zircon/system/ulib/fs/watcher.cpp

// Copyright 2017 The Fuchsia Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#include <stdlib.h>
#include <string.h>
#include <sys/stat.h>
#include <unistd.h>

#ifdef __Fuchsia__
#include <zircon/device/vfs.h>
#include <zircon/syscalls.h>
#include <fbl/auto_lock.h>
#endif

#include <fbl/alloc_checker.h>
#include <fs/vfs.h>
#include <fs/vnode.h>
#include <fs/watcher.h>
#include <lib/zx/channel.h>

#include <utility>

namespace fs {

WatcherContainer::WatcherContainer() = default;
WatcherContainer::~WatcherContainer() = default;

WatcherContainer::VnodeWatcher::VnodeWatcher(zx::channel h, uint32_t mask) : h(std::move(h)),
    mask(mask & ~(fuchsia_io_WATCH_MASK_EXISTING | fuchsia_io_WATCH_MASK_IDLE)) {}

WatcherContainer::VnodeWatcher::~VnodeWatcher() {}

// Transmission buffer for sending directory watcher notifications to clients.
// Allows enqueueing multiple messages in a buffer before sending an IPC message
// to a client.
class WatchBuffer {
public:
    DISALLOW_COPY_ASSIGN_AND_MOVE(WatchBuffer);
    WatchBuffer() = default;

    zx_status_t AddMsg(const zx::channel& c, unsigned event, fbl::StringPiece name);
    zx_status_t Send(const zx::channel& c);

private:
    size_t watch_buf_size_ = 0;
    char watch_buf_[fuchsia_io_MAX_BUF]{};
};

zx_status_t WatchBuffer::AddMsg(const zx::channel& c, unsigned event, fbl::StringPiece name) {
    size_t slen = name.length();
    size_t mlen = sizeof(vfs_watch_msg_t) + slen;
    if (mlen + watch_buf_size_ > sizeof(watch_buf_)) {
        // This message won't fit in the watch_buf; transmit first.
        zx_status_t status = Send(c);
        if (status != ZX_OK) {
            return status;
        }
    }
    vfs_watch_msg_t* vmsg = reinterpret_cast<vfs_watch_msg_t*>((uintptr_t)watch_buf_ + watch_buf_size_);
    vmsg->event = static_cast<uint8_t>(event);
    vmsg->len = static_cast<uint8_t>(slen);
    memcpy(vmsg->name, name.data(), slen);
    watch_buf_size_ += mlen;
    return ZX_OK;
}

zx_status_t WatchBuffer::Send(const zx::channel& c) {
    if (watch_buf_size_ > 0) {
        // Only write if we have something to write
        zx_status_t status = c.write(0, watch_buf_, static_cast<uint32_t>(watch_buf_size_),
                                     nullptr, 0);
        watch_buf_size_ = 0;
        if (status != ZX_OK) {
            return status;
        }
    }
    return ZX_OK;
}

zx_status_t WatcherContainer::WatchDir(Vfs* vfs, Vnode* vn, uint32_t mask, uint32_t options,
                                       zx::channel channel) {
    if ((mask & fuchsia_io_WATCH_MASK_ALL) == 0) {
        // No events to watch
        return ZX_ERR_INVALID_ARGS;
    }

    fbl::AllocChecker ac;
    fbl::unique_ptr<VnodeWatcher> watcher(new (&ac) VnodeWatcher(std::move(channel), mask));
    if (!ac.check()) {
        return ZX_ERR_NO_MEMORY;
    }

    if (mask & fuchsia_io_WATCH_MASK_EXISTING) {
        vdircookie_t dircookie;
        memset(&dircookie, 0, sizeof(dircookie));
        char readdir_buf[FDIO_CHUNK_SIZE];
        WatchBuffer wb;
        {
            // Send "fuchsia_io_WATCH_EVENT_EXISTING" for all entries in readdir
            while (true) {
                size_t actual;
                zx_status_t status = vfs->Readdir(vn, &dircookie, readdir_buf,
                                                  sizeof(readdir_buf), &actual);
                if (status != ZX_OK || actual == 0) {
                    break;
                }
                void* ptr = readdir_buf;
                while (actual >= sizeof(vdirent_t)) {
                    auto dirent = reinterpret_cast<vdirent_t*>(ptr);
                    if (dirent->name[0]) {
                        wb.AddMsg(watcher->h, fuchsia_io_WATCH_EVENT_EXISTING,
                                  fbl::StringPiece(dirent->name, dirent->size));
                    }
                    size_t entry_len = dirent->size + sizeof(vdirent_t);
                    ZX_ASSERT(entry_len <= actual); // Prevent underflow
                    actual -= entry_len;
                    ptr = reinterpret_cast<void*>(
                            static_cast<uintptr_t>(entry_len) +
                            reinterpret_cast<uintptr_t>(ptr));
                }
            }
        }

        // Send fuchsia_io_WATCH_EVENT_IDLE to signify that readdir has completed
        if (mask & fuchsia_io_WATCH_MASK_IDLE) {
            wb.AddMsg(watcher->h, fuchsia_io_WATCH_EVENT_IDLE, "");
        }

        wb.Send(watcher->h);
    }

    fbl::AutoLock lock(&lock_);
    watch_list_.push_back(std::move(watcher));
    return ZX_OK;
}

void WatcherContainer::Notify(fbl::StringPiece name, unsigned event) {
    if (name.length() > fuchsia_io_MAX_FILENAME) {
        return;
    }

    fbl::AutoLock lock(&lock_);

    if (watch_list_.is_empty()) {
        return;
    }

    uint8_t msg[sizeof(vfs_watch_msg_t) + name.length()];
    vfs_watch_msg_t* vmsg = reinterpret_cast<vfs_watch_msg_t*>(msg);
    vmsg->event = static_cast<uint8_t>(event);
    vmsg->len = static_cast<uint8_t>(name.length());
    memcpy(vmsg->name, name.data(), name.length());

    for (auto it = watch_list_.begin(); it != watch_list_.end();) {
        if (!(it->mask & (1 << event))) {
            ++it;
            continue;
        }

        zx_status_t status = it->h.write(0, msg, static_cast<uint32_t>(sizeof(msg)), nullptr, 0);
        if (status < 0) {
            // Lazily remove watchers when their handles cannot accept incoming
            // watch messages.
            auto to_remove = it;
            ++it;
            watch_list_.erase(to_remove);
        } else {
            ++it;
        }
    }
}

} // namespace fs