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moby--moby/pkg/filenotify/poller.go

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package filenotify
import (
"errors"
"fmt"
"os"
"sync"
"time"
"github.com/Sirupsen/logrus"
"github.com/fsnotify/fsnotify"
)
var (
// errPollerClosed is returned when the poller is closed
errPollerClosed = errors.New("poller is closed")
// errNoSuchWatch is returned when trying to remove a watch that doesn't exist
errNoSuchWatch = errors.New("watch does not exist")
)
// watchWaitTime is the time to wait between file poll loops
const watchWaitTime = 200 * time.Millisecond
// filePoller is used to poll files for changes, especially in cases where fsnotify
// can't be run (e.g. when inotify handles are exhausted)
// filePoller satisfies the FileWatcher interface
type filePoller struct {
// watches is the list of files currently being polled, close the associated channel to stop the watch
watches map[string]chan struct{}
// events is the channel to listen to for watch events
events chan fsnotify.Event
// errors is the channel to listen to for watch errors
errors chan error
// mu locks the poller for modification
mu sync.Mutex
// closed is used to specify when the poller has already closed
closed bool
}
// Add adds a filename to the list of watches
// once added the file is polled for changes in a separate goroutine
func (w *filePoller) Add(name string) error {
w.mu.Lock()
defer w.mu.Unlock()
if w.closed == true {
return errPollerClosed
}
f, err := os.Open(name)
if err != nil {
return err
}
fi, err := os.Stat(name)
if err != nil {
return err
}
if w.watches == nil {
w.watches = make(map[string]chan struct{})
}
if _, exists := w.watches[name]; exists {
return fmt.Errorf("watch exists")
}
chClose := make(chan struct{})
w.watches[name] = chClose
go w.watch(f, fi, chClose)
return nil
}
// Remove stops and removes watch with the specified name
func (w *filePoller) Remove(name string) error {
w.mu.Lock()
defer w.mu.Unlock()
return w.remove(name)
}
func (w *filePoller) remove(name string) error {
if w.closed == true {
return errPollerClosed
}
chClose, exists := w.watches[name]
if !exists {
return errNoSuchWatch
}
close(chClose)
delete(w.watches, name)
return nil
}
// Events returns the event channel
// This is used for notifications on events about watched files
func (w *filePoller) Events() <-chan fsnotify.Event {
return w.events
}
// Errors returns the errors channel
// This is used for notifications about errors on watched files
func (w *filePoller) Errors() <-chan error {
return w.errors
}
// Close closes the poller
// All watches are stopped, removed, and the poller cannot be added to
func (w *filePoller) Close() error {
w.mu.Lock()
defer w.mu.Unlock()
if w.closed {
return nil
}
w.closed = true
for name := range w.watches {
w.remove(name)
delete(w.watches, name)
}
return nil
}
// sendEvent publishes the specified event to the events channel
func (w *filePoller) sendEvent(e fsnotify.Event, chClose <-chan struct{}) error {
select {
case w.events <- e:
case <-chClose:
return fmt.Errorf("closed")
}
return nil
}
// sendErr publishes the specified error to the errors channel
func (w *filePoller) sendErr(e error, chClose <-chan struct{}) error {
select {
case w.errors <- e:
case <-chClose:
return fmt.Errorf("closed")
}
return nil
}
// watch is responsible for polling the specified file for changes
// upon finding changes to a file or errors, sendEvent/sendErr is called
func (w *filePoller) watch(f *os.File, lastFi os.FileInfo, chClose chan struct{}) {
defer f.Close()
for {
time.Sleep(watchWaitTime)
select {
case <-chClose:
logrus.Debugf("watch for %s closed", f.Name())
return
default:
}
fi, err := os.Stat(f.Name())
if err != nil {
// if we got an error here and lastFi is not set, we can presume that nothing has changed
// This should be safe since before `watch()` is called, a stat is performed, there is any error `watch` is not called
if lastFi == nil {
continue
}
// If it doesn't exist at this point, it must have been removed
// no need to send the error here since this is a valid operation
if os.IsNotExist(err) {
if err := w.sendEvent(fsnotify.Event{Op: fsnotify.Remove, Name: f.Name()}, chClose); err != nil {
return
}
lastFi = nil
continue
}
// at this point, send the error
if err := w.sendErr(err, chClose); err != nil {
return
}
continue
}
if lastFi == nil {
if err := w.sendEvent(fsnotify.Event{Op: fsnotify.Create, Name: fi.Name()}, chClose); err != nil {
return
}
lastFi = fi
continue
}
if fi.Mode() != lastFi.Mode() {
if err := w.sendEvent(fsnotify.Event{Op: fsnotify.Chmod, Name: fi.Name()}, chClose); err != nil {
return
}
lastFi = fi
continue
}
if fi.ModTime() != lastFi.ModTime() || fi.Size() != lastFi.Size() {
if err := w.sendEvent(fsnotify.Event{Op: fsnotify.Write, Name: fi.Name()}, chClose); err != nil {
return
}
lastFi = fi
continue
}
}
}