MB-61421: Interfaces for supporting online backup of an index (#48)

* transfer interface

* fix interface

* fix comment

index.go

@@ -48,6 +48,15 @@ type Index interface {
 	StatsMap() map[string]interface{}
 }
 
+// CopyIndex is an extended index that supports copying to a new location online.
+// Use the CopyReader method to obtain a reader for initiating the copy operation.
+type CopyIndex interface {
+	Index
+	// Obtain a copy reader for the online copy/backup operation,
+	// to handle necessary bookkeeping, instead of using the regular IndexReader.
+	CopyReader() CopyReader
+}
+
 type IndexReader interface {
 	TermFieldReader(ctx context.Context, term []byte, field string, includeFreq, includeNorm, includeTermVectors bool) (TermFieldReader, error)
 
@@ -79,6 +88,15 @@ type IndexReader interface {
 	Close() error
 }
 
+// CopyReader is an extended index reader for backup or online copy operations, replacing the regular index reader.
+type CopyReader interface {
+	IndexReader
+	// CopyTo performs an online copy or backup of the index to the specified directory.
+	CopyTo(d Directory) error
+	// CloseCopyReader must be used instead of Close() to close the copy reader.
+	CloseCopyReader() error
+}
+
 type IndexReaderRegexp interface {
 	FieldDictRegexp(field string, regex string) (FieldDict, error)
 }