bpo-36256: Fix bug in parsermodule when parsing if statements (GH-12488)

bpo-36256: Fix bug in parsermodule when parsing if statements

In the parser module, when validating nodes before starting the parsing with to create a ST in "parser_newstobject" there is a problem that appears when two arcs in the same DFA state has transitions with labels with the same type. For example, the DFA for if_stmt has a state with
two labels with the same type: "elif" and "else" (type NAME). The algorithm tries one by one the arcs until the label that starts the arc transition has a label with the same type of the current child label we are trying to accept. In this case, the arc for "elif" comes before the arc for "else"and passes this test (because the current child label is "else" and has the same type as "elif"). This lead to expecting a namedexpr_test (305) instead of a colon (11). The solution is to compare also the string representation (in case there is one) of the labels to see if the transition that we have is the correct one.
(cherry picked from commit 9a0000d15d27361eaa47b77600c7c00a9787a894)

Co-authored-by: Pablo Galindo <Pablogsal@gmail.com>

diff --git a/Lib/test/test_parser.py b/Lib/test/test_parser.py
index 274e26061a1991e2b890b2eb754b11eb8360ee94..94e454663573b35ac185abcde76e6cfc72522c18 100644 (file)
--- a/Lib/test/test_parser.py
+++ b/Lib/test/test_parser.py
@@ -318,6 +318,10 @@ class RoundtripLegalSyntaxTestCase(unittest.TestCase):
         self.check_suite("try: pass\nexcept: pass\nelse: pass\n"
                          "finally: pass\n")
 
+    def test_if_stmt(self):
+        self.check_suite("if True:\n  pass\nelse:\n  pass\n")
+        self.check_suite("if True:\n  pass\nelif True:\n  pass\nelse:\n  pass\n")
+
     def test_position(self):
         # An absolutely minimal test of position information.  Better
         # tests would be a big project.

diff --git a/Misc/NEWS.d/next/Core and Builtins/2019-03-21-00-24-18.bpo-12477.OZHa0t.rst b/Misc/NEWS.d/next/Core and Builtins/2019-03-21-00-24-18.bpo-12477.OZHa0t.rst
new file mode 100644 (file)
index 0000000..aada7f9
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2019-03-21-00-24-18.bpo-12477.OZHa0t.rst
@@ -0,0 +1,2 @@
+Fix bug in parsermodule when parsing a state in a DFA that has two or more
+arcs with labels of the same type. Patch by Pablo Galindo.

diff --git a/Modules/parsermodule.c b/Modules/parsermodule.c
index 38e5f750d5723d30742022db0a6ee1ea69d901f6..67c874267f2450a7d86aad81b5919cccb7ab32f3 100644 (file)
--- a/Modules/parsermodule.c
+++ b/Modules/parsermodule.c
@@ -666,7 +666,12 @@ validate_node(node *tree)
         for (arc = 0; arc < dfa_state->s_narcs; ++arc) {
             short a_label = dfa_state->s_arc[arc].a_lbl;
             assert(a_label < _PyParser_Grammar.g_ll.ll_nlabels);
-            if (_PyParser_Grammar.g_ll.ll_label[a_label].lb_type == ch_type) {
+
+            const char *label_str = _PyParser_Grammar.g_ll.ll_label[a_label].lb_str;
+            if ((_PyParser_Grammar.g_ll.ll_label[a_label].lb_type == ch_type)
+                && ((ch->n_str == NULL) || (label_str == NULL)
+                     || (strcmp(ch->n_str, label_str) == 0))
+               ) {
                 /* The child is acceptable; if non-terminal, validate it recursively. */
                 if (ISNONTERMINAL(ch_type) && !validate_node(ch))
                     return 0;
@@ -679,17 +684,24 @@ validate_node(node *tree)
         /* What would this state have accepted? */
         {
             short a_label = dfa_state->s_arc->a_lbl;
-            int next_type;
             if (!a_label) /* Wouldn't accept any more children */
                 goto illegal_num_children;
 
-            next_type = _PyParser_Grammar.g_ll.ll_label[a_label].lb_type;
-            if (ISNONTERMINAL(next_type))
+            int next_type = _PyParser_Grammar.g_ll.ll_label[a_label].lb_type;
+            const char *expected_str = _PyParser_Grammar.g_ll.ll_label[a_label].lb_str;
+
+            if (ISNONTERMINAL(next_type)) {
                 PyErr_Format(parser_error, "Expected node type %d, got %d.",
                              next_type, ch_type);
-            else
+            }
+            else if (expected_str != NULL) {
+                PyErr_Format(parser_error, "Illegal terminal: expected '%s'.",
+                             expected_str);
+            }
+            else {
                 PyErr_Format(parser_error, "Illegal terminal: expected %s.",
                              _PyParser_TokenNames[next_type]);
+            }
             return 0;
         }