path: root/services/core/java/com/android/server/Watchdog.java

/*
 * Copyright (C) 2008 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.android.server;

import android.app.IActivityController;
import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;
import android.content.IntentFilter;
import android.hidl.manager.V1_0.IServiceManager;
import android.os.Binder;
import android.os.Build;
import android.os.Debug;
import android.os.Handler;
import android.os.IPowerManager;
import android.os.Looper;
import android.os.Process;
import android.os.RemoteException;
import android.os.ServiceManager;
import android.os.SystemClock;
import android.system.ErrnoException;
import android.system.Os;
import android.system.OsConstants;
import android.system.StructRlimit;
import android.util.EventLog;
import android.util.Log;
import android.util.Slog;
import android.util.StatsLog;

import com.android.internal.os.ZygoteConnectionConstants;
import com.android.server.am.ActivityManagerService;
import com.android.server.wm.SurfaceAnimationThread;

import java.io.File;
import java.io.FileWriter;
import java.io.IOException;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.HashSet;
import java.util.List;

/** This class calls its monitor every minute. Killing this process if they don't return **/
public class Watchdog extends Thread {
    static final String TAG = "Watchdog";

    /** Debug flag. */
    public static final boolean DEBUG = false;

    // Set this to true to use debug default values.
    static final boolean DB = false;

    // Note 1: Do not lower this value below thirty seconds without tightening the invoke-with
    //         timeout in com.android.internal.os.ZygoteConnection, or wrapped applications
    //         can trigger the watchdog.
    // Note 2: The debug value is already below the wait time in ZygoteConnection. Wrapped
    //         applications may not work with a debug build. CTS will fail.
    static final long DEFAULT_TIMEOUT = DB ? 10*1000 : 60*1000;
    static final long CHECK_INTERVAL = DEFAULT_TIMEOUT / 2;

    // These are temporally ordered: larger values as lateness increases
    static final int COMPLETED = 0;
    static final int WAITING = 1;
    static final int WAITED_HALF = 2;
    static final int OVERDUE = 3;

    // Which native processes to dump into dropbox's stack traces
    public static final String[] NATIVE_STACKS_OF_INTEREST = new String[] {
        "/system/bin/audioserver",
        "/system/bin/cameraserver",
        "/system/bin/drmserver",
        "/system/bin/mediadrmserver",
        "/system/bin/mediaserver",
        "/system/bin/sdcard",
        "/system/bin/surfaceflinger",
        "/system/bin/vold",
        "media.extractor", // system/bin/mediaextractor
        "media.metrics", // system/bin/mediametrics
        "media.codec", // vendor/bin/hw/android.hardware.media.omx@1.0-service
        "media.swcodec", // /apex/com.android.media.swcodec/bin/mediaswcodec
        "com.android.bluetooth",  // Bluetooth service
        "/system/bin/statsd",  // Stats daemon
    };

    public static final List<String> HAL_INTERFACES_OF_INTEREST = Arrays.asList(
            "android.hardware.audio@2.0::IDevicesFactory",
            "android.hardware.audio@4.0::IDevicesFactory",
            "android.hardware.biometrics.face@1.0::IBiometricsFace",
            "android.hardware.bluetooth@1.0::IBluetoothHci",
            "android.hardware.camera.provider@2.4::ICameraProvider",
            "android.hardware.graphics.allocator@2.0::IAllocator",
            "android.hardware.graphics.composer@2.1::IComposer",
            "android.hardware.health@2.0::IHealth",
            "android.hardware.media.c2@1.0::IComponentStore",
            "android.hardware.media.omx@1.0::IOmx",
            "android.hardware.media.omx@1.0::IOmxStore",
            "android.hardware.power.stats@1.0::IPowerStats",
            "android.hardware.sensors@1.0::ISensors",
            "android.hardware.vr@1.0::IVr",
            "android.system.suspend@1.0::ISystemSuspend"
    );

    static Watchdog sWatchdog;

    /* This handler will be used to post message back onto the main thread */
    final ArrayList<HandlerChecker> mHandlerCheckers = new ArrayList<>();
    final HandlerChecker mMonitorChecker;
    ActivityManagerService mActivity;

    int mPhonePid;
    IActivityController mController;
    boolean mAllowRestart = true;
    final OpenFdMonitor mOpenFdMonitor;

    /**
     * Used for checking status of handle threads and scheduling monitor callbacks.
     */
    public final class HandlerChecker implements Runnable {
        private final Handler mHandler;
        private final String mName;
        private final long mWaitMax;
        private final ArrayList<Monitor> mMonitors = new ArrayList<Monitor>();
        private final ArrayList<Monitor> mMonitorQueue = new ArrayList<Monitor>();
        private boolean mCompleted;
        private Monitor mCurrentMonitor;
        private long mStartTime;
        private int mPauseCount;

        HandlerChecker(Handler handler, String name, long waitMaxMillis) {
            mHandler = handler;
            mName = name;
            mWaitMax = waitMaxMillis;
            mCompleted = true;
        }

        void addMonitorLocked(Monitor monitor) {
            // We don't want to update mMonitors when the Handler is in the middle of checking
            // all monitors. We will update mMonitors on the next schedule if it is safe
            mMonitorQueue.add(monitor);
        }

        public void scheduleCheckLocked() {
            if (mCompleted) {
                // Safe to update monitors in queue, Handler is not in the middle of work
                mMonitors.addAll(mMonitorQueue);
                mMonitorQueue.clear();
            }
            if ((mMonitors.size() == 0 && mHandler.getLooper().getQueue().isPolling())
                    || (mPauseCount > 0)) {
                // Don't schedule until after resume OR
                // If the target looper has recently been polling, then
                // there is no reason to enqueue our checker on it since that
                // is as good as it not being deadlocked.  This avoid having
                // to do a context switch to check the thread. Note that we
                // only do this if we have no monitors since those would need to
                // be executed at this point.
                mCompleted = true;
                return;
            }
            if (!mCompleted) {
                // we already have a check in flight, so no need
                return;
            }

            mCompleted = false;
            mCurrentMonitor = null;
            mStartTime = SystemClock.uptimeMillis();
            mHandler.postAtFrontOfQueue(this);
        }

        boolean isOverdueLocked() {
            return (!mCompleted) && (SystemClock.uptimeMillis() > mStartTime + mWaitMax);
        }

        public int getCompletionStateLocked() {
            if (mCompleted) {
                return COMPLETED;
            } else {
                long latency = SystemClock.uptimeMillis() - mStartTime;
                if (latency < mWaitMax/2) {
                    return WAITING;
                } else if (latency < mWaitMax) {
                    return WAITED_HALF;
                }
            }
            return OVERDUE;
        }

        public Thread getThread() {
            return mHandler.getLooper().getThread();
        }

        public String getName() {
            return mName;
        }

        String describeBlockedStateLocked() {
            if (mCurrentMonitor == null) {
                return "Blocked in handler on " + mName + " (" + getThread().getName() + ")";
            } else {
                return "Blocked in monitor " + mCurrentMonitor.getClass().getName()
                        + " on " + mName + " (" + getThread().getName() + ")";
            }
        }

        @Override
        public void run() {
            // Once we get here, we ensure that mMonitors does not change even if we call
            // #addMonitorLocked because we first add the new monitors to mMonitorQueue and
            // move them to mMonitors on the next schedule when mCompleted is true, at which
            // point we have completed execution of this method.
            final int size = mMonitors.size();
            for (int i = 0 ; i < size ; i++) {
                synchronized (Watchdog.this) {
                    mCurrentMonitor = mMonitors.get(i);
                }
                mCurrentMonitor.monitor();
            }

            synchronized (Watchdog.this) {
                mCompleted = true;
                mCurrentMonitor = null;
            }
        }

        /** Pause the HandlerChecker. */
        public void pauseLocked(String reason) {
            mPauseCount++;
            // Mark as completed, because there's a chance we called this after the watchog
            // thread loop called Object#wait after 'WAITED_HALF'. In that case we want to ensure
            // the next call to #getCompletionStateLocked for this checker returns 'COMPLETED'
            mCompleted = true;
            Slog.i(TAG, "Pausing HandlerChecker: " + mName + " for reason: "
                    + reason + ". Pause count: " + mPauseCount);
        }

        /** Resume the HandlerChecker from the last {@link #pauseLocked}. */
        public void resumeLocked(String reason) {
            if (mPauseCount > 0) {
                mPauseCount--;
                Slog.i(TAG, "Resuming HandlerChecker: " + mName + " for reason: "
                        + reason + ". Pause count: " + mPauseCount);
            } else {
                Slog.wtf(TAG, "Already resumed HandlerChecker: " + mName);
            }
        }
    }

    final class RebootRequestReceiver extends BroadcastReceiver {
        @Override
        public void onReceive(Context c, Intent intent) {
            if (intent.getIntExtra("nowait", 0) != 0) {
                rebootSystem("Received ACTION_REBOOT broadcast");
                return;
            }
            Slog.w(TAG, "Unsupported ACTION_REBOOT broadcast: " + intent);
        }
    }

    /** Monitor for checking the availability of binder threads. The monitor will block until
     * there is a binder thread available to process in coming IPCs to make sure other processes
     * can still communicate with the service.
     */
    private static final class BinderThreadMonitor implements Watchdog.Monitor {
        @Override
        public void monitor() {
            Binder.blockUntilThreadAvailable();
        }
    }

    public interface Monitor {
        void monitor();
    }

    public static Watchdog getInstance() {
        if (sWatchdog == null) {
            sWatchdog = new Watchdog();
        }

        return sWatchdog;
    }

    private Watchdog() {
        super("watchdog");
        // Initialize handler checkers for each common thread we want to check.  Note
        // that we are not currently checking the background thread, since it can
        // potentially hold longer running operations with no guarantees about the timeliness
        // of operations there.

        // The shared foreground thread is the main checker.  It is where we
        // will also dispatch monitor checks and do other work.
        mMonitorChecker = new HandlerChecker(FgThread.getHandler(),
                "foreground thread", DEFAULT_TIMEOUT);
        mHandlerCheckers.add(mMonitorChecker);
        // Add checker for main thread.  We only do a quick check since there
        // can be UI running on the thread.
        mHandlerCheckers.add(new HandlerChecker(new Handler(Looper.getMainLooper()),
                "main thread", DEFAULT_TIMEOUT));
        // Add checker for shared UI thread.
        mHandlerCheckers.add(new HandlerChecker(UiThread.getHandler(),
                "ui thread", DEFAULT_TIMEOUT));
        // And also check IO thread.
        mHandlerCheckers.add(new HandlerChecker(IoThread.getHandler(),
                "i/o thread", DEFAULT_TIMEOUT));
        // And the display thread.
        mHandlerCheckers.add(new HandlerChecker(DisplayThread.getHandler(),
                "display thread", DEFAULT_TIMEOUT));
        // And the animation thread.
        mHandlerCheckers.add(new HandlerChecker(AnimationThread.getHandler(),
                "animation thread", DEFAULT_TIMEOUT));
        // And the surface animation thread.
        mHandlerCheckers.add(new HandlerChecker(SurfaceAnimationThread.getHandler(),
                "surface animation thread", DEFAULT_TIMEOUT));

        // Initialize monitor for Binder threads.
        addMonitor(new BinderThreadMonitor());

        mOpenFdMonitor = OpenFdMonitor.create();

        // See the notes on DEFAULT_TIMEOUT.
        assert DB ||
                DEFAULT_TIMEOUT > ZygoteConnectionConstants.WRAPPED_PID_TIMEOUT_MILLIS;
    }

    /**
     * Registers a {@link BroadcastReceiver} to listen to reboot broadcasts and trigger reboot.
     * Should be called during boot after the ActivityManagerService is up and registered
     * as a system service so it can handle registration of a {@link BroadcastReceiver}.
     */
    public void init(Context context, ActivityManagerService activity) {
        mActivity = activity;
        context.registerReceiver(new RebootRequestReceiver(),
                new IntentFilter(Intent.ACTION_REBOOT),
                android.Manifest.permission.REBOOT, null);
    }

    public void processStarted(String name, int pid) {
        synchronized (this) {
            if ("com.android.phone".equals(name)) {
                mPhonePid = pid;
            }
        }
    }

    public void setActivityController(IActivityController controller) {
        synchronized (this) {
            mController = controller;
        }
    }

    public void setAllowRestart(boolean allowRestart) {
        synchronized (this) {
            mAllowRestart = allowRestart;
        }
    }

    public void addMonitor(Monitor monitor) {
        synchronized (this) {
            mMonitorChecker.addMonitorLocked(monitor);
        }
    }

    public void addThread(Handler thread) {
        addThread(thread, DEFAULT_TIMEOUT);
    }

    public void addThread(Handler thread, long timeoutMillis) {
        synchronized (this) {
            final String name = thread.getLooper().getThread().getName();
            mHandlerCheckers.add(new HandlerChecker(thread, name, timeoutMillis));
        }
    }

    /**
     * Pauses Watchdog action for the currently running thread. Useful before executing long running
     * operations that could falsely trigger the watchdog. Each call to this will require a matching
     * call to {@link #resumeWatchingCurrentThread}.
     *
     * <p>If the current thread has not been added to the Watchdog, this call is a no-op.
     *
     * <p>If the Watchdog is already paused for the current thread, this call adds
     * adds another pause and will require an additional {@link #resumeCurrentThread} to resume.
     *
     * <p>Note: Use with care, as any deadlocks on the current thread will be undetected until all
     * pauses have been resumed.
     */
    public void pauseWatchingCurrentThread(String reason) {
        synchronized (this) {
            for (HandlerChecker hc : mHandlerCheckers) {
                if (Thread.currentThread().equals(hc.getThread())) {
                    hc.pauseLocked(reason);
                }
            }
        }
    }

    /**
     * Resumes the last pause from {@link #pauseWatchingCurrentThread} for the currently running
     * thread.
     *
     * <p>If the current thread has not been added to the Watchdog, this call is a no-op.
     *
     * <p>If the Watchdog action for the current thread is already resumed, this call logs a wtf.
     *
     * <p>If all pauses have been resumed, the Watchdog action is finally resumed, otherwise,
     * the Watchdog action for the current thread remains paused until resume is called at least
     * as many times as the calls to pause.
     */
    public void resumeWatchingCurrentThread(String reason) {
        synchronized (this) {
            for (HandlerChecker hc : mHandlerCheckers) {
                if (Thread.currentThread().equals(hc.getThread())) {
                    hc.resumeLocked(reason);
                }
            }
        }
    }

    /**
     * Perform a full reboot of the system.
     */
    void rebootSystem(String reason) {
        Slog.i(TAG, "Rebooting system because: " + reason);
        IPowerManager pms = (IPowerManager)ServiceManager.getService(Context.POWER_SERVICE);
        try {
            pms.reboot(false, reason, false);
        } catch (RemoteException ex) {
        }
    }

    private int evaluateCheckerCompletionLocked() {
        int state = COMPLETED;
        for (int i=0; i<mHandlerCheckers.size(); i++) {
            HandlerChecker hc = mHandlerCheckers.get(i);
            state = Math.max(state, hc.getCompletionStateLocked());
        }
        return state;
    }

    private ArrayList<HandlerChecker> getBlockedCheckersLocked() {
        ArrayList<HandlerChecker> checkers = new ArrayList<HandlerChecker>();
        for (int i=0; i<mHandlerCheckers.size(); i++) {
            HandlerChecker hc = mHandlerCheckers.get(i);
            if (hc.isOverdueLocked()) {
                checkers.add(hc);
            }
        }
        return checkers;
    }

    private String describeCheckersLocked(List<HandlerChecker> checkers) {
        StringBuilder builder = new StringBuilder(128);
        for (int i=0; i<checkers.size(); i++) {
            if (builder.length() > 0) {
                builder.append(", ");
            }
            builder.append(checkers.get(i).describeBlockedStateLocked());
        }
        return builder.toString();
    }

    private static ArrayList<Integer> getInterestingHalPids() {
        try {
            IServiceManager serviceManager = IServiceManager.getService();
            ArrayList<IServiceManager.InstanceDebugInfo> dump =
                    serviceManager.debugDump();
            HashSet<Integer> pids = new HashSet<>();
            for (IServiceManager.InstanceDebugInfo info : dump) {
                if (info.pid == IServiceManager.PidConstant.NO_PID) {
                    continue;
                }

                if (!HAL_INTERFACES_OF_INTEREST.contains(info.interfaceName)) {
                    continue;
                }

                pids.add(info.pid);
            }
            return new ArrayList<Integer>(pids);
        } catch (RemoteException e) {
            return new ArrayList<Integer>();
        }
    }

    static ArrayList<Integer> getInterestingNativePids() {
        ArrayList<Integer> pids = getInterestingHalPids();

        int[] nativePids = Process.getPidsForCommands(NATIVE_STACKS_OF_INTEREST);
        if (nativePids != null) {
            pids.ensureCapacity(pids.size() + nativePids.length);
            for (int i : nativePids) {
                pids.add(i);
            }
        }

        return pids;
    }

    @Override
    public void run() {
        boolean waitedHalf = false;
        while (true) {
            final List<HandlerChecker> blockedCheckers;
            final String subject;
            final boolean allowRestart;
            int debuggerWasConnected = 0;
            synchronized (this) {
                long timeout = CHECK_INTERVAL;
                // Make sure we (re)spin the checkers that have become idle within
                // this wait-and-check interval
                for (int i=0; i<mHandlerCheckers.size(); i++) {
                    HandlerChecker hc = mHandlerCheckers.get(i);
                    hc.scheduleCheckLocked();
                }

                if (debuggerWasConnected > 0) {
                    debuggerWasConnected--;
                }

                // NOTE: We use uptimeMillis() here because we do not want to increment the time we
                // wait while asleep. If the device is asleep then the thing that we are waiting
                // to timeout on is asleep as well and won't have a chance to run, causing a false
                // positive on when to kill things.
                long start = SystemClock.uptimeMillis();
                while (timeout > 0) {
                    if (Debug.isDebuggerConnected()) {
                        debuggerWasConnected = 2;
                    }
                    try {
                        wait(timeout);
                        // Note: mHandlerCheckers and mMonitorChecker may have changed after waiting
                    } catch (InterruptedException e) {
                        Log.wtf(TAG, e);
                    }
                    if (Debug.isDebuggerConnected()) {
                        debuggerWasConnected = 2;
                    }
                    timeout = CHECK_INTERVAL - (SystemClock.uptimeMillis() - start);
                }

                boolean fdLimitTriggered = false;
                if (mOpenFdMonitor != null) {
                    fdLimitTriggered = mOpenFdMonitor.monitor();
                }

                if (!fdLimitTriggered) {
                    final int waitState = evaluateCheckerCompletionLocked();
                    if (waitState == COMPLETED) {
                        // The monitors have returned; reset
                        waitedHalf = false;
                        continue;
                    } else if (waitState == WAITING) {
                        // still waiting but within their configured intervals; back off and recheck
                        continue;
                    } else if (waitState == WAITED_HALF) {
                        if (!waitedHalf) {
                            Slog.i(TAG, "WAITED_HALF");
                            // We've waited half the deadlock-detection interval.  Pull a stack
                            // trace and wait another half.
                            ArrayList<Integer> pids = new ArrayList<Integer>();
                            pids.add(Process.myPid());
                            ActivityManagerService.dumpStackTraces(pids, null, null,
                                getInterestingNativePids());
                            waitedHalf = true;
                        }
                        continue;
                    }

                    // something is overdue!
                    blockedCheckers = getBlockedCheckersLocked();
                    subject = describeCheckersLocked(blockedCheckers);
                } else {
                    blockedCheckers = Collections.emptyList();
                    subject = "Open FD high water mark reached";
                }
                allowRestart = mAllowRestart;
            }

            // If we got here, that means that the system is most likely hung.
            // First collect stack traces from all threads of the system process.
            // Then kill this process so that the system will restart.
            EventLog.writeEvent(EventLogTags.WATCHDOG, subject);

            ArrayList<Integer> pids = new ArrayList<>();
            pids.add(Process.myPid());
            if (mPhonePid > 0) pids.add(mPhonePid);

            final File stack = ActivityManagerService.dumpStackTraces(
                    pids, null, null, getInterestingNativePids());

            // Give some extra time to make sure the stack traces get written.
            // The system's been hanging for a minute, another second or two won't hurt much.
            SystemClock.sleep(5000);

            // Trigger the kernel to dump all blocked threads, and backtraces on all CPUs to the kernel log
            doSysRq('w');
            doSysRq('l');

            // Try to add the error to the dropbox, but assuming that the ActivityManager
            // itself may be deadlocked.  (which has happened, causing this statement to
            // deadlock and the watchdog as a whole to be ineffective)
            Thread dropboxThread = new Thread("watchdogWriteToDropbox") {
                    public void run() {
                        // If a watched thread hangs before init() is called, we don't have a
                        // valid mActivity. So we can't log the error to dropbox.
                        if (mActivity != null) {
                            mActivity.addErrorToDropBox(
                                    "watchdog", null, "system_server", null, null, null,
                                    subject, null, stack, null);
                        }
                        StatsLog.write(StatsLog.SYSTEM_SERVER_WATCHDOG_OCCURRED, subject);
                    }
                };
            dropboxThread.start();
            try {
                dropboxThread.join(2000);  // wait up to 2 seconds for it to return.
            } catch (InterruptedException ignored) {}

            IActivityController controller;
            synchronized (this) {
                controller = mController;
            }
            if (controller != null) {
                Slog.i(TAG, "Reporting stuck state to activity controller");
                try {
                    Binder.setDumpDisabled("Service dumps disabled due to hung system process.");
                    // 1 = keep waiting, -1 = kill system
                    int res = controller.systemNotResponding(subject);
                    if (res >= 0) {
                        Slog.i(TAG, "Activity controller requested to coninue to wait");
                        waitedHalf = false;
                        continue;
                    }
                } catch (RemoteException e) {
                }
            }

            // Only kill the process if the debugger is not attached.
            if (Debug.isDebuggerConnected()) {
                debuggerWasConnected = 2;
            }
            if (debuggerWasConnected >= 2) {
                Slog.w(TAG, "Debugger connected: Watchdog is *not* killing the system process");
            } else if (debuggerWasConnected > 0) {
                Slog.w(TAG, "Debugger was connected: Watchdog is *not* killing the system process");
            } else if (!allowRestart) {
                Slog.w(TAG, "Restart not allowed: Watchdog is *not* killing the system process");
            } else {
                Slog.w(TAG, "*** WATCHDOG KILLING SYSTEM PROCESS: " + subject);
                WatchdogDiagnostics.diagnoseCheckers(blockedCheckers);
                Slog.w(TAG, "*** GOODBYE!");
                Process.killProcess(Process.myPid());
                System.exit(10);
            }

            waitedHalf = false;
        }
    }

    private void doSysRq(char c) {
        try {
            FileWriter sysrq_trigger = new FileWriter("/proc/sysrq-trigger");
            sysrq_trigger.write(c);
            sysrq_trigger.close();
        } catch (IOException e) {
            Slog.w(TAG, "Failed to write to /proc/sysrq-trigger", e);
        }
    }

    public static final class OpenFdMonitor {
        /**
         * Number of FDs below the soft limit that we trigger a runtime restart at. This was
         * chosen arbitrarily, but will need to be at least 6 in order to have a sufficient number
         * of FDs in reserve to complete a dump.
         */
        private static final int FD_HIGH_WATER_MARK = 12;

        private final File mDumpDir;
        private final File mFdHighWaterMark;

        public static OpenFdMonitor create() {
            // Only run the FD monitor on debuggable builds (such as userdebug and eng builds).
            if (!Build.IS_DEBUGGABLE) {
                return null;
            }

            final StructRlimit rlimit;
            try {
                rlimit = android.system.Os.getrlimit(OsConstants.RLIMIT_NOFILE);
            } catch (ErrnoException errno) {
                Slog.w(TAG, "Error thrown from getrlimit(RLIMIT_NOFILE)", errno);
                return null;
            }

            // The assumption we're making here is that FD numbers are allocated (more or less)
            // sequentially, which is currently (and historically) true since open is currently
            // specified to always return the lowest-numbered non-open file descriptor for the
            // current process.
            //
            // We do this to avoid having to enumerate the contents of /proc/self/fd in order to
            // count the number of descriptors open in the process.
            final File fdThreshold = new File("/proc/self/fd/" + (rlimit.rlim_cur - FD_HIGH_WATER_MARK));
            return new OpenFdMonitor(new File("/data/anr"), fdThreshold);
        }

        OpenFdMonitor(File dumpDir, File fdThreshold) {
            mDumpDir = dumpDir;
            mFdHighWaterMark = fdThreshold;
        }

        /**
         * Dumps open file descriptors and their full paths to a temporary file in {@code mDumpDir}.
         */
        private void dumpOpenDescriptors() {
            // We cannot exec lsof to get more info about open file descriptors because a newly
            // forked process will not have the permissions to readlink. Instead list all open
            // descriptors from /proc/pid/fd and resolve them.
            List<String> dumpInfo = new ArrayList<>();
            String fdDirPath = String.format("/proc/%d/fd/", Process.myPid());
            File[] fds = new File(fdDirPath).listFiles();
            if (fds == null) {
                dumpInfo.add("Unable to list " + fdDirPath);
            } else {
                for (File f : fds) {
                    String fdSymLink = f.getAbsolutePath();
                    String resolvedPath = "";
                    try {
                        resolvedPath = Os.readlink(fdSymLink);
                    } catch (ErrnoException ex) {
                        resolvedPath = ex.getMessage();
                    }
                    dumpInfo.add(fdSymLink + "\t" + resolvedPath);
                }
            }

            // Dump the fds & paths to a temp file.
            try {
                File dumpFile = File.createTempFile("anr_fd_", "", mDumpDir);
                Path out = Paths.get(dumpFile.getAbsolutePath());
                Files.write(out, dumpInfo, StandardCharsets.UTF_8);
            } catch (IOException ex) {
                Slog.w(TAG, "Unable to write open descriptors to file: " + ex);
            }
        }

        /**
         * @return {@code true} if the high water mark was breached and a dump was written,
         *     {@code false} otherwise.
         */
        public boolean monitor() {
            if (mFdHighWaterMark.exists()) {
                dumpOpenDescriptors();
                return true;
            }

            return false;
        }
    }
}