bpo-31286, bpo-30024: Fixed stack usage in absolute imports with (#3217)

binding a submodule to a name.

Lib/test/test_import/__init__.py

@@ -237,6 +237,23 @@ class ImportTests(unittest.TestCase):
         import test.support as y
         self.assertIs(y, test.support, y.__name__)
 
+    def test_issue31286(self):
+        # import in a 'finally' block resulted in SystemError
+        try:
+            x = ...
+        finally:
+            import test.support.script_helper as x
+
+        # import in a 'while' loop resulted in stack overflow
+        i = 0
+        while i < 10:
+            import test.support.script_helper as x
+            i += 1
+
+        # import in a 'for' loop resulted in segmentation fault
+        for i in range(2):
+            import test.support.script_helper as x
+
     def test_failing_reload(self):
         # A failing reload should leave the module object in sys.modules.
         source = TESTFN + os.extsep + "py"

Python/compile.c

@@ -2673,28 +2673,34 @@ compiler_import_as(struct compiler *c, identifier name, identifier asname)
        If there is a dot in name, we need to split it and emit a
        IMPORT_FROM for each name.
     */
-    Py_ssize_t dot = PyUnicode_FindChar(name, '.', 0,
+    Py_ssize_t len = PyUnicode_GET_LENGTH(name);
-                                        PyUnicode_GET_LENGTH(name), 1);
+    Py_ssize_t dot = PyUnicode_FindChar(name, '.', 0, len, 1);
     if (dot == -2)
         return 0;
     if (dot != -1) {
         /* Consume the base module name to get the first attribute */
+        while (1) {
             Py_ssize_t pos = dot + 1;
-        while (dot != -1) {
             PyObject *attr;
-            dot = PyUnicode_FindChar(name, '.', pos,
+            dot = PyUnicode_FindChar(name, '.', pos, len, 1);
-                                     PyUnicode_GET_LENGTH(name), 1);
             if (dot == -2)
                 return 0;
-            attr = PyUnicode_Substring(name, pos,
+            attr = PyUnicode_Substring(name, pos, (dot != -1) ? dot : len);
-                                       (dot != -1) ? dot :
-                                       PyUnicode_GET_LENGTH(name));
             if (!attr)
                 return 0;
             ADDOP_O(c, IMPORT_FROM, attr, names);
             Py_DECREF(attr);
-            pos = dot + 1;
+            if (dot == -1) {
+                break;
             }
+            ADDOP(c, ROT_TWO);
+            ADDOP(c, POP_TOP);
+        }
+        if (!compiler_nameop(c, asname, Store)) {
+            return 0;
+        }
+        ADDOP(c, POP_TOP);
+        return 1;
     }
     return compiler_nameop(c, asname, Store);
 }