java/com/google/gerrit/server/util/RequestScopePropagator.java

// Copyright (C) 2012 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.server.util;

import static java.util.Objects.requireNonNull;

import com.google.common.base.Throwables;
import com.google.gerrit.entities.Project;
import com.google.gerrit.server.RequestCleanup;
import com.google.gerrit.server.git.ProjectRunnable;
import com.google.inject.Key;
import com.google.inject.Scope;
import com.google.inject.servlet.ServletScopes;
import java.util.concurrent.Callable;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledThreadPoolExecutor;

/**
 * Base class for propagating request-scoped data between threads.
 *
 * <p>Request scopes are typically linked to a {@link ThreadLocal}, which is only available to the
 * current thread. In order to allow background work involving RequestScoped data, the ThreadLocal
 * data must be copied from the request thread to the new background thread.
 *
 * <p>Every type of RequestScope must provide an implementation of RequestScopePropagator. See
 * {@link #wrap(Callable)} for details on the implementation, usage, and restrictions.
 *
 * @see ThreadLocalRequestScopePropagator
 */
public abstract class RequestScopePropagator {

  private final Scope scope;
  private final ThreadLocalRequestContext local;

  protected RequestScopePropagator(Scope scope, ThreadLocalRequestContext local) {
    this.scope = scope;
    this.local = local;
  }

  /**
   * Ensures that the current request state is available when the passed in Callable is invoked.
   *
   * <p>If needed wraps the passed in Callable in a new {@link Callable} that propagates the current
   * request state when the returned Callable is invoked. The method must be called in a request
   * scope and the returned Callable may only be invoked in a thread that is not already in a
   * request scope or is in the same request scope. The returned Callable will inherit toString()
   * from the passed in Callable. A {@link ScheduledThreadPoolExecutor} does not accept a Callable,
   * so there is no ProjectCallable implementation. Implementations of this method must be
   * consistent with Guice's {@link ServletScopes#continueRequest(Callable, java.util.Map)}.
   *
   * <p>There are some limitations:
   *
   * <ul>
   *   <li>Derived objects (i.e. anything marked created in a request scope) will not be
   *       transported.
   *   <li>State changes to the request scoped context after this method is called will not be seen
   *       in the continued thread.
   * </ul>
   *
   * @param callable the Callable to wrap.
   * @return a new Callable which will execute in the current request scope.
   */
  @SuppressWarnings("javadoc") // See GuiceRequestScopePropagator#wrapImpl
  public final <T> Callable<T> wrap(Callable<T> callable) {
    final RequestContext callerContext = requireNonNull(local.getContext());
    final Callable<T> wrapped = wrapImpl(context(callerContext, cleanup(callable)));
    return new Callable<>() {
      @Override
      public T call() throws Exception {
        if (callerContext == local.getContext()) {
          return callable.call();
        }
        return wrapped.call();
      }

      @Override
      public String toString() {
        return callable.toString();
      }
    };
  }

  /**
   * Wraps runnable in a new {@link Runnable} that propagates the current request state when the
   * runnable is invoked. The method must be called in a request scope and the returned Runnable may
   * only be invoked in a thread that is not already in a request scope. The returned Runnable will
   * inherit toString() from the passed in Runnable. Furthermore, if the passed runnable is of type
   * {@link ProjectRunnable}, the returned runnable will be of the same type with the methods
   * delegated.
   *
   * <p>See {@link #wrap(Callable)} for details on implementation and usage.
   *
   * @param runnable the Runnable to wrap.
   * @return a new Runnable which will execute in the current request scope.
   */
  public final Runnable wrap(Runnable runnable) {
    final Callable<Object> wrapped = wrap(Executors.callable(runnable));

    if (runnable instanceof ProjectRunnable) {
      return new ProjectRunnable() {
        @Override
        public void run() {
          try {
            wrapped.call();
          } catch (Exception e) {
            Throwables.throwIfUnchecked(e);
            throw new RuntimeException(e); // Not possible.
          }
        }

        @Override
        public Project.NameKey getProjectNameKey() {
          return ((ProjectRunnable) runnable).getProjectNameKey();
        }

        @Override
        public String getRemoteName() {
          return ((ProjectRunnable) runnable).getRemoteName();
        }

        @Override
        public boolean hasCustomizedPrint() {
          return ((ProjectRunnable) runnable).hasCustomizedPrint();
        }

        @Override
        public String toString() {
          return runnable.toString();
        }
      };
    }
    return new Runnable() {
      @Override
      public void run() {
        try {
          wrapped.call();
        } catch (RuntimeException e) {
          throw e;
        } catch (Exception e) {
          throw new RuntimeException(e); // Not possible.
        }
      }

      @Override
      public String toString() {
        return runnable.toString();
      }
    };
  }

  /**
   * Ensures that the current request state is available when the passed in Callable is invoked
   *
   * <p>See {@link #wrap(Callable)}
   */
  protected abstract <T> Callable<T> wrapImpl(Callable<T> callable);

  protected <T> Callable<T> context(RequestContext context, Callable<T> callable) {
    return () -> {
      RequestContext old = local.setContext(context);
      try {
        return callable.call();
      } finally {
        local.setContext(old);
      }
    };
  }

  protected <T> Callable<T> cleanup(Callable<T> callable) {
    return () -> {
      RequestCleanup cleanup =
          scope.scope(Key.get(RequestCleanup.class), RequestCleanup::new).get();
      try {
        return callable.call();
      } finally {
        cleanup.run();
      }
    };
  }
}