all: Add filesystem notification support

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3986

lib/fs/filesystem.go

@@ -7,6 +7,7 @@
 package fs
 
 import (
+	"context"
 	"errors"
 	"io"
 	"os"
@@ -33,7 +34,8 @@ type Filesystem interface {
 	Rename(oldname, newname string) error
 	Stat(name string) (FileInfo, error)
 	SymlinksSupported() bool
-	Walk(root string, walkFn WalkFunc) error
+	Walk(name string, walkFn WalkFunc) error
+	Watch(path string, ignore Matcher, ctx context.Context, ignorePerms bool) (<-chan Event, error)
 	Hide(name string) error
 	Unhide(name string) error
 	Glob(pattern string) ([]string, error)
@@ -82,6 +84,42 @@ type Usage struct {
 	Total int64
 }
 
+type Matcher interface {
+	ShouldIgnore(name string) bool
+}
+
+type MatchResult interface {
+	IsIgnored() bool
+}
+
+type Event struct {
+	Name string
+	Type EventType
+}
+
+type EventType int
+
+const (
+	NonRemove EventType = 1 + iota
+	Remove
+	Mixed // Should probably not be necessary to be used in filesystem interface implementation
+)
+
+func (evType EventType) String() string {
+	switch {
+	case evType == NonRemove:
+		return "non-remove"
+	case evType == Remove:
+		return "remove"
+	case evType == Mixed:
+		return "mixed"
+	default:
+		panic("bug: Unknown event type")
+	}
+}
+
+var ErrWatchNotSupported = errors.New("watching is not supported")
+
 // Equivalents from os package.
 
 const ModePerm = FileMode(os.ModePerm)