lib/fs, lib/api, lib/model: Expose mtime remappings as part of /db/file (#7624)

* lib/fs, lib/api, lib/model: Expose mtime remappings as part of /db/file

* Fix wrong error returned by CLI

* Gofmt

* Better names

* Review comments

* Review comments

cmd/syncthing/cli/client.go

@@ -135,11 +135,11 @@ func (c *apiClient) Post(url, body string) (*http.Response, error) {
 }
 
 func checkResponse(response *http.Response) error {
-	if response.StatusCode == 404 {
+	if response.StatusCode == http.StatusNotFound {
 		return errors.New("invalid endpoint or API call")
-	} else if response.StatusCode == 403 {
+	} else if response.StatusCode == http.StatusUnauthorized {
 		return errors.New("invalid API key")
-	} else if response.StatusCode != 200 {
+	} else if response.StatusCode != http.StatusOK {
 		data, err := responseToBArray(response)
 		if err != nil {
 			return err

lib/api/api.go

@@ -32,7 +32,7 @@ import (
 	"unicode"
 
 	"github.com/julienschmidt/httprouter"
-	metrics "github.com/rcrowley/go-metrics"
+	"github.com/rcrowley/go-metrics"
 	"github.com/thejerf/suture/v4"
 	"github.com/vitrun/qart/qr"
 	"golang.org/x/text/runes"
@@ -915,11 +915,16 @@ func (s *service) getDBFile(w http.ResponseWriter, r *http.Request) {
 	if err != nil {
 		http.Error(w, err.Error(), http.StatusInternalServerError)
 	}
+	mtimeMapping, mtimeErr := s.model.GetMtimeMapping(folder, file)
 
 	sendJSON(w, map[string]interface{}{
 		"global":       jsonFileInfo(gf),
 		"local":        jsonFileInfo(lf),
 		"availability": av,
+		"mtime": map[string]interface{}{
+			"err":   mtimeErr,
+			"value": mtimeMapping,
+		},
 	})
 }
 
@@ -934,6 +939,8 @@ func (s *service) getDebugFile(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	mtimeMapping, mtimeErr := s.model.GetMtimeMapping(folder, file)
+
 	lf, _ := snap.Get(protocol.LocalDeviceID, file)
 	gf, _ := snap.GetGlobal(file)
 	av := snap.Availability(file)
@@ -944,6 +951,10 @@ func (s *service) getDebugFile(w http.ResponseWriter, r *http.Request) {
 		"local":          jsonFileInfo(lf),
 		"availability":   av,
 		"globalVersions": vl.String(),
+		"mtime": map[string]interface{}{
+			"err":   mtimeErr,
+			"value": mtimeMapping,
+		},
 	})
 }
 

lib/db/keyer.go

@@ -36,7 +36,7 @@ const (
 	// KeyTypeFolderStatistic <folder ID as string> <some string> = some value
 	KeyTypeFolderStatistic byte = 4
 
-	// KeyTypeVirtualMtime <int32 folder ID> <file name> = dbMtime
+	// KeyTypeVirtualMtime <int32 folder ID> <file name> = mtimeMapping
 	KeyTypeVirtualMtime byte = 5
 
 	// KeyTypeFolderIdx <int32 id> = string value

lib/fs/basicfs.go

@@ -330,6 +330,14 @@ func (f *BasicFilesystem) SameFile(fi1, fi2 FileInfo) bool {
 	return os.SameFile(f1.osFileInfo(), f2.osFileInfo())
 }
 
+func (f *BasicFilesystem) underlying() (Filesystem, bool) {
+	return nil, false
+}
+
+func (f *BasicFilesystem) wrapperType() filesystemWrapperType {
+	return filesystemWrapperTypeNone
+}
+
 // basicFile implements the fs.File interface on top of an os.File
 type basicFile struct {
 	*os.File

lib/fs/casefs.go

@@ -339,6 +339,14 @@ func (f *caseFilesystem) Unhide(name string) error {
 	return f.Filesystem.Unhide(name)
 }
 
+func (f *caseFilesystem) underlying() (Filesystem, bool) {
+	return f.Filesystem, true
+}
+
+func (f *caseFilesystem) wrapperType() filesystemWrapperType {
+	return filesystemWrapperTypeCase
+}
+
 func (f *caseFilesystem) checkCase(name string) error {
 	var err error
 	if name, err = Canonicalize(name); err != nil {

lib/fs/casefs_test.go

@@ -161,7 +161,10 @@ func BenchmarkWalkCaseFakeFS100k(b *testing.B) {
 		b.Fatal(err)
 	}
 	b.Run("rawfs", func(b *testing.B) {
-		fakefs := unwrapFilesystem(fsys).(*fakefs)
+		var fakefs *fakeFS
+		if ffs, ok := unwrapFilesystem(fsys, filesystemWrapperTypeNone); ok {
+			fakefs = ffs.(*fakeFS)
+		}
 		fakefs.resetCounters()
 		benchmarkWalkFakeFS(b, fsys, paths, 0, "")
 		fakefs.reportMetricsPerOp(b)
@@ -174,7 +177,10 @@ func BenchmarkWalkCaseFakeFS100k(b *testing.B) {
 			Filesystem: fsys,
 			realCaser:  newDefaultRealCaser(fsys),
 		}
-		fakefs := unwrapFilesystem(fsys).(*fakefs)
+		var fakefs *fakeFS
+		if ffs, ok := unwrapFilesystem(fsys, filesystemWrapperTypeNone); ok {
+			fakefs = ffs.(*fakeFS)
+		}
 		fakefs.resetCounters()
 		benchmarkWalkFakeFS(b, casefs, paths, 0, "")
 		fakefs.reportMetricsPerOp(b)
@@ -197,7 +203,10 @@ func BenchmarkWalkCaseFakeFS100k(b *testing.B) {
 			Filesystem: fsys,
 			realCaser:  newDefaultRealCaser(fsys),
 		}
-		fakefs := unwrapFilesystem(fsys).(*fakefs)
+		var fakefs *fakeFS
+		if ffs, ok := unwrapFilesystem(fsys, filesystemWrapperTypeNone); ok {
+			fakefs = ffs.(*fakeFS)
+		}
 		fakefs.resetCounters()
 		benchmarkWalkFakeFS(b, casefs, paths, otherOpEvery, otherOpPath)
 		fakefs.reportMetricsPerOp(b)

lib/fs/debug_symlink_unix.go

@@ -18,7 +18,8 @@ import (
 // reason for existence is the Windows version, which allows creating
 // symlinks when non-elevated.
 func DebugSymlinkForTestsOnly(oldFs, newFs Filesystem, oldname, newname string) error {
-	if caseFs, ok := unwrapFilesystem(newFs).(*caseFilesystem); ok {
+	if fs, ok := unwrapFilesystem(newFs, filesystemWrapperTypeCase); ok {
+		caseFs := fs.(*caseFilesystem)
 		if err := caseFs.checkCase(newname); err != nil {
 			return err
 		}

lib/fs/errorfs.go

@@ -52,3 +52,11 @@ func (fs *errorFilesystem) SameFile(fi1, fi2 FileInfo) bool { return false }
 func (fs *errorFilesystem) Watch(path string, ignore Matcher, ctx context.Context, ignorePerms bool) (<-chan Event, <-chan error, error) {
 	return nil, nil, fs.err
 }
+
+func (fs *errorFilesystem) underlying() (Filesystem, bool) {
+	return nil, false
+}
+
+func (fs *errorFilesystem) wrapperType() filesystemWrapperType {
+	return filesystemWrapperTypeError
+}

lib/fs/fakefs.go

@@ -27,7 +27,7 @@ import (
 // see readShortAt()
 const randomBlockShift = 14 // 128k
 
-// fakefs is a fake filesystem for testing and benchmarking. It has the
+// fakeFS is a fake filesystem for testing and benchmarking. It has the
 // following properties:
 //
 // - File metadata is kept in RAM. Specifically, we remember which files and
@@ -37,7 +37,7 @@ const randomBlockShift = 14 // 128k
 // - File contents are generated pseudorandomly with just the file name as
 //   seed. Writes are discarded, other than having the effect of increasing
 //   the file size. If you only write data that you've read from a file with
-//   the same name on a different fakefs, you'll never know the difference...
+//   the same name on a different fakeFS, you'll never know the difference...
 //
 // - We totally ignore permissions - pretend you are root.
 //
@@ -51,10 +51,10 @@ const randomBlockShift = 14 // 128k
 //     insens=b   "true" makes filesystem case-insensitive Windows- or OSX-style (default false)
 //     latency=d  to set the amount of time each "disk" operation takes, where d is time.ParseDuration format
 //
-// - Two fakefs:s pointing at the same root path see the same files.
+// - Two fakeFS:s pointing at the same root path see the same files.
 //
-type fakefs struct {
-	counters    fakefsCounters
+type fakeFS struct {
+	counters    fakeFSCounters
 	uri         string
 	mut         sync.Mutex
 	root        *fakeEntry
@@ -63,7 +63,7 @@ type fakefs struct {
 	latency     time.Duration
 }
 
-type fakefsCounters struct {
+type fakeFSCounters struct {
 	Chmod       int64
 	Lchown      int64
 	Chtimes     int64
@@ -81,13 +81,13 @@ type fakefsCounters struct {
 }
 
 var (
-	fakefsMut sync.Mutex
-	fakefsFs  = make(map[string]*fakefs)
+	fakeFSMut   sync.Mutex
+	fakeFSCache = make(map[string]*fakeFS)
 )
 
-func newFakeFilesystem(rootURI string, _ ...Option) *fakefs {
-	fakefsMut.Lock()
-	defer fakefsMut.Unlock()
+func newFakeFilesystem(rootURI string, _ ...Option) *fakeFS {
+	fakeFSMut.Lock()
+	defer fakeFSMut.Unlock()
 
 	root := rootURI
 	var params url.Values
@@ -97,12 +97,12 @@ func newFakeFilesystem(rootURI string, _ ...Option) *fakefs {
 		params = uri.Query()
 	}
 
-	if fs, ok := fakefsFs[rootURI]; ok {
+	if fs, ok := fakeFSCache[rootURI]; ok {
 		// Already have an fs at this path
 		return fs
 	}
 
-	fs := &fakefs{
+	fs := &fakeFS{
 		uri: "fake://" + rootURI,
 		root: &fakeEntry{
 			name:      "/",
@@ -157,7 +157,7 @@ func newFakeFilesystem(rootURI string, _ ...Option) *fakefs {
 	// the filesystem initially.
 	fs.latency, _ = time.ParseDuration(params.Get("latency"))
 
-	fakefsFs[root] = fs
+	fakeFSCache[root] = fs
 	return fs
 }
 
@@ -183,7 +183,7 @@ type fakeEntry struct {
 	content   []byte
 }
 
-func (fs *fakefs) entryForName(name string) *fakeEntry {
+func (fs *fakeFS) entryForName(name string) *fakeEntry {
 	// bug: lookup doesn't work through symlinks.
 	if fs.insens {
 		name = UnicodeLowercase(name)
@@ -210,7 +210,7 @@ func (fs *fakefs) entryForName(name string) *fakeEntry {
 	return entry
 }
 
-func (fs *fakefs) Chmod(name string, mode FileMode) error {
+func (fs *fakeFS) Chmod(name string, mode FileMode) error {
 	fs.mut.Lock()
 	defer fs.mut.Unlock()
 	fs.counters.Chmod++
@@ -223,7 +223,7 @@ func (fs *fakefs) Chmod(name string, mode FileMode) error {
 	return nil
 }
 
-func (fs *fakefs) Lchown(name string, uid, gid int) error {
+func (fs *fakeFS) Lchown(name string, uid, gid int) error {
 	fs.mut.Lock()
 	defer fs.mut.Unlock()
 	fs.counters.Lchown++
@@ -237,7 +237,7 @@ func (fs *fakefs) Lchown(name string, uid, gid int) error {
 	return nil
 }
 
-func (fs *fakefs) Chtimes(name string, atime time.Time, mtime time.Time) error {
+func (fs *fakeFS) Chtimes(name string, atime time.Time, mtime time.Time) error {
 	fs.mut.Lock()
 	defer fs.mut.Unlock()
 	fs.counters.Chtimes++
@@ -250,7 +250,7 @@ func (fs *fakefs) Chtimes(name string, atime time.Time, mtime time.Time) error {
 	return nil
 }
 
-func (fs *fakefs) create(name string) (*fakeEntry, error) {
+func (fs *fakeFS) create(name string) (*fakeEntry, error) {
 	fs.mut.Lock()
 	defer fs.mut.Unlock()
 	fs.counters.Create++
@@ -296,7 +296,7 @@ func (fs *fakefs) create(name string) (*fakeEntry, error) {
 	return new, nil
 }
 
-func (fs *fakefs) Create(name string) (File, error) {
+func (fs *fakeFS) Create(name string) (File, error) {
 	entry, err := fs.create(name)
 	if err != nil {
 		return nil, err
@@ -307,7 +307,7 @@ func (fs *fakefs) Create(name string) (File, error) {
 	return &fakeFile{fakeEntry: entry}, nil
 }
 
-func (fs *fakefs) CreateSymlink(target, name string) error {
+func (fs *fakeFS) CreateSymlink(target, name string) error {
 	entry, err := fs.create(name)
 	if err != nil {
 		return err
@@ -317,7 +317,7 @@ func (fs *fakefs) CreateSymlink(target, name string) error {
 	return nil
 }
 
-func (fs *fakefs) DirNames(name string) ([]string, error) {
+func (fs *fakeFS) DirNames(name string) ([]string, error) {
 	fs.mut.Lock()
 	defer fs.mut.Unlock()
 	fs.counters.DirNames++
@@ -336,7 +336,7 @@ func (fs *fakefs) DirNames(name string) ([]string, error) {
 	return names, nil
 }
 
-func (fs *fakefs) Lstat(name string) (FileInfo, error) {
+func (fs *fakeFS) Lstat(name string) (FileInfo, error) {
 	fs.mut.Lock()
 	defer fs.mut.Unlock()
 	fs.counters.Lstat++
@@ -355,7 +355,7 @@ func (fs *fakefs) Lstat(name string) (FileInfo, error) {
 	return info, nil
 }
 
-func (fs *fakefs) Mkdir(name string, perm FileMode) error {
+func (fs *fakeFS) Mkdir(name string, perm FileMode) error {
 	fs.mut.Lock()
 	defer fs.mut.Unlock()
 	fs.counters.Mkdir++
@@ -389,7 +389,7 @@ func (fs *fakefs) Mkdir(name string, perm FileMode) error {
 	return nil
 }
 
-func (fs *fakefs) MkdirAll(name string, perm FileMode) error {
+func (fs *fakeFS) MkdirAll(name string, perm FileMode) error {
 	fs.mut.Lock()
 	defer fs.mut.Unlock()
 	fs.counters.MkdirAll++
@@ -426,7 +426,7 @@ func (fs *fakefs) MkdirAll(name string, perm FileMode) error {
 	return nil
 }
 
-func (fs *fakefs) Open(name string) (File, error) {
+func (fs *fakeFS) Open(name string) (File, error) {
 	fs.mut.Lock()
 	defer fs.mut.Unlock()
 	fs.counters.Open++
@@ -443,7 +443,7 @@ func (fs *fakefs) Open(name string) (File, error) {
 	return &fakeFile{fakeEntry: entry}, nil
 }
 
-func (fs *fakefs) OpenFile(name string, flags int, mode FileMode) (File, error) {
+func (fs *fakeFS) OpenFile(name string, flags int, mode FileMode) (File, error) {
 	if flags&os.O_CREATE == 0 {
 		return fs.Open(name)
 	}
@@ -486,7 +486,7 @@ func (fs *fakefs) OpenFile(name string, flags int, mode FileMode) (File, error)
 	return &fakeFile{fakeEntry: newEntry}, nil
 }
 
-func (fs *fakefs) ReadSymlink(name string) (string, error) {
+func (fs *fakeFS) ReadSymlink(name string) (string, error) {
 	fs.mut.Lock()
 	defer fs.mut.Unlock()
 	fs.counters.ReadSymlink++
@@ -501,7 +501,7 @@ func (fs *fakefs) ReadSymlink(name string) (string, error) {
 	return entry.dest, nil
 }
 
-func (fs *fakefs) Remove(name string) error {
+func (fs *fakeFS) Remove(name string) error {
 	fs.mut.Lock()
 	defer fs.mut.Unlock()
 	fs.counters.Remove++
@@ -524,7 +524,7 @@ func (fs *fakefs) Remove(name string) error {
 	return nil
 }
 
-func (fs *fakefs) RemoveAll(name string) error {
+func (fs *fakeFS) RemoveAll(name string) error {
 	fs.mut.Lock()
 	defer fs.mut.Unlock()
 	fs.counters.RemoveAll++
@@ -536,7 +536,7 @@ func (fs *fakefs) RemoveAll(name string) error {
 
 	entry := fs.entryForName(filepath.Dir(name))
 	if entry == nil {
-		return nil // all tested real systems exibit this behaviour
+		return nil // all tested real systems exhibit this behaviour
 	}
 
 	// RemoveAll is easy when the file system uses garbage collection under
@@ -545,7 +545,7 @@ func (fs *fakefs) RemoveAll(name string) error {
 	return nil
 }
 
-func (fs *fakefs) Rename(oldname, newname string) error {
+func (fs *fakeFS) Rename(oldname, newname string) error {
 	fs.mut.Lock()
 	defer fs.mut.Unlock()
 	fs.counters.Rename++
@@ -595,56 +595,56 @@ func (fs *fakefs) Rename(oldname, newname string) error {
 	return nil
 }
 
-func (fs *fakefs) Stat(name string) (FileInfo, error) {
+func (fs *fakeFS) Stat(name string) (FileInfo, error) {
 	return fs.Lstat(name)
 }
 
-func (fs *fakefs) SymlinksSupported() bool {
+func (fs *fakeFS) SymlinksSupported() bool {
 	return false
 }
 
-func (fs *fakefs) Walk(name string, walkFn WalkFunc) error {
+func (fs *fakeFS) Walk(name string, walkFn WalkFunc) error {
 	return errors.New("not implemented")
 }
 
-func (fs *fakefs) Watch(path string, ignore Matcher, ctx context.Context, ignorePerms bool) (<-chan Event, <-chan error, error) {
+func (fs *fakeFS) Watch(path string, ignore Matcher, ctx context.Context, ignorePerms bool) (<-chan Event, <-chan error, error) {
 	return nil, nil, ErrWatchNotSupported
 }
 
-func (fs *fakefs) Hide(name string) error {
+func (fs *fakeFS) Hide(name string) error {
 	return nil
 }
 
-func (fs *fakefs) Unhide(name string) error {
+func (fs *fakeFS) Unhide(name string) error {
 	return nil
 }
 
-func (fs *fakefs) Glob(pattern string) ([]string, error) {
+func (fs *fakeFS) Glob(pattern string) ([]string, error) {
 	// gnnh we don't seem to actually require this in practice
 	return nil, errors.New("not implemented")
 }
 
-func (fs *fakefs) Roots() ([]string, error) {
+func (fs *fakeFS) Roots() ([]string, error) {
 	return []string{"/"}, nil
 }
 
-func (fs *fakefs) Usage(name string) (Usage, error) {
+func (fs *fakeFS) Usage(name string) (Usage, error) {
 	return Usage{}, errors.New("not implemented")
 }
 
-func (fs *fakefs) Type() FilesystemType {
+func (fs *fakeFS) Type() FilesystemType {
 	return FilesystemTypeFake
 }
 
-func (fs *fakefs) URI() string {
+func (fs *fakeFS) URI() string {
 	return fs.uri
 }
 
-func (fs *fakefs) Options() []Option {
+func (fs *fakeFS) Options() []Option {
 	return nil
 }
 
-func (fs *fakefs) SameFile(fi1, fi2 FileInfo) bool {
+func (fs *fakeFS) SameFile(fi1, fi2 FileInfo) bool {
 	// BUG: real systems base file sameness on path, inodes, etc
 	// we try our best, but FileInfo just doesn't have enough data
 	// so there be false positives, especially on Windows
@@ -659,17 +659,25 @@ func (fs *fakefs) SameFile(fi1, fi2 FileInfo) bool {
 	return ok && fi1.ModTime().Equal(fi2.ModTime()) && fi1.Mode() == fi2.Mode() && fi1.IsDir() == fi2.IsDir() && fi1.IsRegular() == fi2.IsRegular() && fi1.IsSymlink() == fi2.IsSymlink() && fi1.Owner() == fi2.Owner() && fi1.Group() == fi2.Group()
 }
 
-func (fs *fakefs) resetCounters() {
+func (fs *fakeFS) underlying() (Filesystem, bool) {
+	return nil, false
+}
+
+func (fs *fakeFS) wrapperType() filesystemWrapperType {
+	return filesystemWrapperTypeNone
+}
+
+func (fs *fakeFS) resetCounters() {
 	fs.mut.Lock()
-	fs.counters = fakefsCounters{}
+	fs.counters = fakeFSCounters{}
 	fs.mut.Unlock()
 }
 
-func (fs *fakefs) reportMetricsPerOp(b *testing.B) {
+func (fs *fakeFS) reportMetricsPerOp(b *testing.B) {
 	fs.reportMetricsPer(b, 1, "op")
 }
 
-func (fs *fakefs) reportMetricsPer(b *testing.B, divisor float64, unit string) {
+func (fs *fakeFS) reportMetricsPer(b *testing.B, divisor float64, unit string) {
 	fs.mut.Lock()
 	defer fs.mut.Unlock()
 	b.ReportMetric(float64(fs.counters.Lstat)/divisor/float64(b.N), "Lstat/"+unit)

lib/fs/fakefs_test.go

@@ -21,7 +21,7 @@ import (
 )
 
 func TestFakeFS(t *testing.T) {
-	// Test some basic aspects of the fakefs
+	// Test some basic aspects of the fakeFS
 
 	fs := newFakeFilesystem("/foo/bar/baz")
 
@@ -131,7 +131,7 @@ func TestFakeFS(t *testing.T) {
 }
 
 func testFakeFSRead(t *testing.T, fs Filesystem) {
-	// Test some basic aspects of the fakefs
+	// Test some basic aspects of the fakeFS
 	// Create
 	fd, _ := fs.Create("test")
 	defer fd.Close()
@@ -201,7 +201,7 @@ func TestFakeFSCaseSensitive(t *testing.T) {
 		{"FileName", testFakeFSFileName},
 	}
 	var filesystems = []testFS{
-		{"fakefs", newFakeFilesystem("/foo")},
+		{"fakeFS", newFakeFilesystem("/foo")},
 	}
 
 	testDir, sensitive := createTestDir(t)
@@ -237,7 +237,7 @@ func TestFakeFSCaseInsensitive(t *testing.T) {
 	}
 
 	var filesystems = []testFS{
-		{"fakefs", newFakeFilesystem("/foobar?insens=true")},
+		{"fakeFS", newFakeFilesystem("/foobar?insens=true")},
 	}
 
 	testDir, sensitive := createTestDir(t)
@@ -891,7 +891,7 @@ func testFakeFSCreateInsens(t *testing.T, fs Filesystem) {
 		t.Errorf("name of created file \"fOo\" is %s", fd2.Name())
 	}
 
-	// one would expect DirNames to show the last variant, but in fact it shows
+	// one would expect DirNames to show the last wrapperType, but in fact it shows
 	// the original one
 	assertDir(t, fs, "/", []string{"FOO"})
 }

lib/fs/filesystem.go

@@ -16,6 +16,17 @@ import (
 	"time"
 )
 
+type filesystemWrapperType int32
+
+const (
+	filesystemWrapperTypeNone filesystemWrapperType = iota
+	filesystemWrapperTypeMtime
+	filesystemWrapperTypeCase
+	filesystemWrapperTypeError
+	filesystemWrapperTypeWalk
+	filesystemWrapperTypeLog
+)
+
 // The Filesystem interface abstracts access to the file system.
 type Filesystem interface {
 	Chmod(name string, mode FileMode) error
@@ -49,6 +60,10 @@ type Filesystem interface {
 	URI() string
 	Options() []Option
 	SameFile(fi1, fi2 FileInfo) bool
+
+	// Used for unwrapping things
+	underlying() (Filesystem, bool)
+	wrapperType() filesystemWrapperType
 }
 
 // The File interface abstracts access to a regular file, being a somewhat
@@ -284,18 +299,16 @@ func wrapFilesystem(fs Filesystem, wrapFn func(Filesystem) Filesystem) Filesyste
 	return fs
 }
 
-// unwrapFilesystem removes "wrapping" filesystems to expose the underlying filesystem.
-func unwrapFilesystem(fs Filesystem) Filesystem {
+// unwrapFilesystem removes "wrapping" filesystems to expose the filesystem of the requested wrapperType, if it exists.
+func unwrapFilesystem(fs Filesystem, wrapperType filesystemWrapperType) (Filesystem, bool) {
+	var ok bool
 	for {
-		switch sfs := fs.(type) {
-		case *logFilesystem:
-			fs = sfs.Filesystem
-		case *walkFilesystem:
-			fs = sfs.Filesystem
-		case *mtimeFS:
-			fs = sfs.Filesystem
-		default:
-			return sfs
+		if fs.wrapperType() == wrapperType {
+			return fs, true
+		}
+		fs, ok = fs.underlying()
+		if !ok {
+			return nil, false
 		}
 	}
 }

lib/fs/logfs.go

@@ -163,3 +163,11 @@ func (fs *logFilesystem) Usage(name string) (Usage, error) {
 	l.Debugln(getCaller(), fs.Type(), fs.URI(), "Usage", name, usage, err)
 	return usage, err
 }
+
+func (fs *logFilesystem) underlying() (Filesystem, bool) {
+	return fs.Filesystem, true
+}
+
+func (fs *logFilesystem) wrapperType() filesystemWrapperType {
+	return filesystemWrapperTypeLog
+}

lib/fs/mtimefs.go

@@ -7,6 +7,7 @@
 package fs
 
 import (
+	"errors"
 	"time"
 )
 
@@ -69,14 +70,14 @@ func (f *mtimeFS) Stat(name string) (FileInfo, error) {
 		return nil, err
 	}
 
-	real, virtual, err := f.load(name)
+	mtimeMapping, err := f.load(name)
 	if err != nil {
 		return nil, err
 	}
-	if real == info.ModTime() {
+	if mtimeMapping.Real == info.ModTime() {
 		info = mtimeFileInfo{
 			FileInfo: info,
-			mtime:    virtual,
+			mtime:    mtimeMapping.Virtual,
 		}
 	}
 
@@ -89,14 +90,14 @@ func (f *mtimeFS) Lstat(name string) (FileInfo, error) {
 		return nil, err
 	}
 
-	real, virtual, err := f.load(name)
+	mtimeMapping, err := f.load(name)
 	if err != nil {
 		return nil, err
 	}
-	if real == info.ModTime() {
+	if mtimeMapping.Real == info.ModTime() {
 		info = mtimeFileInfo{
 			FileInfo: info,
-			mtime:    virtual,
+			mtime:    mtimeMapping.Virtual,
 		}
 	}
 
@@ -106,15 +107,15 @@ func (f *mtimeFS) Lstat(name string) (FileInfo, error) {
 func (f *mtimeFS) Walk(root string, walkFn WalkFunc) error {
 	return f.Filesystem.Walk(root, func(path string, info FileInfo, err error) error {
 		if info != nil {
-			real, virtual, loadErr := f.load(path)
+			mtimeMapping, loadErr := f.load(path)
 			if loadErr != nil && err == nil {
 				// The iterator gets to deal with the error
 				err = loadErr
 			}
-			if real == info.ModTime() {
+			if mtimeMapping.Real == info.ModTime() {
 				info = mtimeFileInfo{
 					FileInfo: info,
-					mtime:    virtual,
+					mtime:    mtimeMapping.Virtual,
 				}
 			}
 		}
@@ -146,8 +147,13 @@ func (f *mtimeFS) OpenFile(name string, flags int, mode FileMode) (File, error)
 	return mtimeFile{fd, f}, nil
 }
 
-// "real" is the on disk timestamp
-// "virtual" is what want the timestamp to be
+func (f *mtimeFS) underlying() (Filesystem, bool) {
+	return f.Filesystem, true
+}
+
+func (f *mtimeFS) wrapperType() filesystemWrapperType {
+	return filesystemWrapperTypeMtime
+}
 
 func (f *mtimeFS) save(name string, real, virtual time.Time) {
 	if f.caseInsensitive {
@@ -161,32 +167,31 @@ func (f *mtimeFS) save(name string, real, virtual time.Time) {
 		return
 	}
 
-	mtime := dbMtime{
-		real:    real,
-		virtual: virtual,
+	mtime := MtimeMapping{
+		Real:    real,
+		Virtual: virtual,
 	}
 	bs, _ := mtime.Marshal() // Can't fail
 	f.db.PutBytes(name, bs)
 }
 
-func (f *mtimeFS) load(name string) (real, virtual time.Time, err error) {
+func (f *mtimeFS) load(name string) (MtimeMapping, error) {
 	if f.caseInsensitive {
 		name = UnicodeLowercase(name)
 	}
 
 	data, exists, err := f.db.Bytes(name)
 	if err != nil {
-		return time.Time{}, time.Time{}, err
+		return MtimeMapping{}, err
 	} else if !exists {
-		return time.Time{}, time.Time{}, nil
+		return MtimeMapping{}, nil
 	}
 
-	var mtime dbMtime
+	var mtime MtimeMapping
 	if err := mtime.Unmarshal(data); err != nil {
-		return time.Time{}, time.Time{}, err
+		return MtimeMapping{}, err
 	}
-
-	return mtime.real, mtime.virtual, nil
+	return mtime, nil
 }
 
 // The mtimeFileInfo is an os.FileInfo that lies about the ModTime().
@@ -211,43 +216,57 @@ func (f mtimeFile) Stat() (FileInfo, error) {
 		return nil, err
 	}
 
-	real, virtual, err := f.fs.load(f.Name())
+	mtimeMapping, err := f.fs.load(f.Name())
 	if err != nil {
 		return nil, err
 	}
-	if real == info.ModTime() {
+	if mtimeMapping.Real == info.ModTime() {
 		info = mtimeFileInfo{
 			FileInfo: info,
-			mtime:    virtual,
+			mtime:    mtimeMapping.Virtual,
 		}
 	}
 
 	return info, nil
 }
 
+// Used by copyRange to unwrap to the real file and access SyscallConn
 func (f mtimeFile) unwrap() File {
 	return f.File
 }
 
-// The dbMtime is our database representation
-
-type dbMtime struct {
-	real    time.Time
-	virtual time.Time
+// MtimeMapping represents the mapping as stored in the database
+type MtimeMapping struct {
+	// "Real" is the on disk timestamp
+	Real time.Time `json:"real"`
+	// "Virtual" is what want the timestamp to be
+	Virtual time.Time `json:"virtual"`
 }
 
-func (t *dbMtime) Marshal() ([]byte, error) {
-	bs0, _ := t.real.MarshalBinary()
-	bs1, _ := t.virtual.MarshalBinary()
+func (t *MtimeMapping) Marshal() ([]byte, error) {
+	bs0, _ := t.Real.MarshalBinary()
+	bs1, _ := t.Virtual.MarshalBinary()
 	return append(bs0, bs1...), nil
 }
 
-func (t *dbMtime) Unmarshal(bs []byte) error {
-	if err := t.real.UnmarshalBinary(bs[:len(bs)/2]); err != nil {
+func (t *MtimeMapping) Unmarshal(bs []byte) error {
+	if err := t.Real.UnmarshalBinary(bs[:len(bs)/2]); err != nil {
 		return err
 	}
-	if err := t.virtual.UnmarshalBinary(bs[len(bs)/2:]); err != nil {
+	if err := t.Virtual.UnmarshalBinary(bs[len(bs)/2:]); err != nil {
 		return err
 	}
 	return nil
 }
+
+func GetMtimeMapping(fs Filesystem, file string) (MtimeMapping, error) {
+	fs, ok := unwrapFilesystem(fs, filesystemWrapperTypeMtime)
+	if !ok {
+		return MtimeMapping{}, errors.New("failed to unwrap")
+	}
+	mtimeFs, ok := fs.(*mtimeFS)
+	if !ok {
+		return MtimeMapping{}, errors.New("unwrapping failed")
+	}
+	return mtimeFs.load(file)
+}

lib/fs/walkfs.go

@@ -149,3 +149,11 @@ func (f *walkFilesystem) Walk(root string, walkFn WalkFunc) error {
 	}
 	return f.walk(root, info, walkFn, ancestors)
 }
+
+func (f *walkFilesystem) underlying() (Filesystem, bool) {
+	return f.Filesystem, true
+}
+
+func (f *walkFilesystem) wrapperType() filesystemWrapperType {
+	return filesystemWrapperTypeWalk
+}

lib/model/mocks/model.go

@@ -8,6 +8,7 @@ import (
 	"time"
 
 	"github.com/syncthing/syncthing/lib/db"
+	"github.com/syncthing/syncthing/lib/fs"
 	"github.com/syncthing/syncthing/lib/model"
 	"github.com/syncthing/syncthing/lib/protocol"
 	"github.com/syncthing/syncthing/lib/stats"
@@ -249,6 +250,20 @@ type Model struct {
 	getHelloReturnsOnCall map[int]struct {
 		result1 protocol.HelloIntf
 	}
+	GetMtimeMappingStub        func(string, string) (fs.MtimeMapping, error)
+	getMtimeMappingMutex       sync.RWMutex
+	getMtimeMappingArgsForCall []struct {
+		arg1 string
+		arg2 string
+	}
+	getMtimeMappingReturns struct {
+		result1 fs.MtimeMapping
+		result2 error
+	}
+	getMtimeMappingReturnsOnCall map[int]struct {
+		result1 fs.MtimeMapping
+		result2 error
+	}
 	GlobalDirectoryTreeStub        func(string, string, int, bool) ([]*model.TreeEntry, error)
 	globalDirectoryTreeMutex       sync.RWMutex
 	globalDirectoryTreeArgsForCall []struct {
@@ -1691,6 +1706,71 @@ func (fake *Model) GetHelloReturnsOnCall(i int, result1 protocol.HelloIntf) {
 	}{result1}
 }
 
+func (fake *Model) GetMtimeMapping(arg1 string, arg2 string) (fs.MtimeMapping, error) {
+	fake.getMtimeMappingMutex.Lock()
+	ret, specificReturn := fake.getMtimeMappingReturnsOnCall[len(fake.getMtimeMappingArgsForCall)]
+	fake.getMtimeMappingArgsForCall = append(fake.getMtimeMappingArgsForCall, struct {
+		arg1 string
+		arg2 string
+	}{arg1, arg2})
+	stub := fake.GetMtimeMappingStub
+	fakeReturns := fake.getMtimeMappingReturns
+	fake.recordInvocation("GetMtimeMapping", []interface{}{arg1, arg2})
+	fake.getMtimeMappingMutex.Unlock()
+	if stub != nil {
+		return stub(arg1, arg2)
+	}
+	if specificReturn {
+		return ret.result1, ret.result2
+	}
+	return fakeReturns.result1, fakeReturns.result2
+}
+
+func (fake *Model) GetMtimeMappingCallCount() int {
+	fake.getMtimeMappingMutex.RLock()
+	defer fake.getMtimeMappingMutex.RUnlock()
+	return len(fake.getMtimeMappingArgsForCall)
+}
+
+func (fake *Model) GetMtimeMappingCalls(stub func(string, string) (fs.MtimeMapping, error)) {
+	fake.getMtimeMappingMutex.Lock()
+	defer fake.getMtimeMappingMutex.Unlock()
+	fake.GetMtimeMappingStub = stub
+}
+
+func (fake *Model) GetMtimeMappingArgsForCall(i int) (string, string) {
+	fake.getMtimeMappingMutex.RLock()
+	defer fake.getMtimeMappingMutex.RUnlock()
+	argsForCall := fake.getMtimeMappingArgsForCall[i]
+	return argsForCall.arg1, argsForCall.arg2
+}
+
+func (fake *Model) GetMtimeMappingReturns(result1 fs.MtimeMapping, result2 error) {
+	fake.getMtimeMappingMutex.Lock()
+	defer fake.getMtimeMappingMutex.Unlock()
+	fake.GetMtimeMappingStub = nil
+	fake.getMtimeMappingReturns = struct {
+		result1 fs.MtimeMapping
+		result2 error
+	}{result1, result2}
+}
+
+func (fake *Model) GetMtimeMappingReturnsOnCall(i int, result1 fs.MtimeMapping, result2 error) {
+	fake.getMtimeMappingMutex.Lock()
+	defer fake.getMtimeMappingMutex.Unlock()
+	fake.GetMtimeMappingStub = nil
+	if fake.getMtimeMappingReturnsOnCall == nil {
+		fake.getMtimeMappingReturnsOnCall = make(map[int]struct {
+			result1 fs.MtimeMapping
+			result2 error
+		})
+	}
+	fake.getMtimeMappingReturnsOnCall[i] = struct {
+		result1 fs.MtimeMapping
+		result2 error
+	}{result1, result2}
+}
+
 func (fake *Model) GlobalDirectoryTree(arg1 string, arg2 string, arg3 int, arg4 bool) ([]*model.TreeEntry, error) {
 	fake.globalDirectoryTreeMutex.Lock()
 	ret, specificReturn := fake.globalDirectoryTreeReturnsOnCall[len(fake.globalDirectoryTreeArgsForCall)]
@@ -3186,6 +3266,8 @@ func (fake *Model) Invocations() map[string][][]interface{} {
 	defer fake.getFolderVersionsMutex.RUnlock()
 	fake.getHelloMutex.RLock()
 	defer fake.getHelloMutex.RUnlock()
+	fake.getMtimeMappingMutex.RLock()
+	defer fake.getMtimeMappingMutex.RUnlock()
 	fake.globalDirectoryTreeMutex.RLock()
 	defer fake.globalDirectoryTreeMutex.RUnlock()
 	fake.indexMutex.RLock()

lib/model/model.go

@@ -95,6 +95,7 @@ type Model interface {
 
 	CurrentFolderFile(folder string, file string) (protocol.FileInfo, bool, error)
 	CurrentGlobalFile(folder string, file string) (protocol.FileInfo, bool, error)
+	GetMtimeMapping(folder string, file string) (fs.MtimeMapping, error)
 	Availability(folder string, file protocol.FileInfo, block protocol.BlockInfo) ([]Availability, error)
 
 	Completion(device protocol.DeviceID, folder string) (FolderCompletion, error)
@@ -2038,12 +2039,12 @@ func (m *model) CurrentFolderFile(folder string, file string) (protocol.FileInfo
 
 func (m *model) CurrentGlobalFile(folder string, file string) (protocol.FileInfo, bool, error) {
 	m.fmut.RLock()
-	fs, ok := m.folderFiles[folder]
+	ffs, ok := m.folderFiles[folder]
 	m.fmut.RUnlock()
 	if !ok {
 		return protocol.FileInfo{}, false, ErrFolderMissing
 	}
-	snap, err := fs.Snapshot()
+	snap, err := ffs.Snapshot()
 	if err != nil {
 		return protocol.FileInfo{}, false, err
 	}
@@ -2052,6 +2053,16 @@ func (m *model) CurrentGlobalFile(folder string, file string) (protocol.FileInfo
 	return f, ok, nil
 }
 
+func (m *model) GetMtimeMapping(folder string, file string) (fs.MtimeMapping, error) {
+	m.fmut.RLock()
+	ffs, ok := m.folderFiles[folder]
+	m.fmut.RUnlock()
+	if !ok {
+		return fs.MtimeMapping{}, ErrFolderMissing
+	}
+	return fs.GetMtimeMapping(ffs.MtimeFS(), file)
+}
+
 // Connection returns the current connection for device, and a boolean whether a connection was found.
 func (m *model) Connection(deviceID protocol.DeviceID) (protocol.Connection, bool) {
 	m.pmut.RLock()