keepalive: implement readinto()

keepalive: implement readinto()

This is part of the standard I/O interface. It is used by the framing
protocol. So we need to implement it so frames can be decoded.

Differential Revision: https://phab.mercurial-scm.org/D2984

--- a/mercurial/keepalive.py	Fri Mar 23 16:24:53 2018 -0700
+++ b/mercurial/keepalive.py	Wed Mar 28 12:44:35 2018 -0700
@@ -349,7 +349,7 @@
 
 class HTTPResponse(httplib.HTTPResponse):
     # we need to subclass HTTPResponse in order to
-    # 1) add readline() and readlines() methods
+    # 1) add readline(), readlines(), and readinto() methods
     # 2) add close_connection() methods
     # 3) add info() and geturl() methods
 
@@ -522,6 +522,14 @@
                 break
         return list
 
+    def readinto(self, dest):
+        res = self.read(len(dest))
+        if not res:
+            return 0
+
+        dest[0:len(res)] = res
+        return len(res)
+
 def safesend(self, str):
     """Send `str' to the server.