SSHD: Prevent double authentication for the same public key

Openssh client sends two requests, one without a key signature to verify
that the public key is acceptable and the second one with the signature
after having loaded the private key and signed some data for actual
verification.

To prevent that the PublickeyAuthenticator#authenticate is called twice
cache the authentication status for session and public key.  Implement
SessionListener to clean up the cache entry when session is destroyed.

This is a workaround for SSHD bug [1].

[1] https://issues.apache.org/jira/browse/SSHD-300

Inspired-By: Guillaume Nodet <gnodet@apache.org>
Change-Id: Ie8caebd6762125a754c46d821b3c7af2a10edd2b

gerrit-sshd/src/main/java/com/google/gerrit/sshd/CachingPublicKeyAuthenticator.java

@@ -0,0 +1,72 @@
+// Copyright (C) 2014 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.sshd;
+
+import com.google.inject.Inject;
+import com.google.inject.Singleton;
+
+import org.apache.sshd.common.Session;
+import org.apache.sshd.common.SessionListener;
+import org.apache.sshd.server.PublickeyAuthenticator;
+import org.apache.sshd.server.session.ServerSession;
+
+import java.security.PublicKey;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
+
+@Singleton
+public class CachingPublicKeyAuthenticator implements PublickeyAuthenticator,
+    SessionListener {
+
+  private final PublickeyAuthenticator authenticator;
+  private final Map<ServerSession, Map<PublicKey, Boolean>> sessionCache;
+
+  @Inject
+  public CachingPublicKeyAuthenticator(DatabasePubKeyAuth authenticator) {
+    this.authenticator = authenticator;
+    this.sessionCache = new ConcurrentHashMap<>();
+  }
+
+  @Override
+  public boolean authenticate(String username, PublicKey key,
+      ServerSession session) {
+    Map<PublicKey, Boolean> m = sessionCache.get(session);
+    if (m == null) {
+      m = new HashMap<>();
+      sessionCache.put(session, m);
+      session.addListener(this);
+    }
+    if (m.containsKey(key)) {
+      return m.get(key);
+    }
+    boolean r = authenticator.authenticate(username, key, session);
+    m.put(key, r);
+    return r;
+  }
+
+  @Override
+  public void sessionCreated(Session session) {
+  }
+
+  @Override
+  public void sessionEvent(Session sesssion, Event event) {
+  }
+
+  @Override
+  public void sessionClosed(Session session) {
+    sessionCache.remove(session);
+  }
+}