backend: move backend implementation helpers to util package

This removes code that is only used within a backend implementation from
the backend package. The latter now only contains code that also has
external users.

internal/backend/util/paths.go

@@ -0,0 +1,29 @@
+package util
+
+import "os"
+
+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
+}