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/config/folderconfiguration.go

@@ -61,8 +61,8 @@ type FolderConfiguration struct {
 	DisableFsync            bool                        `xml:"disableFsync" json:"disableFsync"`
 	BlockPullOrder          BlockPullOrder              `xml:"blockPullOrder" json:"blockPullOrder"`
 	CopyRangeMethod         fs.CopyRangeMethod          `xml:"copyRangeMethod" json:"copyRangeMethod" default:"standard"`
+	CaseSensitiveFS         bool                        `xml:"caseSensitiveFS" json:"caseSensitiveFS"`
 
-	cachedFilesystem    fs.Filesystem
 	cachedModTimeWindow time.Duration
 
 	DeprecatedReadOnly       bool    `xml:"ro,attr,omitempty" json:"-"`
@@ -101,11 +101,11 @@ func (f FolderConfiguration) Copy() FolderConfiguration {
 func (f FolderConfiguration) Filesystem() fs.Filesystem {
 	// This is intentionally not a pointer method, because things like
 	// cfg.Folders["default"].Filesystem() should be valid.
-	if f.cachedFilesystem == nil {
-		l.Infoln("bug: uncached filesystem call (should only happen in tests)")
-		return fs.NewFilesystem(f.FilesystemType, f.Path)
+	filesystem := fs.NewFilesystem(f.FilesystemType, f.Path)
+	if !f.CaseSensitiveFS {
+		filesystem = fs.NewCaseFilesystem(filesystem)
 	}
-	return f.cachedFilesystem
+	return filesystem
 }
 
 func (f FolderConfiguration) ModTimeWindow() time.Duration {
@@ -210,8 +210,6 @@ func (f *FolderConfiguration) DeviceIDs() []protocol.DeviceID {
 }
 
 func (f *FolderConfiguration) prepare() {
-	f.cachedFilesystem = fs.NewFilesystem(f.FilesystemType, f.Path)
-
 	if f.RescanIntervalS > MaxRescanIntervalS {
 		f.RescanIntervalS = MaxRescanIntervalS
 	} else if f.RescanIntervalS < 0 {
@@ -263,7 +261,6 @@ func (f FolderConfiguration) RequiresRestartOnly() FolderConfiguration {
 
 	// Manual handling for things that are not taken care of by the tag
 	// copier, yet should not cause a restart.
-	copy.cachedFilesystem = nil
 
 	blank := FolderConfiguration{}
 	util.CopyMatchingTag(&blank, &copy, "restart", func(v string) bool {