DatabasePubKeyAuth: Exclude comment from peer key line if present

In I9a934bda3 base64 decoding was changed from using commons-codec
library to Guava library. However, this broke users who specify not only
base 64 encoded key, but also provide comment, similar to authorized
keys format without leading algorithm.

Clarify the documentation what format is supported in peer keys file and
skip the comment part if provided.

Also add integration test for peer keys authentication with three use
cases:

1. Test format: <key_material comment>
2. Test format: <key_material>
3. No peer keys file present

Test Plan:

  $ bazel test javatests/com/google/gerrit/integration/ssh:peer-keys-auth

Bug: Issue 12884
Change-Id: Id748cb55497e3347477012911f716ac5b7b69863

javatests/com/google/gerrit/integration/ssh/PeerKeysAuthIT.java

@@ -0,0 +1,104 @@
+// Copyright (C) 2020 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.integration.ssh;
+
+import static com.google.common.truth.Truth.assertThat;
+import static com.google.gerrit.testing.GerritJUnit.assertThrows;
+import static java.nio.charset.StandardCharsets.ISO_8859_1;
+import static java.nio.charset.StandardCharsets.UTF_8;
+
+import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableMap;
+import com.google.gerrit.acceptance.GerritServer.TestSshServerAddress;
+import com.google.gerrit.acceptance.NoHttpd;
+import com.google.gerrit.acceptance.StandaloneSiteTest;
+import com.google.gerrit.acceptance.UseSsh;
+import com.google.gerrit.common.Version;
+import com.google.gerrit.server.PeerDaemonUser;
+import com.google.inject.Inject;
+import java.io.IOException;
+import java.net.InetSocketAddress;
+import java.nio.file.Files;
+import org.junit.Test;
+
+@NoHttpd
+@UseSsh
+public class PeerKeysAuthIT extends StandaloneSiteTest {
+  private static final String[] SSH_KEYGEN_CMD =
+      new String[] {"ssh-keygen", "-t", "rsa", "-q", "-P", "", "-f", "id_rsa"};
+  private static final String[] SSH_COMMAND =
+      new String[] {
+        "ssh",
+        "-o",
+        "UserKnownHostsFile=/dev/null",
+        "-o",
+        "StrictHostKeyChecking=no",
+        "-o",
+        "IdentitiesOnly=yes",
+        "-i"
+      };
+
+  @Inject private @TestSshServerAddress InetSocketAddress sshAddress;
+
+  @Test
+  public void test() throws Exception {
+    try (ServerContext ctx = startServer()) {
+      ctx.getInjector().injectMembers(this);
+      // Generate private/public key for user
+      execute(ImmutableList.<String>builder().add(SSH_KEYGEN_CMD).build());
+
+      String[] parts =
+          new String(Files.readAllBytes(sitePaths.data_dir.resolve("id_rsa.pub")), UTF_8)
+              .split(" ");
+
+      // Loose algorithm at index 0, verify the format: "key comment"
+      Files.write(
+          sitePaths.peer_keys, String.format("%s %s", parts[1], parts[2]).getBytes(ISO_8859_1));
+      assertContent(execGerritVersionCommand());
+
+      // Only preserve the key material: no algorithm and no comment
+      Files.write(sitePaths.peer_keys, parts[1].getBytes(ISO_8859_1));
+      assertContent(execGerritVersionCommand());
+
+      // Wipe out the content of the peer keys file
+      Files.delete(sitePaths.peer_keys);
+      assertThrows(IOException.class, () -> execGerritVersionCommand());
+    }
+  }
+
+  private String execGerritVersionCommand() throws Exception {
+    return execute(
+        ImmutableList.<String>builder()
+            .add(SSH_COMMAND)
+            .add(sitePaths.data_dir.resolve("id_rsa").toString())
+            .add("-p " + sshAddress.getPort())
+            .add(PeerDaemonUser.USER_NAME + "@" + sshAddress.getHostName())
+            .add("suexec")
+            .add("--as")
+            .add("admin")
+            .add("--")
+            .add("gerrit")
+            .add("version")
+            .build());
+  }
+
+  private String execute(ImmutableList<String> cmd) throws Exception {
+    return execute(cmd, sitePaths.data_dir.toFile(), ImmutableMap.of());
+  }
+
+  private static void assertContent(String result) {
+    assertThat(result).contains("gerrit version " + Version.getVersion());
+  }
+}