lib/fs: Add case insensitivity to MtimeFS

This is step one of a hundred fifty on the path to case insensitivity.
It brings in the basic case folding mechanism and adds it to the
mtimefs, as this is something outside the fileset that touches stuff in
the database based on name. No effort to convert or handle existing
entries when the insensitivity is changed, I don't think we need it...

Useless by itself but includes tests and will reduce the review load
along the way.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4521

lib/fs/mtimefs.go

@@ -6,7 +6,9 @@
 
 package fs
 
-import "time"
+import (
+	"time"
+)
 
 // The database is where we store the virtual mtimes
 type database interface {
@@ -20,16 +22,29 @@ type database interface {
 // just does the underlying operations with no additions.
 type MtimeFS struct {
 	Filesystem
-	chtimes func(string, time.Time, time.Time) error
-	db      database
+	chtimes         func(string, time.Time, time.Time) error
+	db              database
+	caseInsensitive bool
 }
 
-func NewMtimeFS(underlying Filesystem, db database) *MtimeFS {
-	return &MtimeFS{
+type MtimeFSOption func(*MtimeFS)
+
+func WithCaseInsensitivity(v bool) MtimeFSOption {
+	return func(f *MtimeFS) {
+		f.caseInsensitive = v
+	}
+}
+
+func NewMtimeFS(underlying Filesystem, db database, options ...MtimeFSOption) *MtimeFS {
+	f := &MtimeFS{
 		Filesystem: underlying,
 		chtimes:    underlying.Chtimes, // for mocking it out in the tests
 		db:         db,
 	}
+	for _, opt := range options {
+		opt(f)
+	}
+	return f
 }
 
 func (f *MtimeFS) Chtimes(name string, atime, mtime time.Time) error {
@@ -72,6 +87,10 @@ func (f *MtimeFS) Lstat(name string) (FileInfo, error) {
 // "virtual" is what want the timestamp to be
 
 func (f *MtimeFS) save(name string, real, virtual time.Time) {
+	if f.caseInsensitive {
+		name = UnicodeLowercase(name)
+	}
+
 	if real.Equal(virtual) {
 		// If the virtual time and the real on disk time are equal we don't
 		// need to store anything.
@@ -88,6 +107,10 @@ func (f *MtimeFS) save(name string, real, virtual time.Time) {
 }
 
 func (f *MtimeFS) load(name string) (real, virtual time.Time) {
+	if f.caseInsensitive {
+		name = UnicodeLowercase(name)
+	}
+
 	data, exists := f.db.Bytes(name)
 	if !exists {
 		return