Fix GH-21683: Handle empty result set in single-row mode

ext/pdo_pgsql/pgsql_statement.c

@@ -573,6 +573,12 @@ static int pgsql_stmt_fetch(pdo_stmt_t *stmt,
 			}
 
 			S->result = PQgetResult(S->H->server);
+			if (!S->result) {
+				S->is_running_unbuffered = false;
+				stmt->row_count = 0;
+				S->current_row = 0;
+				return 0;
+			}
 			status = PQresultStatus(S->result);
 
 			if (status != PGRES_COMMAND_OK && status != PGRES_TUPLES_OK && status != PGRES_SINGLE_TUPLE) {

ext/pdo_pgsql/tests/gh21683.phpt

@@ -0,0 +1,48 @@
+--TEST--
+PDO PgSQL single-row mode (ATTR_PREFETCH=0) on empty result set
+--EXTENSIONS--
+pdo
+pdo_pgsql
+--SKIPIF--
+<?php
+require __DIR__ . '/config.inc';
+require __DIR__ . '/../../../ext/pdo/tests/pdo_test.inc';
+PDOTest::skip();
+?>
+--FILE--
+<?php
+
+require __DIR__ . '/../../../ext/pdo/tests/pdo_test.inc';
+$pdo = PDOTest::test_factory(__DIR__ . '/common.phpt');
+$pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
+$pdo->setAttribute(PDO::ATTR_PREFETCH, 0);
+
+$pdo->exec("CREATE TEMP TABLE empty_t (id int, val text)");
+
+echo "=== query / fetch ===\n";
+$stmt = $pdo->query("SELECT * FROM empty_t");
+var_dump($stmt->fetch());
+
+echo "=== prepare / fetchAll ===\n";
+$stmt = $pdo->prepare("SELECT * FROM empty_t");
+$stmt->execute();
+var_dump($stmt->fetchAll());
+
+echo "=== connection still works ===\n";
+$stmt = $pdo->query("SELECT 1 AS alive");
+var_dump($stmt->fetch(PDO::FETCH_ASSOC));
+
+echo "Done\n";
+?>
+--EXPECT--
+=== query / fetch ===
+bool(false)
+=== prepare / fetchAll ===
+array(0) {
+}
+=== connection still works ===
+array(1) {
+  ["alive"]=>
+  string(1) "1"
+}
+Done