fix: No panic if filter condition on indexed field is empty (#2929)

## Relevant issue(s)

Resolves #2916

## Description

Check if given filter condition is empty before proceeding with
secondary index fetching.

internal/db/fetcher/indexer.go

@@ -31,7 +31,6 @@ type IndexFetcher struct {
 	col           client.Collection
 	txn           datastore.Txn
 	indexFilter   *mapper.Filter
-	docFilter     *mapper.Filter
 	doc           *encodedDocument
 	mapping       *core.DocumentMapping
 	indexedFields []client.FieldDefinition
@@ -71,7 +70,6 @@ func (f *IndexFetcher) Init(
 	f.resetState()
 
 	f.col = col
-	f.docFilter = filter
 	f.doc = &encodedDocument{}
 	f.mapping = docMapper
 	f.txn = txn
@@ -100,15 +98,20 @@ outer:
 	}
 	f.indexIter = iter
 
-	if f.docFetcher != nil && len(f.docFields) > 0 {
+	// if it turns out that we can't use the index, we need to fall back to the document fetcher
+	if f.indexIter == nil {
+		f.docFields = fields
+	}
+
+	if len(f.docFields) > 0 {
 		err = f.docFetcher.Init(
 			ctx,
 			identity,
 			f.txn,
 			acp,
 			f.col,
 			f.docFields,
-			f.docFilter,
+			filter,
 			f.mapping,
 			false,
 			false,
@@ -119,14 +122,16 @@ outer:
 }
 
 func (f *IndexFetcher) Start(ctx context.Context, spans core.Spans) error {
-	err := f.indexIter.Init(ctx, f.txn.Datastore())
-	if err != nil {
-		return err
+	if f.indexIter == nil {
+		return f.docFetcher.Start(ctx, spans)
 	}
-	return nil
+	return f.indexIter.Init(ctx, f.txn.Datastore())
 }
 
 func (f *IndexFetcher) FetchNext(ctx context.Context) (EncodedDocument, ExecInfo, error) {
+	if f.indexIter == nil {
+		return f.docFetcher.FetchNext(ctx)
+	}
 	totalExecInfo := f.execInfo
 	defer func() { f.execInfo.Add(totalExecInfo) }()
 	f.execInfo.Reset()
@@ -176,7 +181,7 @@ func (f *IndexFetcher) FetchNext(ctx context.Context) (EncodedDocument, ExecInfo
 			}
 		}
 
-		if f.docFetcher != nil && len(f.docFields) > 0 {
+		if len(f.docFields) > 0 {
 			targetKey := base.MakeDataStoreKeyWithCollectionAndDocID(f.col.Description(), string(f.doc.id))
 			spans := core.NewSpans(core.NewSpan(targetKey, targetKey.PrefixEnd()))
 			err := f.docFetcher.Start(ctx, spans)
@@ -204,10 +209,10 @@ func (f *IndexFetcher) FetchNext(ctx context.Context) (EncodedDocument, ExecInfo
 }
 
 func (f *IndexFetcher) Close() error {
-	if f.indexIter != nil {
-		return f.indexIter.Close()
+	if f.indexIter == nil {
+		return f.docFetcher.Close()
 	}
-	return nil
+	return f.indexIter.Close()
 }
 
 // resetState resets the mutable state of this IndexFetcher, returning the state to how it
@@ -217,7 +222,6 @@ func (f *IndexFetcher) resetState() {
 
 	f.col = nil
 	f.txn = nil
-	f.docFilter = nil
 	f.doc = nil
 	f.mapping = nil
 	f.indexedFields = nil

internal/db/fetcher/indexer_iterators.go

@@ -538,6 +538,11 @@ func (f *IndexFetcher) createIndexIterator() (indexIterator, error) {
 		return nil, err
 	}
 
+	// this can happen if a query contains an empty condition like User(filter: {name: {}})
+	if len(fieldConditions) == 0 {
+		return nil, nil
+	}
+
 	matchers, err := createValueMatchers(fieldConditions)
 	if err != nil {
 		return nil, err

tests/integration/index/query_with_index_only_filter_test.go

@@ -677,3 +677,44 @@ func TestQueryWithIndex_WithNotLikeFilter_ShouldFetch(t *testing.T) {
 
 	testUtils.ExecuteTestCase(t, test)
 }
+
+func TestQueryWithIndex_EmptyFilterOnIndexedField_ShouldSucceed(t *testing.T) {
+	test := testUtils.TestCase{
+		Actions: []any{
+			testUtils.SchemaUpdate{
+				Schema: `
+					type User {
+						name: String @index
+						age: Int 
+					}`,
+			},
+			testUtils.CreateDoc{
+				Doc: `{
+					"name": "Islam",
+					"age": 33
+				}`,
+			},
+			testUtils.CreateDoc{
+				Doc: `{
+					"name": "John",
+					"age": 21
+				}`,
+			},
+			testUtils.Request{
+				Request: `query {
+					User(filter: {name: {}}) {
+						name
+					}
+				}`,
+				Results: map[string]any{
+					"User": []map[string]any{
+						{"name": "Islam"},
+						{"name": "John"},
+					},
+				},
+			},
+		},
+	}
+
+	testUtils.ExecuteTestCase(t, test)
+}