/*
 * Copyright (C) 2025 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 android.os;

import static android.os.Message.*;

import android.annotation.IntDef;
import android.annotation.NonNull;
import android.annotation.Nullable;
import android.annotation.SuppressLint;
import android.annotation.TestApi;
import android.app.ActivityThread;
import android.compat.annotation.UnsupportedAppUsage;
import android.ravenwood.annotation.RavenwoodKeepWholeClass;
import android.ravenwood.annotation.RavenwoodRedirect;
import android.ravenwood.annotation.RavenwoodRedirectionClass;
import android.ravenwood.annotation.RavenwoodReplace;
import android.ravenwood.annotation.RavenwoodThrow;
import android.util.Log;
import android.util.Printer;
import android.util.SparseArray;
import android.util.proto.ProtoOutputStream;

import dalvik.annotation.optimization.NeverCompile;

import java.io.FileDescriptor;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.invoke.MethodHandles;
import java.lang.invoke.VarHandle;
import java.util.ArrayList;
import java.util.Comparator;
import java.util.NoSuchElementException;
import java.util.concurrent.ConcurrentSkipListSet;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicLong;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.LockSupport;
import java.util.concurrent.locks.ReentrantLock;

/**
 * Low-level class holding the list of messages to be dispatched by a
 * {@link Looper}.  Messages are not added directly to a MessageQueue,
 * but rather through {@link Handler} objects associated with the Looper.
 *
 * <p>You can retrieve the MessageQueue for the current thread with
 * {@link Looper#myQueue() Looper.myQueue()}.
 */
@RavenwoodKeepWholeClass
@RavenwoodRedirectionClass("MessageQueue_ravenwood")
public final class MessageQueue {
    private static final String TAG = "DeliQueue";
    private static final boolean DEBUG = false;

    // True if the message queue can be quit.
    @UnsupportedAppUsage
    private final boolean mQuitAllowed;

    @UnsupportedAppUsage
    @SuppressWarnings("unused")
    private long mPtr; // used by native code

    @RavenwoodRedirect
    private native static long nativeInit();
    @RavenwoodRedirect
    private native static void nativeDestroy(long ptr);
    @UnsupportedAppUsage
    @RavenwoodRedirect
    private native void nativePollOnce(long ptr, int timeoutMillis); /*non-static for callbacks*/

    @RavenwoodRedirect
    private native static void nativeWake(long ptr);
    @RavenwoodRedirect
    private native static boolean nativeIsPolling(long ptr);
    @RavenwoodRedirect
    private native static void nativeSetFileDescriptorEvents(long ptr, int fd, int events);
    @RavenwoodRedirect
    private native static void nativeSetSkipEpollWaitForZeroTimeout(long ptr);

    @UnsupportedAppUsage
    private final ArrayList<IdleHandler> mIdleHandlers = new ArrayList<IdleHandler>();
    private SparseArray<FileDescriptorRecord> mFileDescriptorRecords;
    private IdleHandler[] mPendingIdleHandlers;

    private final Object mIdleHandlersLock = new Object();
    private final Object mFileDescriptorRecordsLock = new Object();

    private final AtomicLong mMessageCount = new AtomicLong();
    private final Thread mLooperThread;
    private final String mThreadName;
    private final long mTid;

    MessageStack mStack = new MessageStack();

    /*
     * This helps us ensure that messages with the same timestamp are inserted in FIFO order.
     * Increments on each insert, starting at 0. MessageNode.compareTo() will compare sequences
     * when delivery timestamps are identical.
     */
    private static final VarHandle sNextInsertSeq;
    private volatile long mNextInsertSeqValue = 0;
    /*
     * The exception to the FIFO order rule is sendMessageAtFrontOfQueue().
     * Those messages must be in LIFO order.
     * Decrements on each front of queue insert.
     */
    private static final VarHandle sNextFrontInsertSeq;
    private volatile long mNextFrontInsertSeqValue = -1;

    private static final VarHandle sWaitState;
    private volatile long mWaitState;

    /*
     * Ref count our access to mPtr.
     * next() doesn't want to dispose of mPtr until after quit() is called.
     * isPolling() also needs to ensure safe access to mPtr.
     * So keep a ref count of access to mPtr. If quitting is set, we disallow new refs.
     * next() will only proceed with disposing of the pointer once all refs are dropped.
     */
    private static final VarHandle sMptrRefCount;
    private volatile long mMptrRefCountValue = 0;

    private static final VarHandle sSyncBarrier;
    private volatile Message mSyncBarrier = null;

    static {
        try {
            // We need to use VarHandle rather than java.util.concurrent.atomic.*
            // for performance reasons. See: b/421437036
            MethodHandles.Lookup l = MethodHandles.lookup();
            sNextInsertSeq = l.findVarHandle(MessageQueue.class, "mNextInsertSeqValue",
                    long.class);
            sNextFrontInsertSeq = l.findVarHandle(MessageQueue.class, "mNextFrontInsertSeqValue",
                    long.class);
            sWaitState = l.findVarHandle(MessageQueue.class, "mWaitState",
                    long.class);
            sMptrRefCount = l.findVarHandle(MessageQueue.class, "mMptrRefCountValue",
                    long.class);
            sSyncBarrier = l.findVarHandle(MessageQueue.class, "mSyncBarrier",
                    Message.class);
        } catch (ReflectiveOperationException e) {
            throw new ExceptionInInitializerError(e);
        }
    }

    // Use MSB to indicate mPtr teardown state. Lower 63 bits hold ref count.
    private static final long MPTR_TEARDOWN_MASK = 1L << 63;

    /**
     * Increment the mPtr ref count.
     *
     * If this method returns true then the caller may use mPtr until they call
     * {@link #decrementMptrRefs()}.
     * If this method returns false then the caller must not use mPtr, and must
     * instead assume that the MessageQueue is quitting or has already quit and
     * act accordingly.
     */
    private boolean incrementMptrRefs() {
        while (true) {
            final long oldVal = mMptrRefCountValue;
            if ((oldVal & MPTR_TEARDOWN_MASK) != 0) {
                // If we're quitting then we're not allowed to increment the ref count.
                return false;
            }
            if (sMptrRefCount.compareAndSet(this, oldVal, oldVal + 1)) {
                // Successfully incremented the ref count without quitting.
                return true;
            }
        }
    }

    /**
     * Decrement the mPtr ref count.
     *
     * Call after {@link #incrementMptrRefs()} to release the ref on mPtr.
     */
    private void decrementMptrRefs() {
        long oldVal = (long) sMptrRefCount.getAndAdd(this, -1);
        // If quitting and we were the last ref, wake up looper thread
        if (oldVal - 1 == MPTR_TEARDOWN_MASK) {
            LockSupport.unpark(mLooperThread);
        }
    }

    /**
     * Wake the looper thread.
     *
     * {@link #nativeWake(long)} may be called directly only by the looper thread.
     * Otherwise, call this method to ensure safe access to mPtr.
     */
    private void concurrentWake() {
        if (incrementMptrRefs()) {
            try {
                nativeWake(mPtr);
            } finally {
                decrementMptrRefs();
            }
        }
    }

    // Must only be called from looper thread
    private void setMptrTeardownAndWaitForRefsToDrop() {
        if (DEBUG && Thread.currentThread() != mLooperThread) {
            throw new IllegalStateException(
                    "setMptrTeardownAndWaitForRefsToDrop must only be called from looper thread");
        }
        while (true) {
            final long oldVal = mMptrRefCountValue;
            if (sMptrRefCount.compareAndSet(this, oldVal, oldVal | MPTR_TEARDOWN_MASK)) {
                // Successfully set teardown state.
                break;
            }
        }

        boolean wasInterrupted = false;
        try {
            while ((mMptrRefCountValue & ~MPTR_TEARDOWN_MASK) != 0) {
                LockSupport.park();
                wasInterrupted |= Thread.interrupted();
            }
        } finally {
            if (wasInterrupted) {
                mLooperThread.interrupt();
            }
        }
    }

    static boolean getUseConcurrent() {
        return true;
    }

    private static boolean isBarrier(Message msg) {
        return msg != null && msg.target == null;
    }

    MessageQueue(boolean quitAllowed) {
        mQuitAllowed = quitAllowed;
        mPtr = nativeInit();
        mLooperThread = Thread.currentThread();
        mThreadName = mLooperThread.getName();
        mTid = Process.myTid();
        long now = SystemClock.uptimeMillis();
        mWaitState = WaitState.composeDeadline(now + INDEFINITE_TIMEOUT_MS, false);
    }

    // Disposes of the underlying message queue.
    // Must only be called on the looper thread or the finalizer.
    private void dispose() {
        if (mPtr != 0) {
            nativeDestroy(mPtr);
            mPtr = 0;
        }
    }

    @SuppressWarnings("Finalize")
    @Override
    protected void finalize() throws Throwable {
        try {
            dispose();
        } finally {
            super.finalize();
        }
    }

    @NeverCompile
    private static void logDeadThread(Message msg) {
        IllegalStateException e = new IllegalStateException(
                msg.target + " sending message to a Handler on a dead thread");
        Log.w(TAG, e.getMessage(), e);
        msg.recycleUnchecked();
    }

    private boolean enqueueMessageUnchecked(@NonNull Message msg, long when) {
        long seq = when != 0 ? ((long) sNextInsertSeq.getAndAdd(this, 1L) + 1L)
                : ((long) sNextFrontInsertSeq.getAndAdd(this, -1L) - 1L);
        msg.when = when;
        msg.insertSeq = seq;
        msg.markInUse();
        incAndTraceMessageCount(msg, when);

        if (!mStack.pushMessage(msg)) {
            logDeadThread(msg);
            decAndTraceMessageCount();
            return false;
        }

        if (DEBUG) {
            Log.d(TAG, "Insert message"
                    + " what: " + msg.what
                    + " when: " + msg.when
                    + " seq: " + msg.insertSeq
                    + " barrier: " + isBarrier(msg)
                    + " async: " + msg.isAsynchronous()
                    + " now: " + SystemClock.uptimeMillis());
        }

        while (true) {
            long waitState = mWaitState;
            long newWaitState;
            boolean needWake = false;
            Message barrier = msg.isAsynchronous() ? null :
                    (Message) sSyncBarrier.getVolatile(this);
            boolean reCheckBarrier = false;

            if (WaitState.isCounter(waitState)) {
                newWaitState = WaitState.incrementCounter(waitState);
            } else {
                final long TSmillis = WaitState.getTSMillis(waitState);
                boolean weComeBeforeBarrier = barrier != null && msg.when <= barrier.when;
                if (weComeBeforeBarrier || (msg.when < TSmillis
                        && (!WaitState.hasSyncBarrier(waitState) || msg.isAsynchronous()))) {
                    newWaitState = WaitState.initCounter();
                    needWake = true;
                } else {
                    newWaitState = WaitState.incrementDeadline(waitState);
                    reCheckBarrier = true;
                }
            }
            if (sWaitState.compareAndSet(this, waitState, newWaitState)) {
                if (reCheckBarrier && barrier != (Message) sSyncBarrier.getVolatile(this)) {
                    /*
                     * If barrier state changed underneath us and we chose not to wake the
                     * looper thread, we have to recheck to ensure that the barrier we saw was
                     * actually in place while we did the CAS.
                     */
                    continue;
                }
                if (needWake) {
                    concurrentWake();
                }
                return true;
            }
        }
    }

    boolean enqueueMessage(Message msg, long when) {
        if (msg.target == null) {
            throw new IllegalArgumentException("Message must have a target.");
        }
        if (msg.isInUse()) {
            throw new IllegalStateException(msg + " This message is already in use.");
        }

        return enqueueMessageUnchecked(msg, when);
    }

    /* These are only read/written from the looper thread */
    private int mNextPollTimeoutMillis;
    private static int INDEFINITE_TIMEOUT_MS = 600_000_000;
    private boolean mWorkerShouldQuit;
    Message nextMessage(boolean peek, boolean returnEarliest) {
        while (true) {
            /*
             * Once we are converted to a counter only the looper thread can change waitstate back
             * to a timestamp
             */
            long oldWaitState = mWaitState;
            final long zeroCounter = WaitState.initCounter();
            while (!WaitState.isCounter(oldWaitState)) {
                if (sWaitState.compareAndSet(this, oldWaitState, zeroCounter)) {
                    oldWaitState = zeroCounter;
                    break;
                }
                oldWaitState = mWaitState;
            }

            mStack.heapSweep();
            mStack.drainFreelist();

            boolean shouldRemoveMessages = false;
            if (mStack.isQuitting()) {
                if (!mWorkerShouldQuit) {
                    mWorkerShouldQuit = true;
                    long TS = mStack.getQuittingTimestamp();
                    if (TS == 0) {
                        removeAllMessages();
                    } else {
                        removeAllFutureMessages(TS);
                    }
                }
            }

            Message msg = mStack.peek(false);
            Message asyncMsg = mStack.peek(true);
            final long now = SystemClock.uptimeMillis();

            if (DEBUG) {
                if (msg != null) {
                    Log.d(TAG, "Next found node"
                            + " what: " + msg.what
                            + " when: " + msg.when
                            + " seq: " + msg.insertSeq
                            + " barrier: " + isBarrier(msg)
                            + " now: " + now);
                }
                if (asyncMsg != null) {
                    Log.d(TAG, "Next found async node"
                            + " what: " + asyncMsg.what
                            + " when: " + asyncMsg.when
                            + " seq: " + asyncMsg.insertSeq
                            + " barrier: " + isBarrier(asyncMsg)
                            + " now: " + now);
                }
            }

            /*
            * the node which we will return, null if none are ready
            */
            Message found = null;
            /*
            * The node from which we will determine our next wakeup time.
            * Null indicates there is no next message ready. If we found a node,
            * we can leave this null as Looper will call us again after delivering
            * the message.
            */
            Message next = null;
            Message syncBarrier = null;
            /*
            * If we have a barrier we should return the async node (if it exists and is ready)
            */
            if (isBarrier(msg)) {
                if (asyncMsg != null && (returnEarliest || now >= asyncMsg.when)) {
                    found = asyncMsg;
                } else {
                    syncBarrier = msg;
                    next = asyncMsg;
                }
            } else { /* No barrier. */
                // Pick the earliest of the next sync and async messages, if any.
                Message earliest = msg;
                if (msg == null) {
                    earliest = asyncMsg;
                } else if (asyncMsg != null) {
                    if (Message.compareMessages(msg, asyncMsg) > 0) {
                        earliest = asyncMsg;
                    }
                }

                if (earliest != null) {
                    if (returnEarliest || now >= earliest.when) {
                        found = earliest;
                    } else {
                        next = earliest;
                    }
                }
            }

            if (DEBUG) {
                if (found != null) {
                    Log.d(TAG, "Will deliver node"
                            + " what: " + found.what
                            + " when: " + found.when
                            + " seq: " + found.insertSeq
                            + " barrier: " + isBarrier(found)
                            + " async: " + found.isAsynchronous()
                            + " now: " + now);
                } else {
                    Log.d(TAG, "No node to deliver");
                }
                if (next != null) {
                    Log.d(TAG, "Next node"
                            + " what: " + next.what
                            + " when: " + next.when
                            + " seq: " + next.insertSeq
                            + " barrier: " + isBarrier(next)
                            + " async: " + next.isAsynchronous()
                            + " now: " + now);
                } else {
                    Log.d(TAG, "No next node");
                }
            }

            /*
             * If we have a found message, we will get called again so there's no need to set
             * state.
             *
             * Otherwise we should determine how to park the thread.
             * mNextPollTimeoutMillis has some special meanings which don't translate directly to
             * the deadline required in our WaitState. Use a separate variable to track waitstate
             * deadline.
             */
            long nextDeadline = 0;
            if (found == null) {
                if (mWorkerShouldQuit) {
                    // Set to zero so we don't block the looper and can quit immediately.
                    mNextPollTimeoutMillis = 0;
                } else if (next == null) {
                    /* No message to deliver, sleep indefinitely */
                    mNextPollTimeoutMillis = INDEFINITE_TIMEOUT_MS;
                    nextDeadline = now + INDEFINITE_TIMEOUT_MS;
                    if (DEBUG) {
                        Log.d(TAG, "nextMessage next wait state is INDEFINITE_TIMEOUT_MS");
                    }
                } else {
                    /* Message not ready, or we found one to deliver already, set a timeout */
                    long nextMessageWhen = next.when;
                    if (nextMessageWhen > now) {
                        mNextPollTimeoutMillis = (int) Math.min(nextMessageWhen - now,
                                Integer.MAX_VALUE);
                        nextDeadline = now + mNextPollTimeoutMillis;
                    } else {
                        mNextPollTimeoutMillis = 0;
                    }

                    if (DEBUG) {
                        Log.d(TAG, "nextMessage next wait "
                                + " timeout ms " + mNextPollTimeoutMillis
                                + " now: " + now);
                    }
                }
            }

            sSyncBarrier.setVolatile(this, syncBarrier);
            /*
             * Try to swap waitstate back from a counter to a deadline. If we can't then that means
             * the counter was incremented and we need to loop back to pick up any new items.
             */
            if (!sWaitState.compareAndSet(this, oldWaitState,
                    WaitState.composeDeadline(nextDeadline, syncBarrier != null))) {
                continue;
            }
            if (found != null || nextDeadline != 0) {
                if (found != null && !peek) {
                    if (!found.markRemoved()) {
                        continue;
                    }
                    mStack.remove(found);
                }
                return found;
            }
            return null;
        }
    }

    Message next() {
        final long ptr = mPtr;
        if (ptr == 0) {
            return null;
        }

        mNextPollTimeoutMillis = 0;
        int pendingIdleHandlerCount = -1; // -1 only during first iteration
        while (true) {
            if (mNextPollTimeoutMillis != 0) {
                Binder.flushPendingCommands();
            }

            // TODO: mMessageDirectlyQueued = false; when implementing direct enqueue
            nativePollOnce(ptr, mNextPollTimeoutMillis);

            Message msg = nextMessage(false, false);
            if (msg != null) {
                msg.markInUse();
                decAndTraceMessageCount();
                return msg;
            }

            // Prevent any race between quit()/nativeWake(), dispose() and other users of mPtr
            if (mWorkerShouldQuit) {
                setMptrTeardownAndWaitForRefsToDrop();
                dispose();
                return null;
            }

            synchronized (mIdleHandlersLock) {
                // If first time idle, then get the number of idlers to run.
                // Idle handles only run if the queue is empty or if the first message
                // in the queue (possibly a barrier) is due to be handled in the future.
                if (pendingIdleHandlerCount < 0
                        && isIdle()) {
                    pendingIdleHandlerCount = mIdleHandlers.size();
                }
                if (pendingIdleHandlerCount <= 0) {
                    // No idle handlers to run.  Loop and wait some more.
                    continue;
                }

                if (mPendingIdleHandlers == null) {
                    mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];
                }
                mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);
            }

            // Run the idle handlers.
            // We only ever reach this code block during the first iteration.
            for (int i = 0; i < pendingIdleHandlerCount; i++) {
                final IdleHandler idler = mPendingIdleHandlers[i];
                mPendingIdleHandlers[i] = null; // release the reference to the handler

                boolean keep = false;
                try {
                    keep = idler.queueIdle();
                } catch (Throwable t) {
                    Log.wtf(TAG, "IdleHandler threw exception", t);
                }

                if (!keep) {
                    synchronized (mIdleHandlersLock) {
                        mIdleHandlers.remove(idler);
                    }
                }
            }

            // Reset the idle handler count to 0 so we do not run them again.
            pendingIdleHandlerCount = 0;

            // While calling an idle handler, a new message could have been delivered
            // so go back and look again for a pending message without waiting.
            mNextPollTimeoutMillis = 0;
        }
    }

    /**
     * Returns true if the looper has no pending messages which are due to be processed
     * and is not blocked on a sync barrier.
     *
     * <p>This method is safe to call from any thread.
     *
     * @return True if the looper is idle.
     */
    public boolean isIdle() {
        final long now = SystemClock.uptimeMillis();
        return !mStack.hasMessages(sMatchDeliverableMessages, null, -1, null, null, now);
    }

    /**
     * Returns whether this looper's thread is currently polling for more work to do.
     * This is a good signal that the loop is still alive rather than being stuck
     * handling a callback.  Note that this method is intrinsically racy, since the
     * state of the loop can change before you get the result back.
     *
     * <p>This method is safe to call from any thread.
     *
     * @return True if the looper is currently polling for events.
     * @hide
     */
    public boolean isPolling() {
        // If the loop is quitting then it must not be idling.
        if (!mStack.isQuitting() && incrementMptrRefs()) {
            try {
                return nativeIsPolling(mPtr);
            } finally {
                decrementMptrRefs();
            }
        }
        return false;
    }

    /**
     * Returns the message with the latest scheduled execution time.
     *
     *
     * Caller must ensure that this doesn't race 'next' from the Looper thread.
     * @hide
     */
    public @Nullable Message peekLastMessageForTest() {
        ActivityThread.throwIfNotInstrumenting();
        return mStack.peekLastMessageForTest();
    }

    /**
     * Resets this queue's state.
     *
     * @hide
     */
    public void resetForTest() {
        ActivityThread.throwIfNotInstrumenting();
        // This queue is already quitting, so we can't reset its state and continue using it.
        if (mWorkerShouldQuit) {
            return;
        }
        synchronized (mIdleHandlersLock) {
            mIdleHandlers.clear();
        }
        synchronized (mFileDescriptorRecordsLock) {
            removeAllFdRecords();
        }
        removeAllMessages();
        mStack.drainFreelist();

        // We reset the sync barrier tokens to reflect the queue's state reset. This helps ensure
        // that the queue's behavior is deterministic in both individual tests and in a test suite.
        resetSyncBarrierTokens();
    }

    private void removeAllFdRecords() {
        if (mFileDescriptorRecords != null) {
            while (mFileDescriptorRecords.size() > 0) {
                removeOnFileDescriptorEventListener(mFileDescriptorRecords.valueAt(0).mDescriptor);
            }
        }
    }

    private void resetSyncBarrierTokens() {
        mNextBarrierTokenAtomic.set(1);
        mNextBarrierToken = 0;
    }

    void quit(boolean safe) {
        if (!mQuitAllowed) {
            throw new IllegalStateException("Main thread not allowed to quit.");
        }

        /*
         * TODO: FIX - It's possible to get an ordering where the handler's post succeeds (returns
         * true) but does not run. That's because you could read different timestamps as "now" in
         * each one, and the quit observes a sooner timestamp despite losing the first CAS.
         */
        long ts = safe ? SystemClock.uptimeMillis() : 0;
        if (mStack.pushQuitting(ts)) {
            if (incrementMptrRefs()) {
                try {
                    nativeWake(mPtr);
                } finally {
                    decrementMptrRefs();
                }
            }
        }
    }

    // The next barrier token.
    // Barriers are indicated by messages with a null target whose arg1 field carries the token.
    private final AtomicInteger mNextBarrierTokenAtomic = new AtomicInteger(1);

    // Must retain this for compatibility reasons.
    @UnsupportedAppUsage
    private int mNextBarrierToken;

    /**
     * Posts a synchronization barrier to the Looper's message queue.
     *
     * Message processing occurs as usual until the message queue encounters the
     * synchronization barrier that has been posted.  When the barrier is encountered,
     * later synchronous messages in the queue are stalled (prevented from being executed)
     * until the barrier is released by calling {@link #removeSyncBarrier} and specifying
     * the token that identifies the synchronization barrier.
     *
     * This method is used to immediately postpone execution of all subsequently posted
     * synchronous messages until a condition is met that releases the barrier.
     * Asynchronous messages (see {@link Message#isAsynchronous} are exempt from the barrier
     * and continue to be processed as usual.
     *
     * This call must be always matched by a call to {@link #removeSyncBarrier} with
     * the same token to ensure that the message queue resumes normal operation.
     * Otherwise the application will probably hang!
     *
     * @return A token that uniquely identifies the barrier.  This token must be
     * passed to {@link #removeSyncBarrier} to release the barrier.
     *
     * @hide
     */
    @UnsupportedAppUsage
    @TestApi
    public int postSyncBarrier() {
        long when = SystemClock.uptimeMillis();
        final int token = mNextBarrierTokenAtomic.getAndIncrement();

        // b/376573804: apps and tests may expect to be able to use reflection
        // to read this value. Make some effort to support this legacy use case.
        mNextBarrierToken = token + 1;

        final Message msg = Message.obtain();

        msg.markInUse();
        msg.arg1 = token;

        if (!enqueueMessageUnchecked(msg, when)) {
            Log.wtf(TAG, "Unexpected error while adding sync barrier!");
            return -1;
        }

        return token;
    }

    /**
     * Removes a synchronization barrier.
     *
     * @param token The synchronization barrier token that was returned by
     * {@link #postSyncBarrier}.
     *
     * @throws IllegalStateException if the barrier was not found.
     *
     * @hide
     */
    @UnsupportedAppUsage
    @TestApi
    public void removeSyncBarrier(int token) {
        final MatchBarrierToken matchBarrierToken = new MatchBarrierToken(token);

        final boolean removed = mStack.moveMatchingToFreelist(matchBarrierToken, null, -1, null,
                null, 0);
        if (!removed) {
            throw new IllegalStateException("The specified message queue synchronization "
                    + " barrier token has not been posted or has already been removed.");
        }
        maybeDrainFreelist();

        boolean needWake;
        while (true) {
            long waitState = mWaitState;
            long newWaitState;

            if (WaitState.isCounter(waitState)) {
                // Thread is already awake and processing messages
                newWaitState = WaitState.incrementCounter(waitState);
                needWake = false;
            } else if (!WaitState.hasSyncBarrier(waitState)) {
                // Thread is asleep but not waiting on sync barrier
                newWaitState = WaitState.incrementDeadline(waitState);
                needWake = false;
            } else {
                // Thread is asleep, wake up
                newWaitState = WaitState.initCounter();
                needWake = true;
            }
            if (sWaitState.compareAndSet(this, waitState, newWaitState)) {
                break;
            }
        }
        if (needWake) {
            // Wake up next() in case it was sleeping on this barrier.
            concurrentWake();
        }
    }

    /**
     * Get the timestamp of the next executable message in our priority queue.
     * Returns null if there are no messages ready for delivery.
     *
     * Caller must ensure that this doesn't race 'next' from the Looper thread.
     */
    @SuppressLint("VisiblySynchronized") // Legacy MessageQueue synchronizes on this
    Long peekWhenForTest() {
        ActivityThread.throwIfNotInstrumenting();
        Message ret = nextMessage(true, true);
        return ret != null ? ret.when : null;
    }

    /**
     * Return the next executable message in our priority queue.
     * Returns null if there are no messages ready for delivery
     *
     * Caller must ensure that this doesn't race 'next' from the Looper thread.
     */
    @SuppressLint("VisiblySynchronized") // Legacy MessageQueue synchronizes on this
    @Nullable
    Message pollForTest() {
        ActivityThread.throwIfNotInstrumenting();
        return nextMessage(false, true);
    }

    /**
     * @return true if we are blocked on a sync barrier
     *
     * Calls to this method must not be allowed to race with `next`.
     * Specifically, the Looper thread must be paused before calling this method,
     * and may not be resumed until after returning from this method.
     */
    boolean isBlockedOnSyncBarrier() {
        ActivityThread.throwIfNotInstrumenting();
        // Call nextMessage to process any pending barriers
        nextMessage(true, false);
        Message asyncMsg = mStack.peek(true);

        return mSyncBarrier != null &&
                (asyncMsg == null || asyncMsg.when <= mSyncBarrier.when);
    }

    void maybeDrainFreelist() {
        if (Thread.currentThread() == mLooperThread) {
            mStack.drainFreelist();
        }
    }

    boolean hasMessages(Handler h, int what, Object object) {
        if (h == null) {
            return false;
        }
        return mStack.hasMessages(sMatchHandlerWhatAndObject, h, what, object, null, 0);
    }

    boolean hasEqualMessages(Handler h, int what, Object object) {
        if (h == null) {
            return false;
        }
        return mStack.hasMessages(sMatchHandlerWhatAndObjectEquals, h, what, object, null, 0);
    }

    @UnsupportedAppUsage(maxTargetSdk = Build.VERSION_CODES.R, trackingBug = 170729553)
    boolean hasMessages(Handler h, Runnable r, Object object) {
        if (h == null) {
            return false;
        }
        return mStack.hasMessages(sMatchHandlerRunnableAndObject, h, -1, object, r, 0);
    }

    boolean hasMessages(Handler h) {
        if (h == null) {
            return false;
        }
        return mStack.hasMessages(sMatchHandler, h, -1, null, null, 0);
    }

    void removeMessages(Handler h, int what, Object object) {
        if (h == null) {
            return;
        }
        mStack.moveMatchingToFreelist(sMatchHandlerWhatAndObject, h, what, object, null, 0);
        maybeDrainFreelist();
    }

    void removeEqualMessages(Handler h, int what, Object object) {
        if (h == null) {
            return;
        }
        mStack.moveMatchingToFreelist(sMatchHandlerWhatAndObjectEquals, h, what, object, null, 0);
        maybeDrainFreelist();
    }

    void removeMessages(Handler h, Runnable r, Object object) {
        if (h == null || r == null) {
            return;
        }
        mStack.moveMatchingToFreelist(sMatchHandlerRunnableAndObject, h, -1, object, r, 0);
        maybeDrainFreelist();
    }

    void removeEqualMessages(Handler h, Runnable r, Object object) {
        if (h == null || r == null) {
            return;
        }
        mStack.moveMatchingToFreelist(sMatchHandlerRunnableAndObjectEquals, h, -1, object, r, 0);
        maybeDrainFreelist();
    }

    void removeCallbacksAndMessages(Handler h, Object object) {
        if (h == null) {
            return;
        }
        mStack.moveMatchingToFreelist(sMatchHandlerAndObject, h, -1, object, null, 0);
        maybeDrainFreelist();
    }

    void removeCallbacksAndEqualMessages(Handler h, Object object) {
        if (h == null) {
            return;
        }
        mStack.moveMatchingToFreelist(sMatchHandlerAndObjectEquals, h, -1, object, null, 0);
        maybeDrainFreelist();
    }

    private void removeAllMessages() {
        mStack.moveMatchingToFreelist(sMatchAllMessages, null, -1, null, null, 0);
        maybeDrainFreelist();
    }

    private void removeAllFutureMessages(long when) {
        mStack.moveMatchingToFreelist(sMatchAllFutureMessages, null, -1, null, null, when);
        maybeDrainFreelist();
    }

    /**
     * Callback interface for discovering when a thread is going to block
     * waiting for more messages.
     */
    public static interface IdleHandler {
        /**
         * Called when the message queue has run out of messages and will now
         * wait for more.  Return true to keep your idle handler active, false
         * to have it removed.  This may be called if there are still messages
         * pending in the queue, but they are all scheduled to be dispatched
         * after the current time.
         */
        boolean queueIdle();
    }

    /**
     * Add a new {@link IdleHandler} to this message queue.  This may be
     * removed automatically for you by returning false from
     * {@link IdleHandler#queueIdle IdleHandler.queueIdle()} when it is
     * invoked, or explicitly removing it with {@link #removeIdleHandler}.
     *
     * <p>This method is safe to call from any thread.
     *
     * @param handler The IdleHandler to be added.
     */
    public void addIdleHandler(@NonNull IdleHandler handler) {
        if (handler == null) {
            throw new NullPointerException("Can't add a null IdleHandler");
        }
        synchronized (mIdleHandlersLock) {
            mIdleHandlers.add(handler);
        }
    }

    /**
     * Remove an {@link IdleHandler} from the queue that was previously added
     * with {@link #addIdleHandler}.  If the given object is not currently
     * in the idle list, nothing is done.
     *
     * <p>This method is safe to call from any thread.
     *
     * @param handler The IdleHandler to be removed.
     */
    public void removeIdleHandler(@NonNull IdleHandler handler) {
        synchronized (mIdleHandlersLock) {
            mIdleHandlers.remove(handler);
        }
    }

    /**
     * A listener which is invoked when file descriptor related events occur.
     */
    public interface OnFileDescriptorEventListener {
        /**
         * File descriptor event: Indicates that the file descriptor is ready for input
         * operations, such as reading.
         * <p>
         * The listener should read all available data from the file descriptor
         * then return <code>true</code> to keep the listener active or <code>false</code>
         * to remove the listener.
         * </p><p>
         * In the case of a socket, this event may be generated to indicate
         * that there is at least one incoming connection that the listener
         * should accept.
         * </p><p>
         * This event will only be generated if the {@link #EVENT_INPUT} event mask was
         * specified when the listener was added.
         * </p>
         */
        public static final int EVENT_INPUT = 1 << 0;

        /**
         * File descriptor event: Indicates that the file descriptor is ready for output
         * operations, such as writing.
         * <p>
         * The listener should write as much data as it needs.  If it could not
         * write everything at once, then it should return <code>true</code> to
         * keep the listener active.  Otherwise, it should return <code>false</code>
         * to remove the listener then re-register it later when it needs to write
         * something else.
         * </p><p>
         * This event will only be generated if the {@link #EVENT_OUTPUT} event mask was
         * specified when the listener was added.
         * </p>
         */
        public static final int EVENT_OUTPUT = 1 << 1;

        /**
         * File descriptor event: Indicates that the file descriptor encountered a
         * fatal error.
         * <p>
         * File descriptor errors can occur for various reasons.  One common error
         * is when the remote peer of a socket or pipe closes its end of the connection.
         * </p><p>
         * This event may be generated at any time regardless of whether the
         * {@link #EVENT_ERROR} event mask was specified when the listener was added.
         * </p>
         */
        public static final int EVENT_ERROR = 1 << 2;

        /** @hide */
        @Retention(RetentionPolicy.SOURCE)
        @IntDef(flag = true, prefix = { "EVENT_" }, value = {
                EVENT_INPUT,
                EVENT_OUTPUT,
                EVENT_ERROR
        })
        public @interface Events {}

        /**
         * Called when a file descriptor receives events.
         *
         * @param fd The file descriptor.
         * @param events The set of events that occurred: a combination of the
         * {@link #EVENT_INPUT}, {@link #EVENT_OUTPUT}, and {@link #EVENT_ERROR} event masks.
         * @return The new set of events to watch, or 0 to unregister the listener.
         *
         * @see #EVENT_INPUT
         * @see #EVENT_OUTPUT
         * @see #EVENT_ERROR
         */
        @Events int onFileDescriptorEvents(@NonNull FileDescriptor fd, @Events int events);
    }

    static final class FileDescriptorRecord {
        public final FileDescriptor mDescriptor;
        public int mEvents;
        public OnFileDescriptorEventListener mListener;
        public int mSeq;

        public FileDescriptorRecord(FileDescriptor descriptor,
                int events, OnFileDescriptorEventListener listener) {
            mDescriptor = descriptor;
            mEvents = events;
            mListener = listener;
        }
    }

    /**
     * Adds a file descriptor listener to receive notification when file descriptor
     * related events occur.
     * <p>
     * If the file descriptor has already been registered, the specified events
     * and listener will replace any that were previously associated with it.
     * It is not possible to set more than one listener per file descriptor.
     * </p><p>
     * It is important to always unregister the listener when the file descriptor
     * is no longer of use.
     * </p>
     *
     * @param fd The file descriptor for which a listener will be registered.
     * @param events The set of events to receive: a combination of the
     * {@link OnFileDescriptorEventListener#EVENT_INPUT},
     * {@link OnFileDescriptorEventListener#EVENT_OUTPUT}, and
     * {@link OnFileDescriptorEventListener#EVENT_ERROR} event masks.  If the requested
     * set of events is zero, then the listener is unregistered.
     * @param listener The listener to invoke when file descriptor events occur.
     *
     * @see OnFileDescriptorEventListener
     * @see #removeOnFileDescriptorEventListener
     */
    @RavenwoodThrow(blockedBy = android.os.ParcelFileDescriptor.class)
    public void addOnFileDescriptorEventListener(@NonNull FileDescriptor fd,
            @OnFileDescriptorEventListener.Events int events,
            @NonNull OnFileDescriptorEventListener listener) {
        if (fd == null) {
            throw new IllegalArgumentException("fd must not be null");
        }
        if (listener == null) {
            throw new IllegalArgumentException("listener must not be null");
        }

        synchronized (mFileDescriptorRecordsLock) {
            updateOnFileDescriptorEventListenerLocked(fd, events, listener);
        }
    }

    /**
     * Removes a file descriptor listener.
     * <p>
     * This method does nothing if no listener has been registered for the
     * specified file descriptor.
     * </p>
     *
     * @param fd The file descriptor whose listener will be unregistered.
     *
     * @see OnFileDescriptorEventListener
     * @see #addOnFileDescriptorEventListener
     */
    @RavenwoodThrow(blockedBy = android.os.ParcelFileDescriptor.class)
    public void removeOnFileDescriptorEventListener(@NonNull FileDescriptor fd) {
        if (fd == null) {
            throw new IllegalArgumentException("fd must not be null");
        }
        synchronized (mFileDescriptorRecordsLock) {
            updateOnFileDescriptorEventListenerLocked(fd, 0, null);
        }
    }

    private void setFileDescriptorEvents(int fdNum, int events) {
        if (incrementMptrRefs()) {
            try {
                nativeSetFileDescriptorEvents(mPtr, fdNum, events);
            } finally {
                decrementMptrRefs();
            }
        }
    }

    @RavenwoodThrow(blockedBy = android.os.ParcelFileDescriptor.class)
    private void updateOnFileDescriptorEventListenerLocked(FileDescriptor fd, int events,
            OnFileDescriptorEventListener listener) {
        final int fdNum = fd.getInt$();

        int index = -1;
        FileDescriptorRecord record = null;
        if (mFileDescriptorRecords != null) {
            index = mFileDescriptorRecords.indexOfKey(fdNum);
            if (index >= 0) {
                record = mFileDescriptorRecords.valueAt(index);
                if (record != null && record.mEvents == events) {
                    return;
                }
            }
        }

        if (events != 0) {
            events |= OnFileDescriptorEventListener.EVENT_ERROR;
            if (record == null) {
                if (mFileDescriptorRecords == null) {
                    mFileDescriptorRecords = new SparseArray<FileDescriptorRecord>();
                }
                record = new FileDescriptorRecord(fd, events, listener);
                mFileDescriptorRecords.put(fdNum, record);
            } else {
                record.mListener = listener;
                record.mEvents = events;
                record.mSeq += 1;
            }
            setFileDescriptorEvents(fdNum, events);
        } else if (record != null) {
            record.mEvents = 0;
            mFileDescriptorRecords.removeAt(index);

            setFileDescriptorEvents(fdNum, 0);
        }
    }

    // Called from native code.
    @UnsupportedAppUsage(maxTargetSdk = Build.VERSION_CODES.R, trackingBug = 170729553)
    private int dispatchEvents(int fd, int events) {
        // Get the file descriptor record and any state that might change.
        final FileDescriptorRecord record;
        final int oldWatchedEvents;
        final OnFileDescriptorEventListener listener;
        final int seq;
        synchronized (mFileDescriptorRecordsLock) {
            record = mFileDescriptorRecords.get(fd);
            if (record == null) {
                return 0; // spurious, no listener registered
            }

            oldWatchedEvents = record.mEvents;
            events &= oldWatchedEvents; // filter events based on current watched set
            if (events == 0) {
                return oldWatchedEvents; // spurious, watched events changed
            }

            listener = record.mListener;
            seq = record.mSeq;
        }
        // Invoke the listener outside of the lock.
        int newWatchedEvents = listener.onFileDescriptorEvents(
                record.mDescriptor, events);
        if (newWatchedEvents != 0) {
            newWatchedEvents |= OnFileDescriptorEventListener.EVENT_ERROR;
        }

        // Update the file descriptor record if the listener changed the set of
        // events to watch and the listener itself hasn't been updated since.
        if (newWatchedEvents != oldWatchedEvents) {
            synchronized (mFileDescriptorRecordsLock) {
                int index = mFileDescriptorRecords.indexOfKey(fd);
                if (index >= 0 && mFileDescriptorRecords.valueAt(index) == record
                        && record.mSeq == seq) {
                    record.mEvents = newWatchedEvents;
                    if (newWatchedEvents == 0) {
                        mFileDescriptorRecords.removeAt(index);
                    }
                }
            }
        }

        // Return the new set of events to watch for native code to take care of.
        return newWatchedEvents;
    }

    private void decAndTraceMessageCount() {
        mMessageCount.decrementAndGet();
        if (PerfettoTrace.isMQCategoryEnabled()) {
            traceMessageCount();
        }
    }

    @NeverCompile
    void dump(Printer pw, String prefix, Handler h) {
        pw.println(prefix + "(MessageQueue is using DeliQueue implementation)");
        final int n = mStack.dump(pw, prefix, h);
        pw.println(prefix + "(Total messages: " + n + ", polling=" + isPolling()
                + ", quitting=" + mStack.isQuitting() + ")");
    }

    @NeverCompile
    void dumpDebug(ProtoOutputStream proto, long fieldId) {
        final long messageQueueToken = proto.start(fieldId);
        mStack.dumpDebug(proto);
        proto.write(MessageQueueProto.IS_POLLING_LOCKED, isPolling());
        proto.write(MessageQueueProto.IS_QUITTING, mStack.isQuitting());
        proto.end(messageQueueToken);
    }

    private void incAndTraceMessageCount(Message msg, long when) {
        mMessageCount.incrementAndGet();
        if (PerfettoTrace.isMQCategoryEnabled()) {
            msg.sendingThreadName = Thread.currentThread().getName();
            final long eventId = msg.eventId = PerfettoTrace.getFlowId();

            traceMessageCount();
            final long messageDelayMs = Math.max(0L, when - SystemClock.uptimeMillis());
            if (PerfettoTrace.IS_USE_SDK_TRACING_API_V3) {
                com.android.internal.dev.perfetto.sdk.PerfettoTrace.instant(
                                PerfettoTrace.MQ_CATEGORY_V3, "message_queue_send")
                        .setFlow(eventId)
                        .beginProto()
                        .beginNested(2004 /* message_queue */)
                        .addField(2 /* receiving_thread_name */, mThreadName)
                        .addField(3 /* message_code */, msg.what)
                        .addField(4 /* message_delay_ms */, messageDelayMs)
                        .endNested()
                        .endProto()
                        .emit();
            } else {
                PerfettoTrace.instant(PerfettoTrace.MQ_CATEGORY, "message_queue_send")
                        .setFlow(eventId)
                        .beginProto()
                        .beginNested(2004 /* message_queue */)
                        .addField(2 /* receiving_thread_name */, mThreadName)
                        .addField(3 /* message_code */, msg.what)
                        .addField(4 /* message_delay_ms */, messageDelayMs)
                        .endNested()
                        .endProto()
                        .emit();
            }
        }
    }

    private void traceMessageCount() {
        if (PerfettoTrace.IS_USE_SDK_TRACING_API_V3) {
            com.android.internal.dev.perfetto.sdk.PerfettoTrace.counter(
                            PerfettoTrace.MQ_CATEGORY_V3, mMessageCount.get())
                    .usingThreadCounterTrack(mTid, mThreadName)
                    .emit();
        } else {
            PerfettoTrace.counter(PerfettoTrace.MQ_CATEGORY, mMessageCount.get())
                    .usingThreadCounterTrack(mTid, mThreadName)
                    .emit();
        }
    }
}