--- a/mercurial/utils/cborutil.py	Fri Aug 31 15:54:17 2018 -0700
+++ b/mercurial/utils/cborutil.py	Wed Aug 29 14:29:01 2018 -0700
@@ -898,6 +898,48 @@
         self._decodedvalues = []
         return l
 
+class bufferingdecoder(object):
+    """A CBOR decoder that buffers undecoded input.
+
+    This is a glorified wrapper around ``sansiodecoder`` that adds a buffering
+    layer. All input that isn't consumed by ``sansiodecoder`` will be buffered
+    and concatenated with any new input that arrives later.
+
+    TODO consider adding limits as to the maximum amount of data that can
+    be buffered.
+    """
+    def __init__(self):
+        self._decoder = sansiodecoder()
+        self._leftover = None
+
+    def decode(self, b):
+        """Attempt to decode bytes to CBOR values.
+
+        Returns a tuple with the following fields:
+
+        * Bool indicating whether new values are available for retrieval.
+        * Integer number of bytes decoded from the new input.
+        * Integer number of bytes wanted to decode the next value.
+        """
+
+        if self._leftover:
+            oldlen = len(self._leftover)
+            b = self._leftover + b
+            self._leftover = None
+        else:
+            b = b
+            oldlen = 0
+
+        available, readcount, wanted = self._decoder.decode(b)
+
+        if readcount < len(b):
+            self._leftover = b[readcount:]
+
+        return available, readcount - oldlen, wanted
+
+    def getavailable(self):
+        return self._decoder.getavailable()
+
 def decodeall(b):
     """Decode all CBOR items present in an iterable of bytes.