diff -r 220a220ed088 -r be930f16a52a mercurial/util.py
--- a/mercurial/util.py	Wed May 27 22:11:37 2015 -0700
+++ b/mercurial/util.py	Sat May 30 23:55:24 2015 -0700
@@ -232,6 +232,103 @@
 import subprocess
 closefds = os.name == 'posix'
 
+_chunksize = 4096
+
+class bufferedinputpipe(object):
+    """a manually buffered input pipe
+
+    Python will not let us use buffered IO and lazy reading with 'polling' at
+    the same time. We cannot probe the buffer state and select will not detect
+    that data are ready to read if they are already buffered.
+
+    This class let us work around that by implementing its own buffering
+    (allowing efficient readline) while offering a way to know if the buffer is
+    empty from the output (allowing collaboration of the buffer with polling).
+
+    This class lives in the 'util' module because it makes use of the 'os'
+    module from the python stdlib.
+    """
+
+    def __init__(self, input):
+        self._input = input
+        self._buffer = []
+        self._eof = False
+
+    @property
+    def hasbuffer(self):
+        """True is any data is currently buffered
+
+        This will be used externally a pre-step for polling IO. If there is
+        already data then no polling should be set in place."""
+        return bool(self._buffer)
+
+    @property
+    def closed(self):
+        return self._input.closed
+
+    def fileno(self):
+        return self._input.fileno()
+
+    def close(self):
+        return self._input.close()
+
+    def read(self, size):
+        while (not self._eof) and (self._lenbuf < size):
+            self._fillbuffer()
+        return self._frombuffer(size)
+
+    def readline(self, *args, **kwargs):
+        if 1 < len(self._buffer):
+            # this should not happen because both read and readline end with a
+            # _frombuffer call that collapse it.
+            self._buffer = [''.join(self._buffer)]
+        lfi = -1
+        if self._buffer:
+            lfi = self._buffer[-1].find('\n')
+        while (not self._eof) and lfi < 0:
+            self._fillbuffer()
+            if self._buffer:
+                lfi = self._buffer[-1].find('\n')
+        size = lfi + 1
+        if lfi < 0: # end of file
+            size = self._lenbuf
+        elif 1 < len(self._buffer):
+            # we need to take previous chunks into account
+            size += self._lenbuf - len(self._buffer[-1])
+        return self._frombuffer(size)
+
+    @property
+    def _lenbuf(self):
+        """return the current lengh of buffered data"""
+        return sum(len(d) for d in self._buffer)
+
+    def _frombuffer(self, size):
+        """return at most 'size' data from the buffer
+
+        The data are removed from the buffer."""
+        if size == 0 or not self._buffer:
+            return ''
+        buf = self._buffer[0]
+        if 1 < len(self._buffer):
+            buf = ''.join(self._buffer)
+
+        data = buf[:size]
+        buf = buf[len(data):]
+        if buf:
+            self._buffer = [buf]
+        else:
+            self._buffer = []
+        return data
+
+    def _fillbuffer(self):
+        """read data to the buffer"""
+        data = os.read(self._input.fileno(), _chunksize)
+        if not data:
+            self._eof = True
+        else:
+            # inefficient add
+            self._buffer.append(data)
+
 def popen2(cmd, env=None, newlines=False):
     # Setting bufsize to -1 lets the system decide the buffer size.
     # The default for bufsize is 0, meaning unbuffered. This leads to