Use config file modes to derive new dir/file modes

Fixes #2351

internal/backend/paths.go

@@ -21,6 +21,28 @@ var Paths = struct {
 	"config",
 }
 
-// Modes holds the default modes for directories and files for file-based
-// backends.
-var Modes = struct{ Dir, File os.FileMode }{0700, 0600}
+type Modes struct {
+	Dir  os.FileMode
+	File os.FileMode
+}
+
+// DefaultModes defines the default permissions to apply to new repository
+// files and directories stored on file-based backends.
+var DefaultModes = Modes{Dir: 0700, File: 0600}
+
+// DeriveModesFromFileInfo will, given the mode of a regular file, compute
+// the mode we should use for new files and directories. If the passed
+// error is non-nil DefaultModes are returned.
+func DeriveModesFromFileInfo(fi os.FileInfo, err error) Modes {
+	m := DefaultModes
+	if err != nil {
+		return m
+	}
+
+	if fi.Mode()&0040 != 0 { // Group has read access
+		m.Dir |= 0070
+		m.File |= 0060
+	}
+
+	return m
+}