py: Handle case of return within the finally block of try-finally.

Addresses issue #1636.

py/vm.c

@@ -1032,6 +1032,14 @@ unwind_jump:;
 
                 ENTRY(MP_BC_RETURN_VALUE):
                     MARK_EXC_IP_SELECTIVE();
+                    // These next 3 lines pop a try-finally exception handler, if one
+                    // is there on the exception stack.  Without this the finally block
+                    // is executed a second time when the return is executed, because
+                    // the try-finally exception handler is still on the stack.
+                    // TODO Possibly find a better way to handle this case.
+                    if (currently_in_except_block) {
+                        POP_EXC_BLOCK();
+                    }
 unwind_return:
                     while (exc_sp >= exc_stack) {
                         if (MP_TAGPTR_TAG1(exc_sp->val_sp)) {

tests/basics/try_finally_return2.py

+# test 'return' within the finally block
+# it should swallow the exception
+
+# simple case
+def f():
+    try:
+        raise ValueError()
+    finally:
+        print('finally')
+        return 0
+    print('got here')
+print(f())
+
+# nested, return in outer
+def f():
+    try:
+        try:
+            raise ValueError
+        finally:
+            print('finally 1')
+        print('got here')
+    finally:
+        print('finally 2')
+        return 2
+    print('got here')
+print(f())
+
+# nested, return in inner
+def f():
+    try:
+        try:
+            raise ValueError
+        finally:
+            print('finally 1')
+            return 1
+        print('got here')
+    finally:
+        print('finally 2')
+    print('got here')
+print(f())
+
+# nested, return in inner and outer
+def f():
+    try:
+        try:
+            raise ValueError
+        finally:
+            print('finally 1')
+            return 1
+        print('got here')
+    finally:
+        print('finally 2')
+        return 2
+    print('got here')
+print(f())
+
+# nested with reraise
+def f():
+    try:
+        try:
+            raise ValueError
+        except:
+            raise
+        print('got here')
+    finally:
+        print('finally')
+        return 0
+    print('got here')
+print(f())
+
+# triple nesting with reraise
+def f():
+    try:
+        try:
+            try:
+                raise ValueError
+            except:
+                raise
+        except:
+            raise
+    finally:
+        print('finally')
+        return 0
+print(f())
+
+# exception when matching exception
+def f():
+    try:
+        raise ValueError
+    except NonExistingError:
+        pass
+    finally:
+        print('finally')
+        return 0
+print(f())
+
+# raising exception class, not instance
+def f():
+    try:
+        raise ValueError
+    finally:
+        print('finally')
+        return 0
+print(f())