mount: Map slashes in tags to underscores

Suggested-by: greatroar <>

changelog/unreleased/issue-2907

@@ -6,5 +6,10 @@ formatting of the time for a snapshot is now controlled using `--time-template`
 and supports subdirectories to for example group snapshots by year. Please
 refer to the help output of the `mount` command for further details.
 
+Characters in tag names which are not allowed in a filename are replaced by
+underscores `_`. For example a tag `foo/bar` will result in a directory name
+of `foo_bar`.
+
 https://github.com/restic/restic/issues/2907
 https://github.com/restic/restic/pull/2913
+https://github.com/restic/restic/pull/3691

internal/fuse/snapshots_dirstruct.go

@@ -97,6 +97,7 @@ func pathsFromSn(pathTemplate string, timeTemplate string, sn *restic.Snapshot)
 				// needs special treatment: Rebuild the string builders
 				newout := make([]strings.Builder, len(out)*len(sn.Tags))
 				for i, tag := range sn.Tags {
+					tag = filenameFromTag(tag)
 					for j := range out {
 						newout[i*len(out)+j].WriteString(out[j].String() + tag)
 					}
@@ -139,6 +140,24 @@ func pathsFromSn(pathTemplate string, timeTemplate string, sn *restic.Snapshot)
 	return paths, timeSuffix
 }
 
+// Some tags are problematic when used as filenames:
+//
+//	""
+//	".", ".."
+//	anything containing '/'
+//
+// Replace all special character by underscores "_", an empty tag is also represented as a underscore.
+func filenameFromTag(tag string) string {
+	switch tag {
+	case "", ".":
+		return "_"
+	case "..":
+		return "__"
+	}
+
+	return strings.ReplaceAll(tag, "/", "_")
+}
+
 // determine static path prefix
 func staticPrefix(pathTemplate string) (prefix string) {
 	inVerb := false

internal/fuse/snapshots_dirstruct_test.go

@@ -273,3 +273,19 @@ func TestMakeEmptyDirs(t *testing.T) {
 
 	verifyEntries(t, expNames, expLatest, sds.entries)
 }
+
+func TestFilenameFromTag(t *testing.T) {
+	for _, c := range []struct {
+		tag, filename string
+	}{
+		{"", "_"},
+		{".", "_"},
+		{"..", "__"},
+		{"%.", "%."},
+		{"foo", "foo"},
+		{"foo ", "foo "},
+		{"foo/bar_baz", "foo_bar_baz"},
+	} {
+		test.Equals(t, c.filename, filenameFromTag(c.tag))
+	}
+}