ext/pdo_pgsql: add failing regression test for GH-21869.

Pre-deallocating the server-side prepared statement before PDO's
destructor fires reproduces the data-loss path: DEALLOCATE errors
inside the transaction, COMMIT is silently converted to ROLLBACK,
and the row is not persisted.

Author: devnexen

ext/pdo_pgsql/tests/gh21869.phpt

@@ -0,0 +1,47 @@
+--TEST--
+GH-21869 pdo_pgsql: DEALLOCATE on statement destruct silently aborts enclosing transaction
+--EXTENSIONS--
+pdo
+pdo_pgsql
+--SKIPIF--
+<?php
+require __DIR__ . '/config.inc';
+require __DIR__ . '/../../../ext/pdo/tests/pdo_test.inc';
+PDOTest::skip();
+$pdo = PDOTest::factory();
+if (version_compare($pdo->getAttribute(PDO::ATTR_CLIENT_VERSION), '17', '>=')) {
+    die('skip libpq >= 17 uses PQclosePrepared, not the DEALLOCATE query path');
+}
+?>
+--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);
+$pdo->setAttribute(PDO::ATTR_EMULATE_PREPARES, false);
+
+$pdo->exec('CREATE TABLE gh21869 (a integer not null)');
+
+$pdo->beginTransaction();
+$stmt = $pdo->prepare('INSERT INTO gh21869 (a) VALUES (?)');
+$stmt->execute([1]);
+
+foreach ($pdo->query('SELECT name FROM pg_prepared_statements') as $row) {
+    $pdo->exec('DEALLOCATE ' . $row['name']);
+}
+
+unset($stmt);
+
+$pdo->commit();
+
+$count = (int) $pdo->query('SELECT count(*) FROM gh21869')->fetchColumn();
+var_dump($count);
+?>
+--CLEAN--
+<?php
+require __DIR__ . '/../../../ext/pdo/tests/pdo_test.inc';
+$pdo = PDOTest::test_factory(__DIR__ . '/common.phpt');
+$pdo->exec('DROP TABLE IF EXISTS gh21869');
+?>
+--EXPECT--
+int(1)