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_test.go

@@ -10,6 +10,7 @@ import (
 	"errors"
 	"io/ioutil"
 	"os"
+	"runtime"
 	"testing"
 	"time"
 )
@@ -80,6 +81,51 @@ func TestMtimeFS(t *testing.T) {
 	}
 }
 
+func TestMtimeFSInsensitive(t *testing.T) {
+	switch runtime.GOOS {
+	case "darwin", "windows":
+		// blatantly assume file systems here are case insensitive. Might be
+		// a spurious failure on oddly configured systems.
+	default:
+		t.Skip("need case insensitive FS")
+	}
+
+	theTest := func(t *testing.T, fs *MtimeFS, shouldSucceed bool) {
+		os.RemoveAll("testdata")
+		defer os.RemoveAll("testdata")
+		os.Mkdir("testdata", 0755)
+		ioutil.WriteFile("testdata/FiLe", []byte("hello"), 0644)
+
+		// a random time with nanosecond precision
+		testTime := time.Unix(1234567890, 123456789)
+
+		// Do one call that gets struck by an exceptionally evil Chtimes, with a
+		// different case from what is on disk.
+		fs.chtimes = evilChtimes
+		if err := fs.Chtimes("testdata/fIlE", testTime, testTime); err != nil {
+			t.Error("Should not have failed:", err)
+		}
+
+		// Check that we get back the mtime we set, if we were supposed to succed.
+		info, err := fs.Lstat("testdata/FILE")
+		if err != nil {
+			t.Error("Lstat shouldn't fail:", err)
+		} else if info.ModTime().Equal(testTime) != shouldSucceed {
+			t.Errorf("Time mismatch; got %v, comparison %v, expected equal=%v", info.ModTime(), testTime, shouldSucceed)
+		}
+	}
+
+	// The test should fail with a case sensitive mtimefs
+	t.Run("with case sensitive mtimefs", func(t *testing.T) {
+		theTest(t, NewMtimeFS(newBasicFilesystem("."), make(mapStore)), false)
+	})
+
+	// And succeed with a case insensitive one.
+	t.Run("with case insensitive mtimefs", func(t *testing.T) {
+		theTest(t, NewMtimeFS(newBasicFilesystem("."), make(mapStore), WithCaseInsensitivity(true)), true)
+	})
+}
+
 // The mapStore is a simple database
 
 type mapStore map[string][]byte