Reorganize forwarded agent cxns to avoid errors

Fixes [Fabric #562](https://github.com/fabric/fabric/issues/562).
(cherry picked from commit 58aa52085bc664fbfce61c5981ba54c1452f7abf)

2 files changed, 27 insertions, 7 deletions

diff --git a/CHANGES b/CHANGES
index be3b2ad7..e529a666 100644
--- a/CHANGES
+++ b/CHANGES
@@ -7,6 +7,12 @@ Temporary, post-Paramiko changelog while we get our sh!t together.
 * #24: Switch some internal type checking to use `isinstance` to help prevent
   problems with client libraries using subclasses of builtin types. Thanks to
   Alex Morega for the patch.
+* [Fabric #562](https://github.com/fabric/fabric/issues/562): Agent forwarding
+  would error out (with `Authentication response too long`) or freeze, when more
+  than one remote connection to the local agent was active at the same time.
+  This has been fixed. Thanks to Steven McDonald for assisting in
+  troubleshooting/patching, and to GitHub user `@lynxis` for providing the
+  final version of the patch.
 
 ## ssh 1.7.13 (2012-02-13)
 
diff --git a/paramiko/agent.py b/paramiko/agent.py
index 31dcaf8c..7115f17b 100644
--- a/paramiko/agent.py
+++ b/paramiko/agent.py
@@ -186,10 +186,11 @@ class AgentClientProxy(object):
        the remote fake agent and the local agent
        -> Communication occurs ...
     """
-    def __init__(self, chanClient):
+    def __init__(self, chanRemote):
         self._conn = None
-        self.__chanC = chanClient
-        chanClient.request_forward_agent(self._forward_agent_handler)
+        self.__chanR = chanRemote
+        self.thread = AgentRemoteProxy(self, chanRemote)
+        self.thread.start()
 
     def __del__(self):
         self.close()
@@ -227,10 +228,6 @@ class AgentClientProxy(object):
         if self._conn is not None:
             self._conn.close()
 
-    def _forward_agent_handler(self, chanRemote):
-        self.thread = AgentRemoteProxy(self, chanRemote)
-        self.thread.start()
-
 class AgentServerProxy(AgentSSH):
     """
     @param t : transport used for the Forward for SSH Agent communication
@@ -281,6 +278,23 @@ class AgentServerProxy(AgentSSH):
     def _get_filename(self):
         return self._file
 
+class AgentRequestHandler(object):
+    def __init__(self, chanClient):
+        self._conn = None
+        self.__chanC = chanClient
+        chanClient.request_forward_agent(self._forward_agent_handler)
+        self.__clientProxys = []
+
+    def _forward_agent_handler(self, chanRemote):
+        self.__clientProxys.append(AgentClientProxy(chanRemote))
+
+    def __del__(self):
+        self.close()
+
+    def close(self):
+        for p in self.__clientProxys:
+            p.close()
+
 class Agent(AgentSSH):
     """
     Client interface for using private keys from an SSH agent running on the