upload android base code part6

android/libnativehelper/AsynchronousCloseMonitor.cpp

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+/*
+ * Copyright (C) 2010 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.
+ */
+
+#define LOG_TAG "AsynchronousCloseMonitor"
+
+#include "AsynchronousCloseMonitor.h"
+#include "cutils/log.h"
+
+#include <errno.h>
+#include <signal.h>
+#include <string.h>
+
+#include <mutex>
+
+/**
+ * We use an intrusive doubly-linked list to keep track of blocked threads.
+ * This gives us O(1) insertion and removal, and means we don't need to do any allocation.
+ * (The objects themselves are stack-allocated.)
+ * Waking potentially-blocked threads when a file descriptor is closed is O(n) in the total number
+ * of blocked threads (not the number of threads actually blocked on the file descriptor in
+ * question). For now at least, this seems like a good compromise for Android.
+ */
+static std::mutex blockedThreadListMutex;
+static AsynchronousCloseMonitor* blockedThreadList = NULL;
+
+/**
+ * The specific signal chosen here is arbitrary, but bionic needs to know so that SIGRTMIN
+ * starts at a higher value.
+ */
+static const int BLOCKED_THREAD_SIGNAL = __SIGRTMIN + 2;
+
+static void blockedThreadSignalHandler(int /*signal*/) {
+    // Do nothing. We only sent this signal for its side-effect of interrupting syscalls.
+}
+
+void AsynchronousCloseMonitor::init() {
+    // Ensure that the signal we send interrupts system calls but doesn't kill threads.
+    // Using sigaction(2) lets us ensure that the SA_RESTART flag is not set.
+    // (The whole reason we're sending this signal is to unblock system calls!)
+    struct sigaction sa;
+    memset(&sa, 0, sizeof(sa));
+    sa.sa_handler = blockedThreadSignalHandler;
+    sa.sa_flags = 0;
+    int rc = sigaction(BLOCKED_THREAD_SIGNAL, &sa, NULL);
+    if (rc == -1) {
+        ALOGE("setting blocked thread signal handler failed: %s", strerror(errno));
+    }
+}
+
+void AsynchronousCloseMonitor::signalBlockedThreads(int fd) {
+    std::lock_guard<std::mutex> lock(blockedThreadListMutex);
+    for (AsynchronousCloseMonitor* it = blockedThreadList; it != NULL; it = it->mNext) {
+        if (it->mFd == fd) {
+            it->mSignaled = true;
+            pthread_kill(it->mThread, BLOCKED_THREAD_SIGNAL);
+            // Keep going, because there may be more than one thread...
+        }
+    }
+}
+
+bool AsynchronousCloseMonitor::wasSignaled() const {
+    return mSignaled;
+}
+
+AsynchronousCloseMonitor::AsynchronousCloseMonitor(int fd) {
+    std::lock_guard<std::mutex> lock(blockedThreadListMutex);
+    // Who are we, and what are we waiting for?
+    mThread = pthread_self();
+    mFd = fd;
+    mSignaled = false;
+    // Insert ourselves at the head of the intrusive doubly-linked list...
+    mPrev = NULL;
+    mNext = blockedThreadList;
+    if (mNext != NULL) {
+        mNext->mPrev = this;
+    }
+    blockedThreadList = this;
+}
+
+AsynchronousCloseMonitor::~AsynchronousCloseMonitor() {
+    std::lock_guard<std::mutex> lock(blockedThreadListMutex);
+    // Unlink ourselves from the intrusive doubly-linked list...
+    if (mNext != NULL) {
+        mNext->mPrev = mPrev;
+    }
+    if (mPrev == NULL) {
+        blockedThreadList = mNext;
+    } else {
+        mPrev->mNext = mNext;
+    }
+}