backup: Keep mountpoints for --one-file-system

When a file system is mounted at a directory, lstat() returns attributes
of the root node of the mounted file system, including the device ID of
the other file system. The previous code used when --one-file-system is
specified excluded the directory itself because of that.

This commit changes the code so that mountpoints are kept as empty
directories, its attributes set to the root note of the mounted file
system. The behavior mimics `tar`, which does the same.

cmd/restic/exclude.go

@@ -288,7 +288,49 @@ func rejectByDevice(samples []string) (RejectFunc, error) {
 			panic(fmt.Sprintf("error checking device ID of %v: %v", item, err))
 		}
 
-		return !allowed
+		if allowed {
+			// accept item
+			return false
+		}
+
+		// reject everything except directories
+		if !fi.IsDir() {
+			return true
+		}
+
+		// special case: make sure we keep mountpoints (directories which
+		// contain a mounted file system). Test this by checking if the parent
+		// directory would be included.
+		parentDir := filepath.Dir(filepath.Clean(item))
+
+		parentFI, err := fs.Lstat(parentDir)
+		if err != nil {
+			debug.Log("item %v: error running lstat() on parent directory: %v", item, err)
+			// if in doubt, reject
+			return true
+		}
+
+		parentDeviceID, err := fs.DeviceID(parentFI)
+		if err != nil {
+			debug.Log("item %v: getting device ID of parent directory: %v", item, err)
+			// if in doubt, reject
+			return true
+		}
+
+		parentAllowed, err := deviceMap.IsAllowed(parentDir, parentDeviceID)
+		if err != nil {
+			debug.Log("item %v: error checking parent directory: %v", item, err)
+			// if in doubt, reject
+			return true
+		}
+
+		if parentAllowed {
+			// we found a mount point, so accept the directory
+			return false
+		}
+
+		// reject everything else
+		return true
 	}, nil
 }