realm-js/apple/external_commit_helper.cpp

////////////////////////////////////////////////////////////////////////////
//
// Copyright 2015 Realm Inc.
//
// 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.
//
////////////////////////////////////////////////////////////////////////////

#include "external_commit_helper.hpp"

#include "shared_realm.hpp"

#include <assert.h>
#include <sys/event.h>
#include <sys/stat.h>
#include <sys/time.h>
#include <system_error>
#include <fcntl.h>
#include <unistd.h>
#include <sstream>

using namespace realm;

namespace {
// Write a byte to a pipe to notify anyone waiting for data on the pipe
void notify_fd(int fd)
{
    while (true) {
        char c = 0;
        ssize_t ret = write(fd, &c, 1);
        if (ret == 1) {
            break;
        }

        // If the pipe's buffer is full, we need to read some of the old data in
        // it to make space. We don't just read in the code waiting for
        // notifications so that we can notify multiple waiters with a single
        // write.
        assert(ret == -1 && errno == EAGAIN);
        char buff[1024];
        read(fd, buff, sizeof buff);
    }
}
} // anonymous namespace

void ExternalCommitHelper::FdHolder::close()
{
        if (m_fd != -1) {
            ::close(m_fd);
        }
        m_fd = -1;

}

// Inter-thread and inter-process notifications of changes are done using a
// named pipe in the filesystem next to the Realm file. Everyone who wants to be
// notified of commits waits for data to become available on the pipe, and anyone
// who commits a write transaction writes data to the pipe after releasing the
// write lock. Note that no one ever actually *reads* from the pipe: the data
// actually written is meaningless, and trying to read from a pipe from multiple
// processes at once is fraught with race conditions.

// When a RLMRealm instance is created, we add a CFRunLoopSource to the current
// thread's runloop. On each cycle of the run loop, the run loop checks each of
// its sources for work to do, which in the case of CFRunLoopSource is just
// checking if CFRunLoopSourceSignal has been called since the last time it ran,
// and if so invokes the function pointer supplied when the source is created,
// which in our case just invokes `[realm handleExternalChange]`.

// Listening for external changes is done using kqueue() on a background thread.
// kqueue() lets us efficiently wait until the amount of data which can be read
// from one or more file descriptors has changed, and tells us which of the file
// descriptors it was that changed. We use this to wait on both the shared named
// pipe, and a local anonymous pipe. When data is written to the named pipe, we
// signal the runloop source and wake up the target runloop, and when data is
// written to the anonymous pipe the background thread removes the runloop
// source from the runloop and and shuts down.
ExternalCommitHelper::ExternalCommitHelper(Realm* realm)
: m_realm(realm)
, m_run_loop(CFRunLoopGetCurrent())
{
    CFRetain(m_run_loop);

    m_kq = kqueue();
    if (m_kq == -1) {
        throw std::system_error(errno, std::system_category());
    }

    auto path = realm->config().path + ".note";

    // Create and open the named pipe
    int ret = mkfifo(path.c_str(), 0600);
    if (ret == -1) {
        int err = errno;
        if (err == ENOTSUP) {
            // Filesystem doesn't support named pipes, so try putting it in tmp instead
            // Hash collisions are okay here because they just result in doing
            // extra work, as opposed to correctness problems
            std::ostringstream ss;
            ss << getenv("TMPDIR");
            ss << "realm_" << std::hash<std::string>()(path) << ".note";
            path = ss.str();
            ret = mkfifo(path.c_str(), 0600);
            err = errno;
        }
        // the fifo already existing isn't an error
        if (ret == -1 && err != EEXIST) {
            throw std::system_error(err, std::system_category());
        }
    }

    m_notify_fd = open(path.c_str(), O_RDWR);
    if (m_notify_fd == -1) {
        throw std::system_error(errno, std::system_category());
    }

    // Make writing to the pipe return -1 when the pipe's buffer is full
    // rather than blocking until there's space available
    ret = fcntl(m_notify_fd, F_SETFL, O_NONBLOCK);
    if (ret == -1) {
        throw std::system_error(errno, std::system_category());
    }

    // Create the anonymous pipe
    int pipeFd[2];
    ret = pipe(pipeFd);
    if (ret == -1) {
        throw std::system_error(errno, std::system_category());
    }

    m_shutdown_read_fd = pipeFd[0];
    m_shutdown_write_fd = pipeFd[1];

    // Use the minimum allowed stack size, as we need very little in our listener
    // https://developer.apple.com/library/ios/documentation/Cocoa/Conceptual/Multithreading/CreatingThreads/CreatingThreads.html#//apple_ref/doc/uid/10000057i-CH15-SW7
    pthread_attr_t attr;
    pthread_attr_init(&attr);
    pthread_attr_setstacksize(&attr, 16 * 1024);

    auto fn = [](void *self) -> void * {
        static_cast<ExternalCommitHelper *>(self)->listen();
        return nullptr;
    };
    ret = pthread_create(&m_thread, &attr, fn, this);
    pthread_attr_destroy(&attr);
    if (ret != 0) {
        throw std::system_error(errno, std::system_category());
    }
}

ExternalCommitHelper::~ExternalCommitHelper()
{
    notify_fd(m_shutdown_write_fd);
    pthread_join(m_thread, nullptr); // Wait for the thread to exit
}

void ExternalCommitHelper::listen()
{
    pthread_setname_np("RLMRealm notification listener");

    // Create the runloop source
    CFRunLoopSourceContext ctx{};
    ctx.info = this;
    ctx.perform = [](void *info) {
        static_cast<ExternalCommitHelper *>(info)->m_realm->notify();
    };

    CFRunLoopSourceRef signal = CFRunLoopSourceCreate(kCFAllocatorDefault, 0, &ctx);
    CFRunLoopAddSource(m_run_loop, signal, kCFRunLoopDefaultMode);

    // Set up the kqueue
    // EVFILT_READ indicates that we care about data being available to read
    // on the given file descriptor.
    // EV_CLEAR makes it wait for the amount of data available to be read to
    // change rather than just returning when there is any data to read.
    struct kevent ke[2];
    EV_SET(&ke[0], m_notify_fd, EVFILT_READ, EV_ADD | EV_CLEAR, 0, 0, 0);
    EV_SET(&ke[1], m_shutdown_read_fd, EVFILT_READ, EV_ADD | EV_CLEAR, 0, 0, 0);
    int ret = kevent(m_kq, ke, 2, nullptr, 0, nullptr);
    assert(ret == 0);

    while (true) {
        struct kevent event;
        // Wait for data to become on either fd
        // Return code is number of bytes available or -1 on error
        ret = kevent(m_kq, nullptr, 0, &event, 1, nullptr);
        assert(ret >= 0);
        if (ret == 0) {
            // Spurious wakeup; just wait again
            continue;
        }

        // Check which file descriptor had activity: if it's the shutdown
        // pipe, then someone called -stop; otherwise it's the named pipe
        // and someone committed a write transaction
        if (event.ident == (uint32_t)m_shutdown_read_fd) {
            CFRunLoopSourceInvalidate(signal);
            CFRelease(signal);
            CFRelease(m_run_loop);
            return;
        }
        assert(event.ident == (uint32_t)m_notify_fd);

        CFRunLoopSourceSignal(signal);
        // Signalling the source makes it run the next time the runloop gets
        // to it, but doesn't make the runloop start if it's currently idle
        // waiting for events
        CFRunLoopWakeUp(m_run_loop);
    }
}

void ExternalCommitHelper::notify_others()
{
    notify_fd(m_notify_fd);
}