Add test script for simultaneous agent connections.

This script makes 128 connections to your SSH agent at once, and then
sends requests down them in random order to check that the agent is
correctly selecting between all its incoming sockets / named pipes /
whatever.

128 is bigger than MAXIMUM_WAIT_OBJECTS, so a successful run of this
script inside a Windows PuTTY agent-forwarding to a Pageant indicates
that both the PuTTY and the Pageant are managing to handle >64 I/O
subthreads without overloading their event loop.

test/agentmulti.py

@@ -0,0 +1,56 @@
+#!/usr/bin/env python3
+
+import argparse
+import os
+import random
+import socket
+import sys
+
+from ssh import *
+
+def make_connections(n):
+    connections = []
+
+    for _ in range(n):
+        s = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
+        s.connect(os.environ["SSH_AUTH_SOCK"])
+        connections.append(s)
+
+    return connections
+
+def use_connection(s, idstring):
+    print("Trying {}...".format(idstring), end="")
+    sys.stdout.flush()
+
+    s.send(ssh_string(ssh_byte(SSH2_AGENTC_EXTENSION) + ssh_string(
+        b"nonexistent-agent-extension@putty.projects.tartarus.org")))
+    length = ssh_decode_uint32(s.recv(4))
+    assert length < AGENT_MAX_MSGLEN
+    msg = s.recv(length)
+    msgtype = msg[0]
+    msgstring = (
+        "SSH_AGENT_EXTENSION_FAILURE" if msgtype == SSH_AGENT_EXTENSION_FAILURE
+        else "SSH_AGENT_FAILURE" if msgtype == SSH_AGENT_FAILURE
+        else "type {:d}".format(msgtype))
+    print("got", msgstring, "with {:d}-byte payload".format(len(msg)-1))
+
+def randomly_use_connections(connections, iterations):
+    for _ in range(iterations):
+        index = random.randrange(0, len(connections))
+        s = connections[index]
+        use_connection(connections[index], "#{:d}".format(index))
+
+def main():
+    parser = argparse.ArgumentParser(
+        description='Test handling of multiple agent connections.')
+    parser.add_argument("--nsockets", type=int, default=128,
+                        help="Number of simultaneous connections to make.")
+    parser.add_argument("--ntries", type=int, default=1024,
+                        help="Number of messages to send in total.")
+    args = parser.parse_args()
+
+    connections = make_connections(args.nsockets)
+    randomly_use_connections(connections, args.ntries)
+
+if __name__ == '__main__':
+    main()

test/ssh.py

@@ -88,6 +88,7 @@ SSH1_AGENTC_REMOVE_RSA_IDENTITY = 8
 SSH1_AGENTC_REMOVE_ALL_RSA_IDENTITIES = 9
 SSH_AGENT_FAILURE = 5
 SSH_AGENT_SUCCESS = 6
+SSH_AGENT_EXTENSION_FAILURE = 28
 SSH2_AGENTC_REQUEST_IDENTITIES = 11
 SSH2_AGENT_IDENTITIES_ANSWER = 12
 SSH2_AGENTC_SIGN_REQUEST = 13