Fix GH-21869: pdo_pgsql DEALLOCATE in destructor can poison the enclosing transaction.

ext/pdo_pgsql/pgsql_statement.c

@@ -79,9 +79,19 @@ static int pgsql_stmt_dtor(pdo_stmt_t *stmt)
 			// TODO (??) libpq does not support close statement protocol < postgres 17
 			// check if we can circumvent this.
 			char *q = NULL;
-			spprintf(&q, 0, "DEALLOCATE %s", S->stmt_name);
-			res = PQexec(H->server, q);
-			efree(q);
+			PGTransactionStatusType tstatus = PQtransactionStatus(H->server);
+
+			if (tstatus == PQTRANS_IDLE) {
+				spprintf(&q, 0, "DEALLOCATE %s", S->stmt_name);
+				res = PQexec(H->server, q);
+				efree(q);
+			} else {
+				/* Inside a transaction, already aborted, or connection is unusable:
+				 * skip DEALLOCATE. On servers that reject it (e.g. Aurora DSQL) it
+				 * would poison the transaction (GH-21869); on any server the server
+				 * frees the prepared statement when the transaction ends. */
+				res = NULL;
+			}
 #else
 			res = PQclosePrepared(H->server, S->stmt_name);
 #endif

ext/pdo_pgsql/tests/gh21869.phpt

@@ -0,0 +1,46 @@
+--TEST--
+GH-21869 pdo_pgsql: DEALLOCATE failure in destructor must not poison the 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 instead of the DEALLOCATE query');
+}
+?>
+--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();
+
+var_dump((int) $pdo->query('SELECT count(*) FROM gh21869')->fetchColumn());
+?>
+--CLEAN--
+<?php
+require __DIR__ . '/../../../ext/pdo/tests/pdo_test.inc';
+$pdo = PDOTest::test_factory(__DIR__ . '/common.phpt');
+$pdo->exec('DROP TABLE IF EXISTS gh21869');
+?>
+--EXPECT--
+int(1)