// Copyright 2016 The Bazel Authors. All rights reserved.
//
// 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.devtools.build.lib.runtime;

import com.google.common.annotations.VisibleForTesting;
import com.google.common.base.Preconditions;
import com.google.common.collect.ImmutableList;
import com.google.common.flogger.GoogleLogger;
import com.google.devtools.build.lib.bugreport.BugReport;
import com.google.devtools.build.lib.concurrent.ThreadSafety;
import com.google.devtools.build.lib.util.AbruptExitException;
import com.google.devtools.build.lib.util.ExitCode;
import com.sun.management.GarbageCollectionNotificationInfo;
import java.lang.management.GarbageCollectorMXBean;
import java.lang.management.ManagementFactory;
import java.lang.management.MemoryUsage;
import java.util.List;
import java.util.Map;
import java.util.OptionalInt;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.concurrent.atomic.AtomicLong;
import javax.management.ListenerNotFoundException;
import javax.management.Notification;
import javax.management.NotificationEmitter;
import javax.management.NotificationListener;
import javax.management.openmbean.CompositeData;

/**
 * Monitor the size of the retained heap and exit promptly if it grows too large. Specifically,
 * check the size of the tenured space after each major GC; if it exceeds {@link
 * #occupiedHeapPercentageThreshold}%, call {@code System.gc()} to trigger a stop-the-world
 * collection; if it's still more than {@link #occupiedHeapPercentageThreshold}% full, exit with an
 * {@link OutOfMemoryError}.
 */
class RetainedHeapLimiter implements NotificationListener {
  private static final GoogleLogger logger = GoogleLogger.forEnclosingClass();
  private static final long MIN_TIME_BETWEEN_TRIGGERED_GC_MILLISECONDS = 60000;

  private final AtomicBoolean throwingOom = new AtomicBoolean(false);
  private final ImmutableList<NotificationEmitter> tenuredGcEmitters;
  private OptionalInt occupiedHeapPercentageThreshold = OptionalInt.empty();
  private final AtomicLong lastTriggeredGcInMilliseconds = new AtomicLong();

  RetainedHeapLimiter() {
    this(ManagementFactory.getGarbageCollectorMXBeans());
  }

  @VisibleForTesting
  RetainedHeapLimiter(List<GarbageCollectorMXBean> gcBeans) {
    tenuredGcEmitters = findTenuredCollectorBeans(gcBeans);
    Preconditions.checkState(
        !tenuredGcEmitters.isEmpty(),
        "Can't find tenured space; update this class for a new collector");
  }

  @ThreadSafety.ThreadCompatible // Can only be called on the logical main Bazel thread.
  void updateThreshold(int occupiedHeapPercentageThreshold) throws AbruptExitException {
    if (occupiedHeapPercentageThreshold < 0 || occupiedHeapPercentageThreshold > 100) {
      throw new AbruptExitException(
          "--experimental_oom_more_eagerly_threshold must be a percent between 0 and 100 but was "
              + occupiedHeapPercentageThreshold,
          ExitCode.COMMAND_LINE_ERROR);
    }
    boolean alreadyInstalled = this.occupiedHeapPercentageThreshold.isPresent();
    this.occupiedHeapPercentageThreshold =
        occupiedHeapPercentageThreshold < 100
            ? OptionalInt.of(occupiedHeapPercentageThreshold)
            : OptionalInt.empty();
    boolean shouldBeInstalled = this.occupiedHeapPercentageThreshold.isPresent();
    if (alreadyInstalled && !shouldBeInstalled) {
      for (NotificationEmitter emitter : tenuredGcEmitters) {
        try {
          emitter.removeNotificationListener(this, null, null);
        } catch (ListenerNotFoundException e) {
          logger.atWarning().log("Couldn't remove self as listener from %s", emitter);
        }
      }
    } else if (!alreadyInstalled && shouldBeInstalled) {
      tenuredGcEmitters.forEach(e -> e.addNotificationListener(this, null, null));
    }
  }

  @VisibleForTesting
  static ImmutableList<NotificationEmitter> findTenuredCollectorBeans(
      List<GarbageCollectorMXBean> gcBeans) {
    ImmutableList.Builder<NotificationEmitter> builder = ImmutableList.builder();
    // Examine all collectors and register for notifications from those which collect the tenured
    // space. Normally there is one such collector.
    for (GarbageCollectorMXBean gcBean : gcBeans) {
      for (String name : gcBean.getMemoryPoolNames()) {
        if (isTenuredSpace(name)) {
          builder.add((NotificationEmitter) gcBean);
        }
      }
    }
    return builder.build();
  }

  // Can be called concurrently, handles concurrent calls with #updateThreshold gracefully.
  @ThreadSafety.ThreadSafe
  @Override
  public void handleNotification(Notification notification, Object handback) {
    if (!notification
        .getType()
        .equals(GarbageCollectionNotificationInfo.GARBAGE_COLLECTION_NOTIFICATION)) {
      return;
    }
    // Get a local reference to guard against concurrent modifications.
    OptionalInt occupiedHeapPercentageThreshold = this.occupiedHeapPercentageThreshold;
    if (!occupiedHeapPercentageThreshold.isPresent()) {
      // Presumably failure above to uninstall this listener, or a racy GC.
      logger.atInfo().atMostEvery(1, TimeUnit.MINUTES).log(
          "Got notification %s when should be disabled", notification);
      return;
    }
    GarbageCollectionNotificationInfo info =
        GarbageCollectionNotificationInfo.from((CompositeData) notification.getUserData());
    Map<String, MemoryUsage> spaces = info.getGcInfo().getMemoryUsageAfterGc();
    for (Map.Entry<String, MemoryUsage> entry : spaces.entrySet()) {
      if (isTenuredSpace(entry.getKey())) {
        MemoryUsage space = entry.getValue();
        if (space.getMax() == 0) {
          // The collector sometimes passes us nonsense stats.
          continue;
        }

        long percentUsed = 100 * space.getUsed() / space.getMax();
        if (percentUsed > occupiedHeapPercentageThreshold.getAsInt()) {
          if (info.getGcCause().equals("System.gc()") && !throwingOom.getAndSet(true)) {
            // Assume we got here from a GC initiated by the other branch.
            String exitMsg =
                String.format(
                    "RetainedHeapLimiter forcing exit due to GC thrashing: After back-to-back full "
                        + "GCs, the tenured space is more than %s%% occupied (%s out of a tenured "
                        + "space size of %s)",
                    occupiedHeapPercentageThreshold.getAsInt(), space.getUsed(), space.getMax());
            System.err.println(exitMsg);
            logger.atInfo().log(exitMsg);
            // Exits the runtime.
            BugReport.handleCrash(new OutOfMemoryError(exitMsg));
          } else if (System.currentTimeMillis() - lastTriggeredGcInMilliseconds.get()
              > MIN_TIME_BETWEEN_TRIGGERED_GC_MILLISECONDS) {
            logger.atInfo().log(
                "Triggering a full GC with %s tenured space used out of a tenured space size of %s",
                space.getUsed(), space.getMax());
            // Force a full stop-the-world GC and see if it can get us below the threshold.
            System.gc();
            lastTriggeredGcInMilliseconds.set(System.currentTimeMillis());
          }
        }
      }
    }
  }

  private static boolean isTenuredSpace(String name) {
    return "CMS Old Gen".equals(name)
        || "G1 Old Gen".equals(name)
        || "PS Old Gen".equals(name)
        || "Tenured Gen".equals(name);
  }
}