lib/fs: Properly handle case insensitive systems (fixes #1787, fixes #2739, fixes #5708)

With this change we emulate a case sensitive filesystem on top of
insensitive filesystems. This means we correctly pick up case-only renames
and throw a case conflict error when there would be multiple files differing
only in case.

This safety check has a small performance hit (about 20% more filesystem
operations when scanning for changes). The new advanced folder option
`caseSensitiveFS` can be used to disable the safety checks, retaining the
previous behavior on systems known to be fully case sensitive.

Co-authored-by: Jakob Borg <jakob@kastelo.net>

lib/fs/debug_symlink_unix.go

@@ -0,0 +1,47 @@
+// Copyright (C) 2017 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+// +build !windows
+
+package fs
+
+import (
+	"os"
+	"path/filepath"
+)
+
+// DebugSymlinkForTestsOnly is not and should not be used in Syncthing code,
+// hence the cumbersome name to make it obvious if this ever leaks. Its
+// 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 err := caseFs.checkCase(newname); err != nil {
+			return err
+		}
+		caseFs.dropCache()
+	}
+	if err := os.Symlink(filepath.Join(oldFs.URI(), oldname), filepath.Join(newFs.URI(), newname)); err != nil {
+		return err
+	}
+	return nil
+}
+
+// unwrapFilesystem removes "wrapping" filesystems to expose the underlying filesystem.
+func unwrapFilesystem(fs Filesystem) Filesystem {
+	for {
+		switch sfs := fs.(type) {
+		case *logFilesystem:
+			fs = sfs.Filesystem
+		case *walkFilesystem:
+			fs = sfs.Filesystem
+		case *MtimeFS:
+			fs = sfs.Filesystem
+		default:
+			return sfs
+		}
+	}
+}