//go:build linux && !appengine
// +build linux,!appengine
// Note: the documentation on the Watcher type and methods is generated from
// mkdoc.zsh
package fsnotify
import (
"errors"
"fmt"
"io"
"os"
"path/filepath"
"strings"
"sync"
"unsafe"
"golang.org/x/sys/unix"
)
// Watcher watches a set of paths, delivering events on a channel.
//
// A watcher should not be copied (e.g. pass it by pointer, rather than by
// value).
//
// # Linux notes
//
// When a file is removed a Remove event won't be emitted until all file
// descriptors are closed, and deletes will always emit a Chmod. For example:
//
// fp := os.Open("file")
// os.Remove("file") // Triggers Chmod
// fp.Close() // Triggers Remove
//
// This is the event that inotify sends, so not much can be changed about this.
//
// The fs.inotify.max_user_watches sysctl variable specifies the upper limit
// for the number of watches per user, and fs.inotify.max_user_instances
// specifies the maximum number of inotify instances per user. Every Watcher you
// create is an "instance", and every path you add is a "watch".
//
// These are also exposed in /proc as /proc/sys/fs/inotify/max_user_watches and
// /proc/sys/fs/inotify/max_user_instances
//
// To increase them you can use sysctl or write the value to the /proc file:
//
// # Default values on Linux 5.18
// sysctl fs.inotify.max_user_watches=124983
// sysctl fs.inotify.max_user_instances=128
//
// To make the changes persist on reboot edit /etc/sysctl.conf or
// /usr/lib/sysctl.d/50-default.conf (details differ per Linux distro; check
// your distro's documentation):
//
// fs.inotify.max_user_watches=124983
// fs.inotify.max_user_instances=128
//
// Reaching the limit will result in a "no space left on device" or "too many open
// files" error.
//
// # kqueue notes (macOS, BSD)
//
// kqueue requires opening a file descriptor for every file that's being watched;
// so if you're watching a directory with five files then that's six file
// descriptors. You will run in to your system's "max open files" limit faster on
// these platforms.
//
// The sysctl variables kern.maxfiles and kern.maxfilesperproc can be used to
// control the maximum number of open files, as well as /etc/login.conf on BSD
// systems.
//
// # Windows notes
//
// Paths can be added as "C:\path\to\dir", but forward slashes
// ("C:/path/to/dir") will also work.
//
// When a watched directory is removed it will always send an event for the
// directory itself, but may not send events for all files in that directory.
// Sometimes it will send events for all times, sometimes it will send no
// events, and often only for some files.
//
// The default ReadDirectoryChangesW() buffer size is 64K, which is the largest
// value that is guaranteed to work with SMB filesystems. If you have many
// events in quick succession this may not be enough, and you will have to use
// [WithBufferSize] to increase the value.
type Watcher struct {
// Events sends the filesystem change events.
//
// fsnotify can send the following events; a "path" here can refer to a
// file, directory, symbolic link, or special file like a FIFO.
//
// fsnotify.Create A new path was created; this may be followed by one
// or more Write events if data also gets written to a
// file.
//
// fsnotify.Remove A path was removed.
//
// fsnotify.Rename A path was renamed. A rename is always sent with the
// old path as Event.Name, and a Create event will be
// sent with the new name. Renames are only sent for
// paths that are currently watched; e.g. moving an
// unmonitored file into a monitored directory will
// show up as just a Create. Similarly, renaming a file
// to outside a monitored directory will show up as
// only a Rename.
//
// fsnotify.Write A file or named pipe was written to. A Truncate will
// also trigger a Write. A single "write action"
// initiated by the user may show up as one or multiple
// writes, depending on when the system syncs things to
// disk. For example when compiling a large Go program
// you may get hundreds of Write events, and you may
// want to wait until you've stopped receiving them
// (see the dedup example in cmd/fsnotify).
//
// Some systems may send Write event for directories
// when the directory content changes.
//
// fsnotify.Chmod Attributes were changed. On Linux this is also sent
// when a file is removed (or more accurately, when a
// link to an inode is removed). On kqueue it's sent
// when a file is truncated. On Windows it's never
// sent.
Events chan Event
// Errors sends any errors.
//
// ErrEventOverflow is used to indicate there are too many events:
//
// - inotify: There are too many queued events (fs.inotify.max_queued_events sysctl)
// - windows: The buffer size is too small; WithBufferSize() can be used to increase it.
// - kqueue, fen: Not used.
Errors chan error
// Store fd here as os.File.Read() will no longer return on close after
// calling Fd(). See: https://github.com/golang/go/issues/26439
fd int
inotifyFile *os.File
watches *watches
done chan struct{} // Channel for sending a "quit message" to the reader goroutine
closeMu sync.Mutex
doneResp chan struct{} // Channel to respond to Close
}
type (
watches struct {
mu sync.RWMutex
wd map[uint32]*watch // wd → watch
path map[string]uint32 // pathname → wd
}
watch struct {
wd uint32 // Watch descriptor (as returned by the inotify_add_watch() syscall)
flags uint32 // inotify flags of this watch (see inotify(7) for the list of valid flags)
path string // Watch path.
}
)
func newWatches() *watches {
return &watches{
wd: make(map[uint32]*watch),
path: make(map[string]uint32),
}
}
func (w *watches) len() int {
w.mu.RLock()
defer w.mu.RUnlock()
return len(w.wd)
}
func (w *watches) add(ww *watch) {
w.mu.Lock()
defer w.mu.Unlock()
w.wd[ww.wd] = ww
w.path[ww.path] = ww.wd
}
func (w *watches) remove(wd uint32) {
w.mu.Lock()
defer w.mu.Unlock()
delete(w.path, w.wd[wd].path)
delete(w.wd, wd)
}
func (w *watches) removePath(path string) (uint32, bool) {
w.mu.Lock()
defer w.mu.Unlock()
wd, ok := w.path[path]
if !ok {
return 0, false
}
delete(w.path, path)
delete(w.wd, wd)
return wd, true
}
func (w *watches) byPath(path string) *watch {
w.mu.RLock()
defer w.mu.RUnlock()
return w.wd[w.path[path]]
}
func (w *watches) byWd(wd uint32) *watch {
w.mu.RLock()
defer w.mu.RUnlock()
return w.wd[wd]
}
func (w *watches) updatePath(path string, f func(*watch) (*watch, error)) error {
w.mu.Lock()
defer w.mu.Unlock()
var existing *watch
wd, ok := w.path[path]
if ok {
existing = w.wd[wd]
}
upd, err := f(existing)
if err != nil {
return err
}
if upd != nil {
w.wd[upd.wd] = upd
w.path[upd.path] = upd.wd
if upd.wd != wd {
delete(w.wd, wd)
}
}
return nil
}
// NewWatcher creates a new Watcher.
func NewWatcher() (*Watcher, error) {
return NewBufferedWatcher(0)
}
// NewBufferedWatcher creates a new Watcher with a buffered Watcher.Events
// channel.
//
// The main use case for this is situations with a very large number of events
// where the kernel buffer size can't be increased (e.g. due to lack of
// permissions). An unbuffered Watcher will perform better for almost all use
// cases, and whenever possible you will be better off increasing the kernel
// buffers instead of adding a large userspace buffer.
func NewBufferedWatcher(sz uint) (*Watcher, error) {
// Need to set nonblocking mode for SetDeadline to work, otherwise blocking
// I/O operations won't terminate on close.
fd, errno := unix.InotifyInit1(unix.IN_CLOEXEC | unix.IN_NONBLOCK)
if fd == -1 {
return nil, errno
}
w := &Watcher{
fd: fd,
inotifyFile: os.NewFile(uintptr(fd), ""),
watches: newWatches(),
Events: make(chan Event, sz),
Errors: make(chan error),
done: make(chan struct{}),
doneResp: make(chan struct{}),
}
go w.readEvents()
return w, nil
}
// Returns true if the event was sent, or false if watcher is closed.
func (w *Watcher) sendEvent(e Event) bool {
select {
case w.Events <- e:
return true
case <-w.done:
return false
}
}
// Returns true if the error was sent, or false if watcher is closed.
func (w *Watcher) sendError(err error) bool {
select {
case w.Errors <- err:
return true
case <-w.done:
return false
}
}
func (w *Watcher) isClosed() bool {
select {
case <-w.done:
return true
default:
return false
}
}
// Close removes all watches and closes the Events channel.
func (w *Watcher) Close() error {
w.closeMu.Lock()
if w.isClosed() {
w.closeMu.Unlock()
return nil
}
close(w.done)
w.closeMu.Unlock()
// Causes any blocking reads to return with an error, provided the file
// still supports deadline operations.
err := w.inotifyFile.Close()
if err != nil {
return err
}
// Wait for goroutine to close
<-w.doneResp
return nil
}
// Add starts monitoring the path for changes.
//
// A path can only be watched once; watching it more than once is a no-op and will
// not return an error. Paths that do not yet exist on the filesystem cannot be
// watched.
//
// A watch will be automatically removed if the watched path is deleted or
// renamed. The exception is the Windows backend, which doesn't remove the
// watcher on renames.
//
// Notifications on network filesystems (NFS, SMB, FUSE, etc.) or special
// filesystems (/proc, /sys, etc.) generally don't work.
//
// Returns [ErrClosed] if [Watcher.Close] was called.
//
// See [Watcher.AddWith] for a version that allows adding options.
//
// # Watching directories
//
// All files in a directory are monitored, including new files that are created
// after the watcher is started. Subdirectories are not watched (i.e. it's
// non-recursive).
//
// # Watching files
//
// Watching individual files (rather than directories) is generally not
// recommended as many programs (especially editors) update files atomically: it
// will write to a temporary file which is then moved to to destination,
// overwriting the original (or some variant thereof). The watcher on the
// original file is now lost, as that no longer exists.
//
// The upshot of this is that a power failure or crash won't leave a
// half-written file.
//
// Watch the parent directory and use Event.Name to filter out files you're not
// interested in. There is an example of this in cmd/fsnotify/file.go.
func (w *Watcher) Add(name string) error { return w.AddWith(name) }
// AddWith is like [Watcher.Add], but allows adding options. When using Add()
// the defaults described below are used.
//
// Possible options are:
//
// - [WithBufferSize] sets the buffer size for the Windows backend; no-op on
// other platforms. The default is 64K (65536 bytes).
func (w *Watcher) AddWith(name string, opts ...addOpt) error {
if w.isClosed() {
return ErrClosed
}
name = filepath.Clean(name)
_ = getOptions(opts...)
var flags uint32 = unix.IN_MOVED_TO | unix.IN_MOVED_FROM |
unix.IN_CREATE | unix.IN_ATTRIB | unix.IN_MODIFY |
unix.IN_MOVE_SELF | unix.IN_DELETE | unix.IN_DELETE_SELF
return w.watches.updatePath(name, func(existing *watch) (*watch, error) {
if existing != nil {
flags |= existing.flags | unix.IN_MASK_ADD
}
wd, err := unix.InotifyAddWatch(w.fd, name, flags)
if wd == -1 {
return nil, err
}
if existing == nil {
return &watch{
wd: uint32(wd),
path: name,
flags: flags,
}, nil
}
existing.wd = uint32(wd)
existing.flags = flags
return existing, nil
})
}
// Remove stops monitoring the path for changes.
//
// Directories are always removed non-recursively. For example, if you added
// /tmp/dir and /tmp/dir/subdir then you will need to remove both.
//
// Removing a path that has not yet been added returns [ErrNonExistentWatch].
//
// Returns nil if [Watcher.Close] was called.
func (w *Watcher) Remove(name string) error {
if w.isClosed() {
return nil
}
return w.remove(filepath.Clean(name))
}
func (w *Watcher) remove(name string) error {
wd, ok := w.watches.removePath(name)
if !ok {
return fmt.Errorf("%w: %s", ErrNonExistentWatch, name)
}
success, errno := unix.InotifyRmWatch(w.fd, wd)
if success == -1 {
// TODO: Perhaps it's not helpful to return an error here in every case;
// The only two possible errors are:
//
// - EBADF, which happens when w.fd is not a valid file descriptor
// of any kind.
// - EINVAL, which is when fd is not an inotify descriptor or wd
// is not a valid watch descriptor. Watch descriptors are
// invalidated when they are removed explicitly or implicitly;
// explicitly by inotify_rm_watch, implicitly when the file they
// are watching is deleted.
return errno
}
return nil
}
// WatchList returns all paths explicitly added with [Watcher.Add] (and are not
// yet removed).
//
// Returns nil if [Watcher.Close] was called.
func (w *Watcher) WatchList() []string {
if w.isClosed() {
return nil
}
entries := make([]string, 0, w.watches.len())
w.watches.mu.RLock()
for pathname := range w.watches.path {
entries = append(entries, pathname)
}
w.watches.mu.RUnlock()
return entries
}
// readEvents reads from the inotify file descriptor, converts the
// received events into Event objects and sends them via the Events channel
func (w *Watcher) readEvents() {
defer func() {
close(w.doneResp)
close(w.Errors)
close(w.Events)
}()
var (
buf [unix.SizeofInotifyEvent * 4096]byte // Buffer for a maximum of 4096 raw events
errno error // Syscall errno
)
for {
// See if we have been closed.
if w.isClosed() {
return
}
n, err := w.inotifyFile.Read(buf[:])
switch {
case errors.Unwrap(err) == os.ErrClosed:
return
case err != nil:
if !w.sendError(err) {
return
}
continue
}
if n < unix.SizeofInotifyEvent {
var err error
if n == 0 {
err = io.EOF // If EOF is received. This should really never happen.
} else if n < 0 {
err = errno // If an error occurred while reading.
} else {
err = errors.New("notify: short read in readEvents()") // Read was too short.
}
if !w.sendError(err) {
return
}
continue
}
var offset uint32
// We don't know how many events we just read into the buffer
// While the offset points to at least one whole event...
for offset <= uint32(n-unix.SizeofInotifyEvent) {
var (
// Point "raw" to the event in the buffer
raw = (*unix.InotifyEvent)(unsafe.Pointer(&buf[offset]))
mask = uint32(raw.Mask)
nameLen = uint32(raw.Len)
)
if mask&unix.IN_Q_OVERFLOW != 0 {
if !w.sendError(ErrEventOverflow) {
return
}
}
// If the event happened to the watched directory or the watched file, the kernel
// doesn't append the filename to the event, but we would like to always fill the
// the "Name" field with a valid filename. We retrieve the path of the watch from
// the "paths" map.
watch := w.watches.byWd(uint32(raw.Wd))
// inotify will automatically remove the watch on deletes; just need
// to clean our state here.
if watch != nil && mask&unix.IN_DELETE_SELF == unix.IN_DELETE_SELF {
w.watches.remove(watch.wd)
}
// We can't really update the state when a watched path is moved;
// only IN_MOVE_SELF is sent and not IN_MOVED_{FROM,TO}. So remove
// the watch.
if watch != nil && mask&unix.IN_MOVE_SELF == unix.IN_MOVE_SELF {
err := w.remove(watch.path)
if err != nil && !errors.Is(err, ErrNonExistentWatch) {
if !w.sendError(err) {
return
}
}
}
var name string
if watch != nil {
name = watch.path
}
if nameLen > 0 {
// Point "bytes" at the first byte of the filename
bytes := (*[unix.PathMax]byte)(unsafe.Pointer(&buf[offset+unix.SizeofInotifyEvent]))[:nameLen:nameLen]
// The filename is padded with NULL bytes. TrimRight() gets rid of those.
name += "/" + strings.TrimRight(string(bytes[0:nameLen]), "\000")
}
event := w.newEvent(name, mask)
// Send the events that are not ignored on the events channel
if mask&unix.IN_IGNORED == 0 {
if !w.sendEvent(event) {
return
}
}
// Move to the next event in the buffer
offset += unix.SizeofInotifyEvent + nameLen
}
}
}
// newEvent returns an platform-independent Event based on an inotify mask.
func (w *Watcher) newEvent(name string, mask uint32) Event {
e := Event{Name: name}
if mask&unix.IN_CREATE == unix.IN_CREATE || mask&unix.IN_MOVED_TO == unix.IN_MOVED_TO {
e.Op |= Create
}
if mask&unix.IN_DELETE_SELF == unix.IN_DELETE_SELF || mask&unix.IN_DELETE == unix.IN_DELETE {
e.Op |= Remove
}
if mask&unix.IN_MODIFY == unix.IN_MODIFY {
e.Op |= Write
}
if mask&unix.IN_MOVE_SELF == unix.IN_MOVE_SELF || mask&unix.IN_MOVED_FROM == unix.IN_MOVED_FROM {
e.Op |= Rename
}
if mask&unix.IN_ATTRIB == unix.IN_ATTRIB {
e.Op |= Chmod
}
return e
}
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