dolthub/dolt#9984: Fix nil dereference by Dispose() in GROUP BY iterator when Next() is never called

enginetest/queries/script_queries.go

@@ -122,6 +122,30 @@ type ScriptTestAssertion struct {
 // Unlike other engine tests, ScriptTests must be self-contained. No other tables are created outside the definition of
 // the tests.
 var ScriptTests = []ScriptTest{
+	{
+		// https://github.com/dolthub/dolt/issues/9987
+		Name: "GROUP BY nil pointer dereference in Dispose when Next() never called",
+		SetUpScript: []string{
+			"CREATE TABLE test_table (id INT PRIMARY KEY, value INT, category VARCHAR(50))",
+			"INSERT INTO test_table VALUES (1, 100, 'A'), (2, 200, 'B'), (3, 300, 'A')",
+		},
+		Assertions: []ScriptTestAssertion{
+			{
+				// LIMIT 0 causes the iterator to close without ever calling Next() on groupByIter
+				// This leaves all buffer elements as nil causing panic in Dispose(), or empty depending data struct
+				Query:    "SELECT category, SUM(value) FROM test_table GROUP BY category LIMIT 0",
+				Expected: []sql.Row{},
+			},
+			{
+				Query:    "SELECT category, COUNT(*) FROM test_table GROUP BY category LIMIT 0",
+				Expected: []sql.Row{},
+			},
+			{
+				Query:    "SELECT SUM(value) FROM test_table LIMIT 0",
+				Expected: []sql.Row{},
+			},
+		},
+	},
 	{
 		// https://github.com/dolthub/dolt/issues/9935
 		Dialect: "mysql",

sql/rowexec/agg.go

@@ -103,7 +103,9 @@ func (i *groupByIter) Close(ctx *sql.Context) error {
 
 func (i *groupByIter) Dispose() {
 	for _, b := range i.buf {
-		b.Dispose()
+		if b != nil {
+			b.Dispose()
+		}
 	}
 }