From 265fcc5fc25d65afb33fda480c603f1e974e374e Mon Sep 17 00:00:00 2001
From: Serhiy Storchaka <storchaka@gmail.com>
Date: Tue, 29 Aug 2017 15:47:44 +0300
Subject: [PATCH] bpo-31286, bpo-30024: Fixed stack usage in absolute imports
 with (#3217)

binding a submodule to a name.
---
 Lib/test/test_import/__init__.py | 17 +++++++++++++++++
 Python/compile.c                 | 26 ++++++++++++++++----------
 2 files changed, 33 insertions(+), 10 deletions(-)

diff --git a/Lib/test/test_import/__init__.py b/Lib/test/test_import/__init__.py
index 1b04de24dd1..ddef27d9cd4 100644
--- a/Lib/test/test_import/__init__.py
+++ b/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"
diff --git a/Python/compile.c b/Python/compile.c
index 78e797a2c33..e547c2fd591 100644
--- a/Python/compile.c
+++ b/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,
-                                        PyUnicode_GET_LENGTH(name), 1);
+    Py_ssize_t len = PyUnicode_GET_LENGTH(name);
+    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 */
-        Py_ssize_t pos = dot + 1;
-        while (dot != -1) {
+        while (1) {
+            Py_ssize_t pos = dot + 1;
             PyObject *attr;
-            dot = PyUnicode_FindChar(name, '.', pos,
-                                     PyUnicode_GET_LENGTH(name), 1);
+            dot = PyUnicode_FindChar(name, '.', pos, len, 1);
             if (dot == -2)
                 return 0;
-            attr = PyUnicode_Substring(name, pos,
-                                       (dot != -1) ? dot :
-                                       PyUnicode_GET_LENGTH(name));
+            attr = PyUnicode_Substring(name, pos, (dot != -1) ? dot : len);
             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);
 }