/*
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* Copyright (C) 2010 The Android Open Source Project
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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#define LOG_TAG "AsynchronousCloseMonitor"
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#include <log/log.h>
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#include <errno.h>
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#include <signal.h>
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#include <string.h>
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#include <mutex>
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#include "AsynchronousCloseMonitor.h"
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namespace {
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class AsynchronousCloseMonitorImpl {
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public:
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explicit AsynchronousCloseMonitorImpl(int fd);
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~AsynchronousCloseMonitorImpl();
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bool wasSignaled() const;
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static void init();
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static void signalBlockedThreads(int fd);
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private:
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AsynchronousCloseMonitorImpl(const AsynchronousCloseMonitorImpl&) = delete;
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AsynchronousCloseMonitorImpl& operator=(const AsynchronousCloseMonitorImpl&) = delete;
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AsynchronousCloseMonitorImpl* mPrev;
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AsynchronousCloseMonitorImpl* mNext;
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pthread_t mThread;
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int mFd;
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bool mSignaled;
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};
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/**
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* We use an intrusive doubly-linked list to keep track of blocked threads.
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* This gives us O(1) insertion and removal, and means we don't need to do any allocation.
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* (The objects themselves are stack-allocated.)
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* Waking potentially-blocked threads when a file descriptor is closed is O(n) in the total number
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* of blocked threads (not the number of threads actually blocked on the file descriptor in
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* question). For now at least, this seems like a good compromise for Android.
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*/
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static std::mutex blockedThreadListMutex;
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static AsynchronousCloseMonitorImpl* blockedThreadList = NULL;
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/**
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* The specific signal chosen here is arbitrary, but bionic needs to know so that SIGRTMIN
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* starts at a higher value.
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*/
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#if defined(__Fuchsia__)
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static const int BLOCKED_THREAD_SIGNAL = SIGRTMIN + 2;
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#else
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static const int BLOCKED_THREAD_SIGNAL = __SIGRTMIN + 2;
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#endif
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static void blockedThreadSignalHandler(int /*signal*/) {
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// Do nothing. We only sent this signal for its side-effect of interrupting syscalls.
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}
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void AsynchronousCloseMonitorImpl::init() {
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// Ensure that the signal we send interrupts system calls but doesn't kill threads.
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// Using sigaction(2) lets us ensure that the SA_RESTART flag is not set.
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// (The whole reason we're sending this signal is to unblock system calls!)
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struct sigaction sa;
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memset(&sa, 0, sizeof(sa));
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sa.sa_handler = blockedThreadSignalHandler;
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sa.sa_flags = 0;
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int rc = sigaction(BLOCKED_THREAD_SIGNAL, &sa, NULL);
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if (rc == -1) {
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ALOGE("setting blocked thread signal handler failed: %s", strerror(errno));
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}
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}
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void AsynchronousCloseMonitorImpl::signalBlockedThreads(int fd) {
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std::lock_guard<std::mutex> lock(blockedThreadListMutex);
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for (AsynchronousCloseMonitorImpl* it = blockedThreadList; it != NULL; it = it->mNext) {
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if (it->mFd == fd) {
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it->mSignaled = true;
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pthread_kill(it->mThread, BLOCKED_THREAD_SIGNAL);
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// Keep going, because there may be more than one thread...
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}
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}
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}
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bool AsynchronousCloseMonitorImpl::wasSignaled() const {
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return mSignaled;
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}
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AsynchronousCloseMonitorImpl::AsynchronousCloseMonitorImpl(int fd) {
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std::lock_guard<std::mutex> lock(blockedThreadListMutex);
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// Who are we, and what are we waiting for?
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mThread = pthread_self();
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mFd = fd;
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mSignaled = false;
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// Insert ourselves at the head of the intrusive doubly-linked list...
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mPrev = NULL;
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mNext = blockedThreadList;
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if (mNext != NULL) {
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mNext->mPrev = this;
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}
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blockedThreadList = this;
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}
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AsynchronousCloseMonitorImpl::~AsynchronousCloseMonitorImpl() {
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std::lock_guard<std::mutex> lock(blockedThreadListMutex);
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// Unlink ourselves from the intrusive doubly-linked list...
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if (mNext != NULL) {
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mNext->mPrev = mPrev;
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}
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if (mPrev == NULL) {
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blockedThreadList = mNext;
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} else {
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mPrev->mNext = mNext;
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}
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}
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} // namespace
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//
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// C ABI and API boundary
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//
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extern "C" {
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void async_close_monitor_static_init() {
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AsynchronousCloseMonitorImpl::init();
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}
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void async_close_monitor_signal_blocked_threads(int fd) {
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AsynchronousCloseMonitorImpl::signalBlockedThreads(fd);
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}
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void* async_close_monitor_create(int fd) {
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return new AsynchronousCloseMonitorImpl(fd);
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}
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void async_close_monitor_destroy(void* instance) {
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auto monitor = reinterpret_cast<AsynchronousCloseMonitorImpl*>(instance);
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delete monitor;
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}
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int async_close_monitor_was_signalled(const void* instance) {
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auto monitor = reinterpret_cast<const AsynchronousCloseMonitorImpl*>(instance);
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return monitor->wasSignaled() ? 1 : 0;
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}
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}
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