path: root/vendor/github.com/rjeczalik/notify/watcher_inotify.go

// Copyright (c) 2014-2015 The Notify Authors. All rights reserved.
// Use of this source code is governed by the MIT license that can be
// found in the LICENSE file.

// +build linux

package notify

import (
    "bytes"
    "errors"
    "path/filepath"
    "runtime"
    "sync"
    "sync/atomic"
    "syscall"
    "unsafe"
)

// eventBufferSize defines the size of the buffer given to read(2) function. One
// should not depend on this value, since it was arbitrary chosen and may be
// changed in the future.
const eventBufferSize = 64 * (syscall.SizeofInotifyEvent + syscall.PathMax + 1)

// consumersCount defines the number of consumers in producer-consumer based
// implementation. Each consumer is run in a separate goroutine and has read
// access to watched files map.
const consumersCount = 2

const invalidDescriptor = -1

// watched is a pair of file path and inotify mask used as a value in
// watched files map.
type watched struct {
    path string
    mask uint32
}

// inotify implements Watcher interface.
type inotify struct {
    sync.RWMutex                       // protects inotify.m map
    m            map[int32]*watched    // watch descriptor to watched object
    fd           int32                 // inotify file descriptor
    pipefd       []int                 // pipe's read and write descriptors
    epfd         int                   // epoll descriptor
    epes         []syscall.EpollEvent  // epoll events
    buffer       [eventBufferSize]byte // inotify event buffer
    wg           sync.WaitGroup        // wait group used to close main loop
    c            chan<- EventInfo      // event dispatcher channel
}

// NewWatcher creates new non-recursive inotify backed by inotify.
func newWatcher(c chan<- EventInfo) watcher {
    i := &inotify{
        m:      make(map[int32]*watched),
        fd:     invalidDescriptor,
        pipefd: []int{invalidDescriptor, invalidDescriptor},
        epfd:   invalidDescriptor,
        epes:   make([]syscall.EpollEvent, 0),
        c:      c,
    }
    runtime.SetFinalizer(i, func(i *inotify) {
        i.epollclose()
        if i.fd != invalidDescriptor {
            syscall.Close(int(i.fd))
        }
    })
    return i
}

// Watch implements notify.watcher interface.
func (i *inotify) Watch(path string, e Event) error {
    return i.watch(path, e)
}

// Rewatch implements notify.watcher interface.
func (i *inotify) Rewatch(path string, _, newevent Event) error {
    return i.watch(path, newevent)
}

// watch adds a new watcher to the set of watched objects or modifies the existing
// one. If called for the first time, this function initializes inotify filesystem
// monitor and starts producer-consumers goroutines.
func (i *inotify) watch(path string, e Event) (err error) {
    if e&^(All|Event(syscall.IN_ALL_EVENTS)) != 0 {
        return errors.New("notify: unknown event")
    }
    if err = i.lazyinit(); err != nil {
        return
    }
    iwd, err := syscall.InotifyAddWatch(int(i.fd), path, encode(e))
    if err != nil {
        return
    }
    i.RLock()
    wd := i.m[int32(iwd)]
    i.RUnlock()
    if wd == nil {
        i.Lock()
        if i.m[int32(iwd)] == nil {
            i.m[int32(iwd)] = &watched{path: path, mask: uint32(e)}
        }
        i.Unlock()
    } else {
        i.Lock()
        wd.mask = uint32(e)
        i.Unlock()
    }
    return nil
}

// lazyinit sets up all required file descriptors and starts 1+consumersCount
// goroutines. The producer goroutine blocks until file-system notifications
// occur. Then, all events are read from system buffer and sent to consumer
// goroutines which construct valid notify events. This method uses
// Double-Checked Locking optimization.
func (i *inotify) lazyinit() error {
    if atomic.LoadInt32(&i.fd) == invalidDescriptor {
        i.Lock()
        defer i.Unlock()
        if atomic.LoadInt32(&i.fd) == invalidDescriptor {
            fd, err := syscall.InotifyInit()
            if err != nil {
                return err
            }
            i.fd = int32(fd)
            if err = i.epollinit(); err != nil {
                _, _ = i.epollclose(), syscall.Close(int(fd)) // Ignore errors.
                i.fd = invalidDescriptor
                return err
            }
            esch := make(chan []*event)
            go i.loop(esch)
            i.wg.Add(consumersCount)
            for n := 0; n < consumersCount; n++ {
                go i.send(esch)
            }
        }
    }
    return nil
}

// epollinit opens an epoll file descriptor and creates a pipe which will be
// used to wake up the epoll_wait(2) function. Then, file descriptor associated
// with inotify event queue and the read end of the pipe are added to epoll set.
// Note that `fd` member must be set before this function is called.
func (i *inotify) epollinit() (err error) {
    if i.epfd, err = syscall.EpollCreate1(0); err != nil {
        return
    }
    if err = syscall.Pipe(i.pipefd); err != nil {
        return
    }
    i.epes = []syscall.EpollEvent{
        {Events: syscall.EPOLLIN, Fd: i.fd},
        {Events: syscall.EPOLLIN, Fd: int32(i.pipefd[0])},
    }
    if err = syscall.EpollCtl(i.epfd, syscall.EPOLL_CTL_ADD, int(i.fd), &i.epes[0]); err != nil {
        return
    }
    return syscall.EpollCtl(i.epfd, syscall.EPOLL_CTL_ADD, i.pipefd[0], &i.epes[1])
}

// epollclose closes the file descriptor created by the call to epoll_create(2)
// and two file descriptors opened by pipe(2) function.
func (i *inotify) epollclose() (err error) {
    if i.epfd != invalidDescriptor {
        if err = syscall.Close(i.epfd); err == nil {
            i.epfd = invalidDescriptor
        }
    }
    for n, fd := range i.pipefd {
        if fd != invalidDescriptor {
            switch e := syscall.Close(fd); {
            case e != nil && err == nil:
                err = e
            case e == nil:
                i.pipefd[n] = invalidDescriptor
            }
        }
    }
    return
}

// loop blocks until either inotify or pipe file descriptor is ready for I/O.
// All read operations triggered by filesystem notifications are forwarded to
// one of the event's consumers. If pipe fd became ready, loop function closes
// all file descriptors opened by lazyinit method and returns afterwards.
func (i *inotify) loop(esch chan<- []*event) {
    epes := make([]syscall.EpollEvent, 1)
    fd := atomic.LoadInt32(&i.fd)
    for {
        switch _, err := syscall.EpollWait(i.epfd, epes, -1); err {
        case nil:
            switch epes[0].Fd {
            case fd:
                esch <- i.read()
                epes[0].Fd = 0
            case int32(i.pipefd[0]):
                i.Lock()
                defer i.Unlock()
                if err = syscall.Close(int(fd)); err != nil && err != syscall.EINTR {
                    panic("notify: close(2) error " + err.Error())
                }
                atomic.StoreInt32(&i.fd, invalidDescriptor)
                if err = i.epollclose(); err != nil && err != syscall.EINTR {
                    panic("notify: epollclose error " + err.Error())
                }
                close(esch)
                return
            }
        case syscall.EINTR:
            continue
        default: // We should never reach this line.
            panic("notify: epoll_wait(2) error " + err.Error())
        }
    }
}

// read reads events from an inotify file descriptor. It does not handle errors
// returned from read(2) function since they are not critical to watcher logic.
func (i *inotify) read() (es []*event) {
    n, err := syscall.Read(int(i.fd), i.buffer[:])
    if err != nil || n < syscall.SizeofInotifyEvent {
        return
    }
    var sys *syscall.InotifyEvent
    nmin := n - syscall.SizeofInotifyEvent
    for pos, path := 0, ""; pos <= nmin; {
        sys = (*syscall.InotifyEvent)(unsafe.Pointer(&i.buffer[pos]))
        pos += syscall.SizeofInotifyEvent
        if path = ""; sys.Len > 0 {
            endpos := pos + int(sys.Len)
            path = string(bytes.TrimRight(i.buffer[pos:endpos], "\x00"))
            pos = endpos
        }
        es = append(es, &event{
            sys: syscall.InotifyEvent{
                Wd:     sys.Wd,
                Mask:   sys.Mask,
                Cookie: sys.Cookie,
            },
            path: path,
        })
    }
    return
}

// send is a consumer function which sends events to event dispatcher channel.
// It is run in a separate goroutine in order to not block loop method when
// possibly expensive write operations are performed on inotify map.
func (i *inotify) send(esch <-chan []*event) {
    for es := range esch {
        for _, e := range i.transform(es) {
            if e != nil {
                i.c <- e
            }
        }
    }
    i.wg.Done()
}

// transform prepares events read from inotify file descriptor for sending to
// user. It removes invalid events and these which are no longer present in
// inotify map. This method may also split one raw event into two different ones
// when system-dependent result is required.
func (i *inotify) transform(es []*event) []*event {
    var multi []*event
    i.RLock()
    for idx, e := range es {
        if e.sys.Mask&(syscall.IN_IGNORED|syscall.IN_Q_OVERFLOW) != 0 {
            es[idx] = nil
            continue
        }
        wd, ok := i.m[e.sys.Wd]
        if !ok || e.sys.Mask&encode(Event(wd.mask)) == 0 {
            es[idx] = nil
            continue
        }
        if e.path == "" {
            e.path = wd.path
        } else {
            e.path = filepath.Join(wd.path, e.path)
        }
        multi = append(multi, decode(Event(wd.mask), e))
        if e.event == 0 {
            es[idx] = nil
        }
    }
    i.RUnlock()
    es = append(es, multi...)
    return es
}

// encode converts notify system-independent events to valid inotify mask
// which can be passed to inotify_add_watch(2) function.
func encode(e Event) uint32 {
    if e&Create != 0 {
        e = (e ^ Create) | InCreate | InMovedTo
    }
    if e&Remove != 0 {
        e = (e ^ Remove) | InDelete | InDeleteSelf
    }
    if e&Write != 0 {
        e = (e ^ Write) | InModify
    }
    if e&Rename != 0 {
        e = (e ^ Rename) | InMovedFrom | InMoveSelf
    }
    return uint32(e)
}

// decode uses internally stored mask to distinguish whether system-independent
// or system-dependent event is requested. The first one is created by modifying
// `e` argument. decode method sets e.event value to 0 when an event should be
// skipped. System-dependent event is set as the function's return value which
// can be nil when the event should not be passed on.
func decode(mask Event, e *event) (syse *event) {
    if sysmask := uint32(mask) & e.sys.Mask; sysmask != 0 {
        syse = &event{sys: syscall.InotifyEvent{
            Wd:     e.sys.Wd,
            Mask:   e.sys.Mask,
            Cookie: e.sys.Cookie,
        }, event: Event(sysmask), path: e.path}
    }
    imask := encode(mask)
    switch {
    case mask&Create != 0 && imask&uint32(InCreate|InMovedTo)&e.sys.Mask != 0:
        e.event = Create
    case mask&Remove != 0 && imask&uint32(InDelete|InDeleteSelf)&e.sys.Mask != 0:
        e.event = Remove
    case mask&Write != 0 && imask&uint32(InModify)&e.sys.Mask != 0:
        e.event = Write
    case mask&Rename != 0 && imask&uint32(InMovedFrom|InMoveSelf)&e.sys.Mask != 0:
        e.event = Rename
    default:
        e.event = 0
    }
    return
}

// Unwatch implements notify.watcher interface. It looks for watch descriptor
// related to registered path and if found, calls inotify_rm_watch(2) function.
// This method is allowed to return EINVAL error when concurrently requested to
// delete identical path.
func (i *inotify) Unwatch(path string) (err error) {
    iwd := int32(invalidDescriptor)
    i.RLock()
    for iwdkey, wd := range i.m {
        if wd.path == path {
            iwd = iwdkey
            break
        }
    }
    i.RUnlock()
    if iwd == invalidDescriptor {
        return errors.New("notify: path " + path + " is already watched")
    }
    fd := atomic.LoadInt32(&i.fd)
    if _, err = syscall.InotifyRmWatch(int(fd), uint32(iwd)); err != nil {
        return
    }
    i.Lock()
    delete(i.m, iwd)
    i.Unlock()
    return nil
}

// Close implements notify.watcher interface. It removes all existing watch
// descriptors and wakes up producer goroutine by sending data to the write end
// of the pipe. The function waits for a signal from producer which means that
// all operations on current monitoring instance are done.
func (i *inotify) Close() (err error) {
    i.Lock()
    if fd := atomic.LoadInt32(&i.fd); fd == invalidDescriptor {
        i.Unlock()
        return nil
    }
    for iwd := range i.m {
        if _, e := syscall.InotifyRmWatch(int(i.fd), uint32(iwd)); e != nil && err == nil {
            err = e
        }
        delete(i.m, iwd)
    }
    switch _, errwrite := syscall.Write(i.pipefd[1], []byte{0x00}); {
    case errwrite != nil && err == nil:
        err = errwrite
        fallthrough
    case errwrite != nil:
        i.Unlock()
    default:
        i.Unlock()
        i.wg.Wait()
    }
    return
}