Set Gerrit projects read-only during maintenance

During rolling restart for Gerrit upgrades it can be useful to
set all projects to read-only and provide a courtesy message
whenever a Git push is received.

Change-Id: Iad586eafd40a0b4d780a48522cd45e2e4f276c36

admin/README.md

diff --git a/admin/README.md b/admin/README.md
index d59d3eb..b94614f 100644
--- a/admin/README.md
+++ b/admin/README.md
@@ -9,3 +9,4 @@
 -----
 [reindexer-1.0.groovy](/admin/reindexer-1.0.groovy) - Allows to recreate the Lucene index using the on-line reindex
 [warm-cache-1.0.groovy](/admin/warm-cache-1.0.groovy) - Controls the Gerrit cache warm-up via command-line
+[readonly-1.0.groovy](/admin/readonly-1.0.groovy) - Set all Gerrit projects in read-only mode during maintenance

admin/readonly-1.0.groovy

diff --git a/admin/readonly-1.0.groovy b/admin/readonly-1.0.groovy
new file mode 100644
index 0000000..cb4b335
--- /dev/null
+++ b/admin/readonly-1.0.groovy
@@ -0,0 +1,36 @@
+// Copyright (C) 2016 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.
+
+import com.google.gerrit.extensions.annotations.Listen
+import com.google.gerrit.server.events.CommitReceivedEvent
+import com.google.gerrit.server.git.validators.CommitValidationException
+import com.google.gerrit.server.git.validators.CommitValidationListener
+import com.google.gerrit.server.git.validators.CommitValidationMessage
+
+import com.google.inject.Singleton
+import com.google.inject.Inject
+
+@Singleton
+@Listen
+public class DoNotCommitHook implements CommitValidationListener {
+
+  def READ_ONLY_MSG = "Gerrit is under maintenance, all projects are READ ONLY"
+
+  @Override
+  public List<CommitValidationMessage> onCommitReceived(
+      CommitReceivedEvent receiveEvent) throws CommitValidationException {
+    throw new CommitValidationException(READ_ONLY_MSG)
+  }
+}
+