py/parse: Always free lexer even if an exception is raised.

Fixes issue #3843.

Signed-off-by: Damien George <damien@micropython.org>

py/parse.c

@@ -1023,6 +1023,9 @@ STATIC void push_result_rule(parser_t *parser, size_t src_line, uint8_t rule_id,
 }
 
 mp_parse_tree_t mp_parse(mp_lexer_t *lex, mp_parse_input_kind_t input_kind) {
+    // Set exception handler to free the lexer if an exception is raised.
+    MP_DEFINE_NLR_JUMP_CALLBACK_FUNCTION_1(ctx, mp_lexer_free, lex);
+    nlr_push_jump_callback(&ctx.callback, mp_call_function_1_from_nlr_jump_callback);
 
     // initialise parser and allocate memory for its stacks
 
@@ -1370,8 +1373,8 @@ mp_parse_tree_t mp_parse(mp_lexer_t *lex, mp_parse_input_kind_t input_kind) {
     m_del(rule_stack_t, parser.rule_stack, parser.rule_stack_alloc);
     m_del(mp_parse_node_t, parser.result_stack, parser.result_stack_alloc);
 
-    // we also free the lexer on behalf of the caller
-    mp_lexer_free(lex);
+    // Deregister exception handler and free the lexer.
+    nlr_pop_jump_callback(true);
 
     return parser.tree;
 }

tests/extmod/vfs_userfs.py

@@ -68,6 +68,7 @@ user_files = {
     "/data.txt": b"some data in a text file",
     "/usermod1.py": b"print('in usermod1')\nimport usermod2",
     "/usermod2.py": b"print('in usermod2')",
+    "/usermod3.py": b"syntax error",
 }
 os.mount(UserFS(user_files), "/userfs")
 
@@ -79,6 +80,12 @@ print(f.read())
 sys.path.append("/userfs")
 import usermod1
 
+# import a .py file with a syntax error (file should be closed on error)
+try:
+    import usermod3
+except SyntaxError:
+    print("SyntaxError in usermod3")
+
 # unmount and undo path addition
 os.umount("/userfs")
 sys.path.pop()

tests/extmod/vfs_userfs.py.exp

@@ -10,3 +10,8 @@ stat /usermod2.py
 open /usermod2.py rb
 ioctl 4 0
 in usermod2
+stat /usermod3
+stat /usermod3.py
+open /usermod3.py rb
+ioctl 4 0
+SyntaxError in usermod3