#3566: Clean up handling of remote server disconnects.

This changeset does two things: introduces a new RemoteDisconnected exception
(that subclasses ConnectionResetError and BadStatusLine) so that a remote
server disconnection can be detected by client code (and provides a better
error message for debugging purposes), and ensures that the client socket is
closed if a ConnectionError happens, so that the automatic re-connection code
can work if the application handles the error and continues on.

Tests are added that confirm that a connection is re-used or not re-used
as appropriate to the various combinations of protocol version and headers.

Patch by Martin Panter, reviewed by Demian Brecht.  (Tweaked only slightly by
me.)

Lib/http/client.py

@@ -20,10 +20,12 @@ request. This diagram details these state transitions:
       | ( putheader() )*  endheaders()
       v
     Request-sent
-      |
-      | response = getresponse()
-      v
-    Unread-response   [Response-headers-read]
+      |\_____________________________
+      |                              | getresponse() raises
+      | response = getresponse()     | ConnectionError
+      v                              v
+    Unread-response                Idle
+    [Response-headers-read]
       |\____________________
       |                     |
       | response.read()     | putrequest()
@@ -83,7 +85,8 @@ __all__ = ["HTTPResponse", "HTTPConnection",
            "UnknownTransferEncoding", "UnimplementedFileMode",
            "IncompleteRead", "InvalidURL", "ImproperConnectionState",
            "CannotSendRequest", "CannotSendHeader", "ResponseNotReady",
-           "BadStatusLine", "LineTooLong", "error", "responses"]
+           "BadStatusLine", "LineTooLong", "RemoteDisconnected", "error",
+           "responses"]
 
 HTTP_PORT = 80
 HTTPS_PORT = 443
@@ -245,7 +248,8 @@ class HTTPResponse(io.BufferedIOBase):
         if not line:
             # Presumably, the server closed the connection before
             # sending a valid response.
-            raise BadStatusLine(line)
+            raise RemoteDisconnected("Remote end closed connection without"
+                                     " response")
         try:
             version, status, reason = line.split(None, 2)
         except ValueError:
@@ -1160,7 +1164,11 @@ class HTTPConnection:
             response = self.response_class(self.sock, method=self._method)
 
         try:
-            response.begin()
+            try:
+                response.begin()
+            except ConnectionError:
+                self.close()
+                raise
             assert response.will_close != _UNKNOWN
             self.__state = _CS_IDLE
 
@@ -1292,5 +1300,10 @@ class LineTooLong(HTTPException):
         HTTPException.__init__(self, "got more than %d bytes when reading %s"
                                      % (_MAXLINE, line_type))
 
+class RemoteDisconnected(ConnectionResetError, BadStatusLine):
+    def __init__(self, *pos, **kw):
+        BadStatusLine.__init__(self, "")
+        ConnectionResetError.__init__(self, *pos, **kw)
+
 # for backwards compatibility
 error = HTTPException