MB-71516: Add unit test to reproduce result inconsistency issue (#2317)

- Add a unit test that reproduces the issue fixed by
blevesearch/zapx#401

---------

Co-authored-by: Abhinav Dangeti <abhinav@couchbase.com>

Author: CascadingRadium

go.mod

@@ -25,7 +25,7 @@ require (
 	github.com/blevesearch/zapx/v14 v14.4.3
 	github.com/blevesearch/zapx/v15 v15.4.3
 	github.com/blevesearch/zapx/v16 v16.3.4
-	github.com/blevesearch/zapx/v17 v17.0.11
+	github.com/blevesearch/zapx/v17 v17.0.12-0.20260421144655-b9ffe761cd37
 	github.com/couchbase/moss v0.2.0
 	github.com/spf13/cobra v1.10.2
 	go.etcd.io/bbolt v1.4.0

go.sum

@@ -45,8 +45,8 @@ github.com/blevesearch/zapx/v15 v15.4.3 h1:iJiMJOHrz216jyO6lS0m9RTCEkprUnzvqAI2l
 github.com/blevesearch/zapx/v15 v15.4.3/go.mod h1:1pssev/59FsuWcgSnTa0OeEpOzmhtmr/0/11H0Z8+Nw=
 github.com/blevesearch/zapx/v16 v16.3.4 h1:hDAqA8qusZTNbPEL7//w5P65UZ2de6yhSeUaTbp0Po0=
 github.com/blevesearch/zapx/v16 v16.3.4/go.mod h1:zqkPPqs9GS9FzVWzCO3Wf1X044yWAV17+4zb+FTiEHg=
-github.com/blevesearch/zapx/v17 v17.0.11 h1:3OKEe8NpD4n14GW+GY/op5nh8/x8dmYNwgLdRgvv5go=
-github.com/blevesearch/zapx/v17 v17.0.11/go.mod h1:62wlIX0vYZoLoLLKmix4zQvyCevvUt7RLuvcV5D3/N0=
+github.com/blevesearch/zapx/v17 v17.0.12-0.20260421144655-b9ffe761cd37 h1:LaROcKdceOI37F6OMc9pySbdXCAQVqOgcMQk47yosqA=
+github.com/blevesearch/zapx/v17 v17.0.12-0.20260421144655-b9ffe761cd37/go.mod h1:62wlIX0vYZoLoLLKmix4zQvyCevvUt7RLuvcV5D3/N0=
 github.com/couchbase/ghistogram v0.1.0 h1:b95QcQTCzjTUocDXp/uMgSNQi8oj1tGwnJ4bODWZnps=
 github.com/couchbase/ghistogram v0.1.0/go.mod h1:s1Jhy76zqfEecpNWJfWUiKZookAFaiGOEoyzgHt9i7k=
 github.com/couchbase/moss v0.2.0 h1:VCYrMzFwEryyhRSeI+/b3tRBSeTpi/8gn5Kf6dxqn+o=

search_knn_test.go

@@ -2986,3 +2986,68 @@ func TestKNNNullParams(t *testing.T) {
 		}
 	}
 }
+
+func TestVectorIndexExhaustion(t *testing.T) {
+	for optimization := range index.SupportedVectorIndexOptimizations {
+		t.Run(optimization, func(t *testing.T) {
+			tmpIndexPath := createTmpIndexPath(t)
+			defer cleanupTmpIndexPath(t, tmpIndexPath)
+
+			const dims = 8
+
+			vecFieldMapping := mapping.NewVectorFieldMapping()
+			vecFieldMapping.Dims = dims
+			vecFieldMapping.VectorIndexOptimizedFor = optimization
+
+			indexMapping := NewIndexMapping()
+			indexMapping.DefaultMapping.AddFieldMappingsAt("vec", vecFieldMapping)
+
+			idx, err := New(tmpIndexPath, indexMapping)
+			if err != nil {
+				t.Fatal(err)
+			}
+			defer func() {
+				err := idx.Close()
+				if err != nil {
+					t.Fatal(err)
+				}
+			}()
+
+			docs := []struct {
+				id  string
+				vec []float32
+			}{
+				{"doc1", []float32{1, 0, 0, 0, 0, 0, 0, 0}},
+				{"doc2", []float32{0, 1, 0, 0, 0, 0, 0, 0}},
+				{"doc3", []float32{0, 0, 1, 0, 0, 0, 0, 0}},
+			}
+
+			batch := idx.NewBatch()
+			for _, doc := range docs {
+				err = batch.Index(doc.id, map[string]interface{}{
+					"vec": doc.vec,
+				})
+				if err != nil {
+					t.Fatal(err)
+				}
+			}
+			err = idx.Batch(batch)
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			searchReq := NewSearchRequest(query.NewMatchNoneQuery())
+			searchReq.AddKNN("vec", []float32{1, 0, 0, 0, 0, 0, 0, 0}, 10, 1.0)
+			res, err := idx.Search(searchReq)
+			if err != nil {
+				t.Fatal(err)
+			}
+			if len(res.Hits) != 3 {
+				t.Fatalf("expected 3 hits, got %d", len(res.Hits))
+			}
+			if res.Hits[0].ID != "doc1" {
+				t.Fatalf("expected doc1 as top hit, got %s", res.Hits[0].ID)
+			}
+		})
+	}
+}