Add AddWith() to pass options, allow controlling Windows buffer size

This is similar to Add(), except that you can pass options. Ideally this
should just be Add(), but that's not a compatible change, so we're stuck
with this until we do a v2.

There are quite a few enhancements that depend on *some* way to pass
options; as an example I added WithBufferSize() to control the buffer
size for (see #72) for the Windows backend, because that one is fairly
trivial to implement:

	w, err := fsnotify.NewWatcher()
	err = w.AddWith("/path", fsnotify.WithBufferSize(65536*4))

Some other options we might want to add:

	err = w.AddWith("/path",
		fsnotify.WithEvents(fsnotify.Open | fsnotify.Close),  // Filter events
		fsnotify.WithPoll(time.Second),                       // Use poll watcher
		fsnotify.WithFanotify(),                              // Prefer fanotify on Linux
		fsnotify.WithFollowLinks(true),                       // Control symlink follow behaviour
		fsnotify.WithDebounce(100*time.Milliseconds),         // Automatically debounce duplicate events
		fsnotify.WithRetry(1*time.Second, 1*time.Minute),     // Retry every second if the path disappears for a minute
	)

These are just some ideas, nothing fixed here yet. Some of these options
are likely to change once I get around to actually working on it.

This uses "functional options" so we can add more later. Options are
passed to Add() rather than the Watcher itself, so the behaviour can be
modified for every watch, rather than being global. This way you can do
things like watch /nfs-drive with a poll backend, and use the regular OS
backend for ~/dir, without having to create two watchers.

This upgrades fairly nicely to v2 where we rename AddWith() to Add():

	err = w.Add("/path",
		fsnotify.WithBufferSize(65536*4),
		fsnotify.WithEvents(fsnotify.Open | fsnotify.Close))

Folks will just have to s/fsnotify.AddWith/fsnotify.Add/, without having
to change all the option names. Plus having a consistent prefix
autocompletes nicely in editors.

Fixes #72

backend_fen.go

@@ -105,7 +105,7 @@ type Watcher struct {
 	// [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.
+	//  - windows: The buffer size is too small; [WithBufferSize] can be used to increase it.
 	//  - kqueue, fen: not used.
 	Errors chan error
 
@@ -195,6 +195,8 @@ func (w *Watcher) Close() error {
 //
 // Returns [ErrClosed] if [Watcher.Close] was called.
 //
+// See [AddWith] for a version that allows adding options.
+//
 // # Watching directories
 //
 // All files in a directory are monitored, including new files that are created
@@ -212,14 +214,25 @@ func (w *Watcher) Close() error {
 //
 // Instead, 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 {
+func (w *Watcher) Add(name string) error { return w.AddWith(name) }
+
+// AddWith is like [Add], but has the possibility to add 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
 	}
 	if w.port.PathIsWatched(name) {
 		return nil
 	}
 
+	_ = getOptions(opts...)
+
 	// Currently we resolve symlinks that were explicitly requested to be
 	// watched. Otherwise we would use LStat here.
 	stat, err := os.Stat(name)

backend_inotify.go

@@ -108,7 +108,7 @@ type Watcher struct {
 	// [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.
+	//  - windows: The buffer size is too small; [WithBufferSize] can be used to increase it.
 	//  - kqueue, fen: not used.
 	Errors chan error
 
@@ -217,6 +217,8 @@ func (w *Watcher) Close() error {
 //
 // Returns [ErrClosed] if [Watcher.Close] was called.
 //
+// See [AddWith] for a version that allows adding options.
+//
 // # Watching directories
 //
 // All files in a directory are monitored, including new files that are created
@@ -234,12 +236,23 @@ func (w *Watcher) Close() error {
 //
 // Instead, 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 {
-	name = filepath.Clean(name)
+func (w *Watcher) Add(name string) error { return w.AddWith(name) }
+
+// AddWith is like [Add], but has the possibility to add 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

backend_kqueue.go

@@ -106,7 +106,7 @@ type Watcher struct {
 	// [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.
+	//  - windows: The buffer size is too small; [WithBufferSize] can be used to increase it.
 	//  - kqueue, fen: not used.
 	Errors chan error
 
@@ -249,6 +249,8 @@ func (w *Watcher) Close() error {
 //
 // Returns [ErrClosed] if [Watcher.Close] was called.
 //
+// See [AddWith] for a version that allows adding options.
+//
 // # Watching directories
 //
 // All files in a directory are monitored, including new files that are created
@@ -266,7 +268,18 @@ func (w *Watcher) Close() error {
 //
 // Instead, 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 {
+func (w *Watcher) Add(name string) error { return w.AddWith(name) }
+
+// AddWith is like [Add], but has the possibility to add 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 {
+	_ = getOptions(opts...)
+
 	w.mu.Lock()
 	w.userWatches[name] = struct{}{}
 	w.mu.Unlock()

backend_other.go

@@ -100,7 +100,7 @@ type Watcher struct {
 	// [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.
+	//  - windows: The buffer size is too small; [WithBufferSize] can be used to increase it.
 	//  - kqueue, fen: not used.
 	Errors chan error
 }
@@ -133,6 +133,8 @@ func (w *Watcher) WatchList() []string { return nil }
 //
 // Returns [ErrClosed] if [Watcher.Close] was called.
 //
+// See [AddWith] for a version that allows adding options.
+//
 // # Watching directories
 //
 // All files in a directory are monitored, including new files that are created
@@ -152,6 +154,17 @@ func (w *Watcher) WatchList() []string { return nil }
 // you're not interested in. There is an example of this in [cmd/fsnotify/file.go].
 func (w *Watcher) Add(name string) error { return nil }
 
+// AddWith is like [Add], but has the possibility to add 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 {
+	return nil
+}
+
 // Remove stops monitoring the path for changes.
 //
 // Directories are always removed non-recursively. For example, if you added

backend_windows.go

@@ -109,7 +109,7 @@ type Watcher struct {
 	// [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.
+	//  - windows: The buffer size is too small; [WithBufferSize] can be used to increase it.
 	//  - kqueue, fen: not used.
 	Errors chan error
 
@@ -204,6 +204,8 @@ func (w *Watcher) Close() error {
 //
 // Returns [ErrClosed] if [Watcher.Close] was called.
 //
+// See [AddWith] for a version that allows adding options.
+//
 // # Watching directories
 //
 // All files in a directory are monitored, including new files that are created
@@ -221,16 +223,31 @@ func (w *Watcher) Close() error {
 //
 // Instead, 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 {
+func (w *Watcher) Add(name string) error { return w.AddWith(name) }
+
+// AddWith is like [Add], but has the possibility to add 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
 	}
 
+	with := getOptions(opts...)
+	if with.bufsize < 4096 {
+		return fmt.Errorf("fsnotify.WithBufferSize: buffer size cannot be smaller than 4096 bytes")
+	}
+
 	in := &input{
-		op:    opAddWatch,
-		path:  filepath.Clean(name),
-		flags: sysFSALLEVENTS,
-		reply: make(chan error),
+		op:      opAddWatch,
+		path:    filepath.Clean(name),
+		flags:   sysFSALLEVENTS,
+		reply:   make(chan error),
+		bufsize: with.bufsize,
 	}
 	w.input <- in
 	if err := w.wakeupReader(); err != nil {
@@ -329,10 +346,11 @@ const (
 )
 
 type input struct {
-	op    int
-	path  string
-	flags uint32
-	reply chan error
+	op      int
+	path    string
+	flags   uint32
+	bufsize int
+	reply   chan error
 }
 
 type inode struct {
@@ -348,7 +366,7 @@ type watch struct {
 	mask   uint64            // Directory itself is being watched with these notify flags
 	names  map[string]uint64 // Map of names being watched and their notify flags
 	rename string            // Remembers the old name while renaming a file
-	buf    [65536]byte       // 64K buffer
+	buf    []byte            // buffer, allocated later
 }
 
 type (
@@ -421,7 +439,7 @@ func (m watchMap) set(ino *inode, watch *watch) {
 }
 
 // Must run within the I/O thread.
-func (w *Watcher) addWatch(pathname string, flags uint64) error {
+func (w *Watcher) addWatch(pathname string, flags uint64, bufsize int) error {
 	dir, err := w.getDir(pathname)
 	if err != nil {
 		return err
@@ -444,6 +462,7 @@ func (w *Watcher) addWatch(pathname string, flags uint64) error {
 			ino:   ino,
 			path:  dir,
 			names: make(map[string]uint64),
+			buf:   make([]byte, bufsize),
 		}
 		w.mu.Lock()
 		w.watches.set(ino, watchEntry)
@@ -543,8 +562,11 @@ func (w *Watcher) startRead(watch *watch) error {
 		return nil
 	}
 
-	rdErr := windows.ReadDirectoryChanges(watch.ino.handle, &watch.buf[0],
-		uint32(unsafe.Sizeof(watch.buf)), false, mask, nil, &watch.ov, 0)
+	// We need to pass the array, rather than the slice.
+	hdr := (*reflect.SliceHeader)(unsafe.Pointer(&watch.buf))
+	rdErr := windows.ReadDirectoryChanges(watch.ino.handle,
+		(*byte)(unsafe.Pointer(hdr.Data)), uint32(hdr.Len),
+		false, mask, nil, &watch.ov, 0)
 	if rdErr != nil {
 		err := os.NewSyscallError("ReadDirectoryChanges", rdErr)
 		if rdErr == windows.ERROR_ACCESS_DENIED && watch.mask&provisional == 0 {
@@ -603,7 +625,7 @@ func (w *Watcher) readEvents() {
 			case in := <-w.input:
 				switch in.op {
 				case opAddWatch:
-					in.reply <- w.addWatch(in.path, uint64(in.flags))
+					in.reply <- w.addWatch(in.path, uint64(in.flags), in.bufsize)
 				case opRemoveWatch:
 					in.reply <- w.remWatch(in.path)
 				}

fsnotify.go

@@ -101,3 +101,32 @@ func (e Event) Has(op Op) bool { return e.Op.Has(op) }
 func (e Event) String() string {
 	return fmt.Sprintf("%-13s %q", e.Op.String(), e.Name)
 }
+
+type (
+	addOpt   func(opt *withOpts)
+	withOpts struct {
+		bufsize int
+	}
+)
+
+var defaultOpts = withOpts{
+	bufsize: 65536, // 64K
+}
+
+func getOptions(opts ...addOpt) withOpts {
+	with := defaultOpts
+	for _, o := range opts {
+		o(&with)
+	}
+	return with
+}
+
+// WithBufferSize sets the buffer size for the Windows backend. This is a no-op
+// for other backends.
+//
+// The default value is 64K (65536 bytes) which should be enough for most
+// applications, but you can increase it if you're hitting "queue or buffer
+// overflow" errors ([ErrEventOverflow]).
+func WithBufferSize(bytes int) addOpt {
+	return func(opt *withOpts) { opt.bufsize = bytes }
+}

mkdoc.zsh

@@ -80,6 +80,8 @@ add=$(<<EOF
 //
 // Returns [ErrClosed] if [Watcher.Close] was called.
 //
+// See [AddWith] for a version that allows adding options.
+//
 // # Watching directories
 //
 // All files in a directory are monitored, including new files that are created
@@ -100,6 +102,17 @@ add=$(<<EOF
 EOF
 )
 
+addwith=$(<<EOF
+// AddWith is like [Add], but has the possibility to add options. When using
+// Add() the defaults described below are used.
+//
+// Possible options are:
+//
+//   - [WithBufferSize] sets the buffer size for the Windows backend; no-op on
+//     other platforms. The default is 64K (65536 bytes).
+EOF
+)
+
 remove=$(<<EOF
 // Remove stops monitoring the path for changes.
 //
@@ -166,7 +179,7 @@ errors=$(<<EOF
 	// [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.
+	//  - windows: The buffer size is too small; [WithBufferSize] can be used to increase it.
 	//  - kqueue, fen: not used.
 EOF
 )
@@ -202,6 +215,7 @@ set-cmt() {
 set-cmt '^type Watcher struct '             $watcher
 set-cmt '^func NewWatcher('                 $new
 set-cmt '^func (w \*Watcher) Add('          $add
+set-cmt '^func (w \*Watcher) AddWith('      $addwith
 set-cmt '^func (w \*Watcher) Remove('       $remove
 set-cmt '^func (w \*Watcher) Close('        $close
 set-cmt '^func (w \*Watcher) WatchList('    $watchlist