Fix GH-20214: PDO::FETCH_DEFAULT unexpected behavior with PDOStatement::setFetchMode

When setFetchMode(PDO::FETCH_DEFAULT) is called, mode=0
(PDO_FETCH_USE_DEFAULT) gets stored as the statement's default fetch
type. Later, do_fetch() tries to resolve PDO_FETCH_USE_DEFAULT by
reading stmt->default_fetch_type, which is also 0 — circular
reference that on 8.4 silently fell through to FETCH_BOTH and on
master throws a ValueError.

Resolve PDO_FETCH_USE_DEFAULT to the connection-level default early in
pdo_stmt_setup_fetch_mode(), before flags extraction and the mode
switch, so the rest of the function processes the actual fetch mode.

Closes GH-20214

Author: iliaal

ext/pdo/pdo_stmt.c

@@ -1628,6 +1628,10 @@ bool pdo_stmt_setup_fetch_mode(pdo_stmt_t *stmt, zend_long mode, uint32_t mode_a
 
 	stmt->default_fetch_type = PDO_FETCH_BOTH;
 
+	if ((mode & ~PDO_FETCH_FLAGS) == PDO_FETCH_USE_DEFAULT) {
+		mode = stmt->dbh->default_fetch_type;
+	}
+
 	flags = mode & PDO_FETCH_FLAGS;
 
 	if (!pdo_verify_fetch_mode(stmt->default_fetch_type, mode, mode_arg_num, false)) {

ext/pdo/tests/gh20214.phpt

@@ -0,0 +1,38 @@
+--TEST--
+GH-20214 (PDO::FETCH_DEFAULT unexpected behavior with PDOStatement::setFetchMode)
+--EXTENSIONS--
+pdo_sqlite
+--FILE--
+<?php
+$db = new PDO("sqlite::memory:");
+$db->setAttribute(PDO::ATTR_DEFAULT_FETCH_MODE, PDO::FETCH_OBJ);
+
+// setFetchMode with FETCH_DEFAULT should use connection default (FETCH_OBJ)
+$stmt = $db->query("SELECT 'v1' AS c1, 'v2' AS c2");
+$stmt->setFetchMode(PDO::FETCH_DEFAULT);
+$row = $stmt->fetch();
+var_dump($row instanceof stdClass);
+var_dump($row->c1);
+
+// fetch with FETCH_DEFAULT should also use connection default
+$stmt = $db->query("SELECT 'v1' AS c1, 'v2' AS c2");
+$row = $stmt->fetch(PDO::FETCH_DEFAULT);
+var_dump($row instanceof stdClass);
+
+// fetchAll with FETCH_DEFAULT should also use connection default
+$stmt = $db->query("SELECT 'v1' AS c1, 'v2' AS c2");
+$rows = $stmt->fetchAll(PDO::FETCH_DEFAULT);
+var_dump($rows[0] instanceof stdClass);
+
+// setFetchMode then fetch without argument
+$stmt = $db->query("SELECT 'v1' AS c1, 'v2' AS c2");
+$stmt->setFetchMode(PDO::FETCH_DEFAULT);
+$row = $stmt->fetch();
+var_dump($row instanceof stdClass);
+?>
+--EXPECT--
+bool(true)
+string(2) "v1"
+bool(true)
+bool(true)
+bool(true)