeer: try harder to snag stderr when stdout closes unexpectedly

[Augie Fackler]

# HG changeset patch
# User Augie Fackler <augie at google.com>
# Date 1492114180 14400
#      Thu Apr 13 16:09:40 2017 -0400
# Branch stable
# Node ID 8862fd11cc7d27512a4b4c2a56c983d68609660b
# Parent  6e0368b6e0bb2aa5210daec091c0200583553a78
sshpeer: try harder to snag stderr when stdout closes unexpectedly

Resolves test failures on FreeBSD, but I'm not happy about the fix.

A previous version of this also wrapped readline by putting the hack
in the _call method on doublepipe. That was confusing for readers and
wasn't necessary - just doing this on read() is sufficient to fix the
bugs I'm observing. We can always come back and do readline later if
needed.

diff --git a/mercurial/sshpeer.py b/mercurial/sshpeer.py
--- a/mercurial/sshpeer.py
+++ b/mercurial/sshpeer.py
@@ -91,7 +91,15 @@ class doublepipe(object):
         return self._call('write', data)
 
     def read(self, size):
-        return self._call('read', size)
+        r = self._call('read', size)
+        if size != 0 and not r:
+            # We've observed a condition that indicates the
+            # stdout closed unexpectedly. Check stderr one
+            # more time and snag anything that's there before
+            # letting anyone know the main part of the pipe
+            # closed prematurely.
+            _forwardoutput(self._ui, self._side)
+        return r
 
     def readline(self):
         return self._call('readline')