walk: factor Listing helpers into their own file and add tests

fs/walk/helpers.go

@@ -0,0 +1,43 @@
+package walk
+
+import "github.com/rclone/rclone/fs"
+
+// Listing helpers used by backends
+
+// ListRHelper is used in the implementation of ListR to accumulate DirEntries
+type ListRHelper struct {
+	callback fs.ListRCallback
+	entries  fs.DirEntries
+}
+
+// NewListRHelper should be called from ListR with the callback passed in
+func NewListRHelper(callback fs.ListRCallback) *ListRHelper {
+	return &ListRHelper{
+		callback: callback,
+	}
+}
+
+// send sends the stored entries to the callback if there are >= max
+// entries.
+func (lh *ListRHelper) send(max int) (err error) {
+	if len(lh.entries) >= max {
+		err = lh.callback(lh.entries)
+		lh.entries = lh.entries[:0]
+	}
+	return err
+}
+
+// Add an entry to the stored entries and send them if there are more
+// than a certain amount
+func (lh *ListRHelper) Add(entry fs.DirEntry) error {
+	if entry == nil {
+		return nil
+	}
+	lh.entries = append(lh.entries, entry)
+	return lh.send(100)
+}
+
+// Flush the stored entries (if any) sending them to the callback
+func (lh *ListRHelper) Flush() error {
+	return lh.send(1)
+}