// 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 static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.collect.ImmutableList.toImmutableList;
import static java.util.concurrent.TimeUnit.MINUTES;

import com.google.common.annotations.VisibleForTesting;
import com.google.common.collect.ImmutableList;
import com.google.common.flogger.GoogleLogger;
import com.google.devtools.build.lib.bugreport.BugReporter;
import com.google.devtools.build.lib.bugreport.Crash;
import com.google.devtools.build.lib.bugreport.CrashContext;
import com.google.devtools.build.lib.concurrent.ThreadSafety;
import com.google.devtools.build.lib.server.FailureDetails.FailureDetail;
import com.google.devtools.build.lib.server.FailureDetails.MemoryOptions;
import com.google.devtools.build.lib.util.AbruptExitException;
import com.google.devtools.build.lib.util.DetailedExitCode;
import com.sun.management.GarbageCollectionNotificationInfo;
import java.lang.management.GarbageCollectorMXBean;
import java.lang.management.ManagementFactory;
import java.lang.management.MemoryUsage;
import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.OptionalInt;
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.concurrent.atomic.AtomicLong;
import javax.annotation.Nullable;
import javax.management.ListenerNotFoundException;
import javax.management.Notification;
import javax.management.NotificationEmitter;
import javax.management.NotificationListener;
import javax.management.openmbean.CompositeData;

/**
 * Monitors the size of the retained heap and exit promptly if it grows too large.
 *
 * <p>Specifically, checks the size of the tenured space after each major GC; if it exceeds {@link
 * #occupiedHeapPercentageThreshold}%, call {@link System#gc()} to trigger a stop-the-world
 * collection; if it's still more than {@link #occupiedHeapPercentageThreshold}% full, exit with an
 * {@link OutOfMemoryError}.
 */
final 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 AtomicBoolean heapLimiterTriggeredGc = new AtomicBoolean(false);
  private final ImmutableList<NotificationEmitter> tenuredGcEmitters;
  private OptionalInt occupiedHeapPercentageThreshold = OptionalInt.empty();
  private final AtomicLong lastTriggeredGcInMilliseconds = new AtomicLong();
  private final BugReporter bugReporter;

  static RetainedHeapLimiter create(BugReporter bugReporter) {
    return createFromBeans(ManagementFactory.getGarbageCollectorMXBeans(), bugReporter);
  }

  @VisibleForTesting
  static RetainedHeapLimiter createFromBeans(
      List<GarbageCollectorMXBean> gcBeans, BugReporter bugReporter) {
    ImmutableList<NotificationEmitter> tenuredGcEmitters = findTenuredCollectorBeans(gcBeans);
    if (tenuredGcEmitters.isEmpty()) {
      logger.atSevere().log(
          "Unable to find tenured collector from %s: names were %s. "
              + "--experimental_oom_more_eagerly_threshold cannot be specified for this JVM",
          gcBeans,
          gcBeans.stream()
              .map(GarbageCollectorMXBean::getMemoryPoolNames)
              .map(Arrays::asList)
              .collect(toImmutableList()));
    }
    return new RetainedHeapLimiter(tenuredGcEmitters, bugReporter);
  }

  private RetainedHeapLimiter(
      ImmutableList<NotificationEmitter> tenuredGcEmitters, BugReporter bugReporter) {
    this.tenuredGcEmitters = checkNotNull(tenuredGcEmitters);
    this.bugReporter = checkNotNull(bugReporter);
  }

  @ThreadSafety.ThreadCompatible // Can only be called on the logical main Bazel thread.
  void update(int oomMoreEagerlyThreshold) throws AbruptExitException {
    if (tenuredGcEmitters.isEmpty() && oomMoreEagerlyThreshold != 100) {
      throw createExitException(
          "No tenured GC collectors were found: unable to watch for GC events to exit JVM when "
              + oomMoreEagerlyThreshold
              + "% of heap is used",
          MemoryOptions.Code.EXPERIMENTAL_OOM_MORE_EAGERLY_NO_TENURED_COLLECTORS_FOUND);
    }
    if (oomMoreEagerlyThreshold < 0 || oomMoreEagerlyThreshold > 100) {
      throw createExitException(
          "--experimental_oom_more_eagerly_threshold must be a percent between 0 and 100 but was "
              + oomMoreEagerlyThreshold,
          MemoryOptions.Code.EXPERIMENTAL_OOM_MORE_EAGERLY_THRESHOLD_INVALID_VALUE);
    }
    boolean alreadyInstalled = this.occupiedHeapPercentageThreshold.isPresent();
    this.occupiedHeapPercentageThreshold =
        oomMoreEagerlyThreshold < 100
            ? OptionalInt.of(oomMoreEagerlyThreshold)
            : 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().withCause(e).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 #update 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, MINUTES).log(
          "Got notification %s when should be disabled", notification);
      return;
    }
    int threshold = occupiedHeapPercentageThreshold.getAsInt();
    GarbageCollectionNotificationInfo info =
        GarbageCollectionNotificationInfo.from((CompositeData) notification.getUserData());
    boolean manualGc = info.getGcCause().equals("System.gc()");
    if (manualGc && !heapLimiterTriggeredGc.getAndSet(false)) {
      // This was a manually triggered GC, but not from the other branch: short-circuit.
      return;
    }

    @Nullable
    MemoryUsage space = getTenuredSpacedIfFull(info.getGcInfo().getMemoryUsageAfterGc(), threshold);
    if (space == null) {
      return;
    }

    if (manualGc) {
      if (!throwingOom.getAndSet(true)) {
        // We got here from a GC initiated by the other branch.
        OutOfMemoryError oom =
            new OutOfMemoryError(
                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).",
                    threshold, space.getUsed(), space.getMax()));
        // Exits the runtime.
        bugReporter.handleCrash(Crash.from(oom), CrashContext.halt());
      }
    } 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());
      heapLimiterTriggeredGc.set(true);
      // 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)
        || "Shenandoah".equals(name)
        || "ZHeap".equals(name);
  }

  @Nullable
  private static MemoryUsage getTenuredSpacedIfFull(
      Map<String, MemoryUsage> spaces, int threshold) {
    ImmutableList<Map.Entry<String, MemoryUsage>> fullSpaces =
        spaces.entrySet().stream()
            .filter(
                e -> {
                  if (!isTenuredSpace(e.getKey())) {
                    return false;
                  }
                  MemoryUsage space = e.getValue();
                  if (space.getMax() == 0) {
                    // The collector sometimes passes us nonsense stats.
                    return false;
                  }

                  return (100 * space.getUsed() / space.getMax()) > threshold;
                })
            .collect(toImmutableList());
    if (fullSpaces.size() > 1) {
      logger.atInfo().log("Multiple full tenured spaces: %s", fullSpaces);
    }
    return fullSpaces.isEmpty() ? null : fullSpaces.get(0).getValue();
  }

  private static AbruptExitException createExitException(String message, MemoryOptions.Code code) {
    return new AbruptExitException(
        DetailedExitCode.of(
            FailureDetail.newBuilder()
                .setMessage(message)
                .setMemoryOptions(MemoryOptions.newBuilder().setCode(code))
                .build()));
  }
}