Expose ListPostings Length via Len() method (#15678)

tsdb: expose remaining ListPostings Length

Signed-off-by: Ben Ye <benye@amazon.com>

---------

Signed-off-by: Ben Ye <benye@amazon.com>

tsdb/index/postings.go

@@ -863,6 +863,11 @@ func (it *ListPostings) Err() error {
 	return nil
 }
 
+// Len returns the remaining number of postings in the list.
+func (it *ListPostings) Len() int {
+	return len(it.list)
+}
+
 // bigEndianPostings implements the Postings interface over a byte stream of
 // big endian numbers.
 type bigEndianPostings struct {

tsdb/index/postings_test.go

@@ -1244,63 +1244,78 @@ func TestPostingsWithIndexHeap(t *testing.T) {
 func TestListPostings(t *testing.T) {
 	t.Run("empty list", func(t *testing.T) {
 		p := NewListPostings(nil)
+		require.Equal(t, 0, p.(*ListPostings).Len())
 		require.False(t, p.Next())
 		require.False(t, p.Seek(10))
 		require.False(t, p.Next())
 		require.NoError(t, p.Err())
+		require.Equal(t, 0, p.(*ListPostings).Len())
 	})
 
 	t.Run("one posting", func(t *testing.T) {
 		t.Run("next", func(t *testing.T) {
 			p := NewListPostings([]storage.SeriesRef{10})
+			require.Equal(t, 1, p.(*ListPostings).Len())
 			require.True(t, p.Next())
 			require.Equal(t, storage.SeriesRef(10), p.At())
 			require.False(t, p.Next())
 			require.NoError(t, p.Err())
+			require.Equal(t, 0, p.(*ListPostings).Len())
 		})
 		t.Run("seek less", func(t *testing.T) {
 			p := NewListPostings([]storage.SeriesRef{10})
+			require.Equal(t, 1, p.(*ListPostings).Len())
 			require.True(t, p.Seek(5))
 			require.Equal(t, storage.SeriesRef(10), p.At())
 			require.True(t, p.Seek(5))
 			require.Equal(t, storage.SeriesRef(10), p.At())
 			require.False(t, p.Next())
 			require.NoError(t, p.Err())
+			require.Equal(t, 0, p.(*ListPostings).Len())
 		})
 		t.Run("seek equal", func(t *testing.T) {
 			p := NewListPostings([]storage.SeriesRef{10})
+			require.Equal(t, 1, p.(*ListPostings).Len())
 			require.True(t, p.Seek(10))
 			require.Equal(t, storage.SeriesRef(10), p.At())
 			require.False(t, p.Next())
 			require.NoError(t, p.Err())
+			require.Equal(t, 0, p.(*ListPostings).Len())
 		})
 		t.Run("seek more", func(t *testing.T) {
 			p := NewListPostings([]storage.SeriesRef{10})
+			require.Equal(t, 1, p.(*ListPostings).Len())
 			require.False(t, p.Seek(15))
 			require.False(t, p.Next())
 			require.NoError(t, p.Err())
+			require.Equal(t, 0, p.(*ListPostings).Len())
 		})
 		t.Run("seek after next", func(t *testing.T) {
 			p := NewListPostings([]storage.SeriesRef{10})
+			require.Equal(t, 1, p.(*ListPostings).Len())
 			require.True(t, p.Next())
 			require.False(t, p.Seek(15))
 			require.False(t, p.Next())
 			require.NoError(t, p.Err())
+			require.Equal(t, 0, p.(*ListPostings).Len())
 		})
 	})
 
 	t.Run("multiple postings", func(t *testing.T) {
 		t.Run("next", func(t *testing.T) {
 			p := NewListPostings([]storage.SeriesRef{10, 20})
+			require.Equal(t, 2, p.(*ListPostings).Len())
 			require.True(t, p.Next())
 			require.Equal(t, storage.SeriesRef(10), p.At())
 			require.True(t, p.Next())
 			require.Equal(t, storage.SeriesRef(20), p.At())
 			require.False(t, p.Next())
 			require.NoError(t, p.Err())
+			require.Equal(t, 0, p.(*ListPostings).Len())
 		})
 		t.Run("seek", func(t *testing.T) {
 			p := NewListPostings([]storage.SeriesRef{10, 20})
+			require.Equal(t, 2, p.(*ListPostings).Len())
 			require.True(t, p.Seek(5))
 			require.Equal(t, storage.SeriesRef(10), p.At())
 			require.True(t, p.Seek(5))
@@ -1315,23 +1330,30 @@ func TestListPostings(t *testing.T) {
 			require.Equal(t, storage.SeriesRef(20), p.At())
 			require.False(t, p.Next())
 			require.NoError(t, p.Err())
+			require.Equal(t, 0, p.(*ListPostings).Len())
 		})
 		t.Run("seek lest than last", func(t *testing.T) {
 			p := NewListPostings([]storage.SeriesRef{10, 20, 30, 40, 50})
+			require.Equal(t, 5, p.(*ListPostings).Len())
 			require.True(t, p.Seek(45))
 			require.Equal(t, storage.SeriesRef(50), p.At())
 			require.False(t, p.Next())
+			require.Equal(t, 0, p.(*ListPostings).Len())
 		})
 		t.Run("seek exactly last", func(t *testing.T) {
 			p := NewListPostings([]storage.SeriesRef{10, 20, 30, 40, 50})
+			require.Equal(t, 5, p.(*ListPostings).Len())
 			require.True(t, p.Seek(50))
 			require.Equal(t, storage.SeriesRef(50), p.At())
 			require.False(t, p.Next())
+			require.Equal(t, 0, p.(*ListPostings).Len())
 		})
 		t.Run("seek more than last", func(t *testing.T) {
 			p := NewListPostings([]storage.SeriesRef{10, 20, 30, 40, 50})
+			require.Equal(t, 5, p.(*ListPostings).Len())
 			require.False(t, p.Seek(60))
 			require.False(t, p.Next())
+			require.Equal(t, 0, p.(*ListPostings).Len())
 		})
 	})