diff --git a/mercurial/sshpeer.py b/mercurial/sshpeer.py
--- a/mercurial/sshpeer.py
+++ b/mercurial/sshpeer.py
@@ -36,6 +36,71 @@ def _forwardoutput(ui, pipe):
         for l in s.splitlines():
             ui.status(_("remote: "), l, '\n')
 
+class doublepipe(object):
+    """Operate a side-channel pipe in addition of a main one
+
+    The side-channel pipe contains server output to be forwarded to the user
+    input. The double pipe will behave as the "main" pipe, but will ensure the
+    content of the "side" pipe is properly processed while we wait for blocking
+    call on the "main" pipe.
+
+    If large amounts of data are read from "main", the forward will cease after
+    the first bytes start to appear. This simplifies the implementation
+    without affecting actual output of sshpeer too much as we rarely issue
+    large read for data not yet emitted by the server.
+
+    The main pipe is expected to be a 'bufferedinputpipe' from the util module
+    that handle all the os specific bites. This class lives in this module
+    because it focus on behavior specifig to the ssh protocol."""
+
+    def __init__(self, ui, main, side):
+        self._ui = ui
+        self._main = main
+        self._side = side
+
+    def _wait(self):
+        """wait until some data are available on main or side
+
+        return a pair of boolean (ismainready, issideready)
+
+        (This will only wait for data if the setup is supported by `util.poll`)
+        """
+        if self._main.hasbuffer:
+            return (True, True) # main has data, assume side is worth poking at.
+        fds = [self._main.fileno(), self._side.fileno()]
+        try:
+            act = util.poll(fds)
+        except NotImplementedError:
+            # non supported yet case, assume all have data.
+            act = fds
+        return (self._main.fileno() in act, self._side.fileno() in act)
+
+    def read(self, size):
+        return self._call('read', size)
+
+    def readline(self):
+        return self._call('readline')
+
+    def _call(self, methname, size=None):
+        """call <methname> on "main", forward output of "side" while blocking
+        """
+        if size == 0 or self._main.closed:
+            _forwardoutput(self._ui, self._side)
+            return ''
+        while True:
+            mainready, sideready = self._wait()
+            if sideready:
+                _forwardoutput(self._ui, self._side)
+            if mainready:
+                meth = getattr(self._main, methname)
+                if size is None:
+                    return meth()
+                else:
+                    return meth(size)
+
+    def close(self):
+        return self._main.close()
+
 class sshpeer(wireproto.wirepeer):
     def __init__(self, ui, path, create=False):
         self._url = path