Return statements inside of nested try-except statements lead to an incorrect...

Return statements inside of nested try-except statements lead to an incorrect control flow graph that did not take the outer 'finally' clauses into account.
Closes https://github.com/cython/cython/issues/3666

CHANGES.rst

@@ -8,6 +8,10 @@ Cython Changelog
 Bugs fixed
 ----------
 
+* Nested try-except statements with multiple ``return`` statements could crash
+  due to incorrect deletion of the ``except as`` target variable.
+  (Github issue #3666)
+
 * The ``@classmethod`` decorator no longer rejects unknown input from other decorators.
   Patch by David Woods.  (Github issue #3660)
 

Cython/Compiler/FlowControl.py

@@ -1198,8 +1198,6 @@ class ControlFlowAnalysis(CythonTransform):
         if self.flow.loops:
             self.flow.loops[-1].exceptions.append(descr)
         self.flow.block = body_block
-        ## XXX: Is it still required
-        body_block.add_child(entry_point)
         self.flow.nextblock()
         self._visit(node.body)
         self.flow.exceptions.pop()
@@ -1233,11 +1231,18 @@ class ControlFlowAnalysis(CythonTransform):
         self.mark_position(node)
         self.visitchildren(node)
 
-        for exception in self.flow.exceptions[::-1]:
-            if exception.finally_enter:
-                self.flow.block.add_child(exception.finally_enter)
-                if exception.finally_exit:
-                    exception.finally_exit.add_child(self.flow.exit_point)
+        outer_exception_handlers = iter(self.flow.exceptions[::-1])
+        for handler in outer_exception_handlers:
+            if handler.finally_enter:
+                self.flow.block.add_child(handler.finally_enter)
+                if handler.finally_exit:
+                    # 'return' goes to function exit, or to the next outer 'finally' clause
+                    exit_point = self.flow.exit_point
+                    for next_handler in outer_exception_handlers:
+                        if next_handler.finally_enter:
+                            exit_point = next_handler.finally_enter
+                            break
+                    handler.finally_exit.add_child(exit_point)
                 break
         else:
             if self.flow.block:

tests/run/cython3.pyx

@@ -258,6 +258,32 @@ def except_as_deletes_target_in_gen(x, a):
         yield 6
 
 
+def nested_except_gh3666(a=False, b=False):
+    """
+    >>> nested_except_gh3666()
+    'A'
+    >>> nested_except_gh3666(a=True)
+    'B-V'
+    >>> nested_except_gh3666(a=True, b=True)
+    'B-V-T'
+    """
+    try:
+        if a:
+            raise ValueError
+        return "A"
+    except TypeError as exc:
+        return "A-T"
+    except ValueError as exc:
+        try:
+            if b:
+                raise TypeError
+            return "B-V"
+        except ValueError as exc:
+            return "B-V-V"
+        except TypeError as exc:
+            return "B-V-T"
+
+
 ### Py3 feature tests
 
 def print_function(*args):