Plugin API: Add datasource interception extension point

This change extends the plugin API to support monitoring of Gerrit's
SQL activity by providing the extension point to create a DataSource
proxy to intercept execution of SQL statements.

To activate SQL monitoring the following must be done:

Add the interceptor class to gerrit.config:

  dataSourceInterceptorClass = javamelody.JavamelodyInterceptor

Implement the interface DataSourceInterceptor:

  public class JavamelodyInterceptor implements DataSourceInterceptor {
    @Override
    public DataSource intercept(String name, DataSource dataSource) {
      return JdbcWrapper.SINGLETON.createDataSourceProxy(name, dataSource);
    }
  }

Put the jar containing the interceptor into $gerrit_site/lib directory.

Working example of this concept can be found in javamelody-plugin.

Change-Id: I5f0b5ffe072394eae77e3b1735a76dceff4bdc42

Documentation/config-gerrit.txt

@@ -1221,6 +1221,13 @@ Default is `30 seconds`.
 This setting only applies if
 <<database.connectionPool,database.connectionPool>> is true.
 
+[[database.dataSourceInterceptorClass]]database.dataSourceInterceptorClass::
+
+Class that implements DataSourceInterceptor interface to monitor SQL activity.
+This class must have default constructor and be available on Gerrit's bootstrap
+classpath, e. g. in `$gerrit_site/lib` directory. Example implementation of
+SQL monitoring can be found in javamelody-plugin.
+
 [[download]]
 === Section download
 

gerrit-extension-api/src/main/java/com/google/gerrit/extensions/persistence/DataSourceInterceptor.java

@@ -0,0 +1,21 @@
+// 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.extensions.persistence;
+
+import javax.sql.DataSource;
+
+public interface DataSourceInterceptor {
+  DataSource intercept(String name, DataSource dataSource);
+}

gerrit-server/src/main/java/com/google/gerrit/server/schema/DataSourceProvider.java

@@ -19,6 +19,7 @@ import static java.util.concurrent.TimeUnit.SECONDS;
 
 import com.google.common.base.Strings;
 import com.google.gerrit.extensions.events.LifecycleListener;
+import com.google.gerrit.extensions.persistence.DataSourceInterceptor;
 import com.google.gerrit.server.config.ConfigSection;
 import com.google.gerrit.server.config.ConfigUtil;
 import com.google.gerrit.server.config.GerritServerConfig;
@@ -32,6 +33,8 @@ import com.google.inject.Singleton;
 import org.apache.commons.dbcp.BasicDataSource;
 import org.eclipse.jgit.lib.Config;
 
+import java.lang.reflect.Constructor;
+import java.lang.reflect.InvocationTargetException;
 import java.sql.SQLException;
 import java.util.Properties;
 
@@ -102,6 +105,7 @@ public class DataSourceProvider implements Provider<DataSource>,
 
     String username = dbs.optional("username");
     String password = dbs.optional("password");
+    String interceptor = dbs.optional("dataSourceInterceptorClass");
 
     boolean usePool;
     if (context == Context.SINGLE_USER) {
@@ -126,7 +130,7 @@ public class DataSourceProvider implements Provider<DataSource>,
       ds.setMaxWait(ConfigUtil.getTimeUnit(cfg, "database", null,
           "poolmaxwait", MILLISECONDS.convert(30, SECONDS), MILLISECONDS));
       ds.setInitialSize(ds.getMinIdle());
-      return ds;
+      return intercept(interceptor, ds);
 
     } else {
       // Don't use the connection pool.
@@ -141,10 +145,26 @@ public class DataSourceProvider implements Provider<DataSource>,
         if (password != null) {
           p.setProperty("password", password);
         }
-        return new SimpleDataSource(p);
+        return intercept(interceptor, new SimpleDataSource(p));
       } catch (SQLException se) {
         throw new ProvisionException("Database unavailable", se);
       }
     }
   }
+
+  private DataSource intercept(String interceptor, DataSource ds) {
+    if (interceptor == null) {
+      return ds;
+    }
+    try {
+      Constructor<?> c = Class.forName(interceptor).getConstructor();
+      DataSourceInterceptor datasourceInterceptor =
+          (DataSourceInterceptor) c.newInstance();
+      return datasourceInterceptor.intercept("reviewDb", ds);
+    } catch (ClassNotFoundException | SecurityException | NoSuchMethodException
+        | IllegalArgumentException | InstantiationException
+        | IllegalAccessException | InvocationTargetException e) {
+      throw new ProvisionException("Cannot intercept datasource", e);
+    }
+  }
 }