CollectOneRow prefers PostgreSQL error over pgx.ErrorNoRows

fixes https://github.com/jackc/pgx/issues/1334

rows.go

@@ -433,6 +433,9 @@ func CollectOneRow[T any](rows Rows, fn RowToFunc[T]) (T, error) {
 	var err error
 
 	if !rows.Next() {
+		if err = rows.Err(); err != nil {
+			return value, err
+		}
 		return value, ErrNoRows
 	}
 

rows_test.go

@@ -218,6 +218,35 @@ func TestCollectOneRowIgnoresExtraRows(t *testing.T) {
 	})
 }
 
+// https://github.com/jackc/pgx/issues/1334
+func TestCollectOneRowPrefersPostgreSQLErrorOverErrNoRows(t *testing.T) {
+	defaultConnTestRunner.RunTest(context.Background(), t, func(ctx context.Context, t testing.TB, conn *pgx.Conn) {
+		_, err := conn.Exec(ctx, `create temporary table t (name text not null unique)`)
+		require.NoError(t, err)
+
+		var name string
+		rows, _ := conn.Query(ctx, `insert into t (name) values ('foo') returning name`)
+		name, err = pgx.CollectOneRow(rows, func(row pgx.CollectableRow) (string, error) {
+			var n string
+			err := row.Scan(&n)
+			return n, err
+		})
+		require.NoError(t, err)
+		require.Equal(t, "foo", name)
+
+		rows, _ = conn.Query(ctx, `insert into t (name) values ('foo') returning name`)
+		name, err = pgx.CollectOneRow(rows, func(row pgx.CollectableRow) (string, error) {
+			var n string
+			err := row.Scan(&n)
+			return n, err
+		})
+		require.Error(t, err)
+		var pgErr *pgconn.PgError
+		require.ErrorAs(t, err, &pgErr)
+		require.Equal(t, "23505", pgErr.Code)
+	})
+}
+
 func TestRowTo(t *testing.T) {
 	defaultConnTestRunner.RunTest(context.Background(), t, func(ctx context.Context, t testing.TB, conn *pgx.Conn) {
 		rows, _ := conn.Query(ctx, `select n from generate_series(0, 99) n`)