fix(python): free allocated string pointers (#393)

* fix(python): free allocated string pointers

* Update python/src/deltachat/cutil.py

Co-Authored-By: holger krekel  <holger@merlinux.eu>

* Update cutil.py

* probably the proper way to handle ffi.strings

python/src/deltachat/__init__.py

@@ -34,13 +34,14 @@ def py_dc_callback(ctx, evt, data1, data2):
     if data1 and event_sig_types & 1:
         data1 = ffi.string(ffi.cast('char*', data1)).decode("utf8")
     if data2 and event_sig_types & 2:
+        data2 = ffi.string(ffi.cast('char*', data2)).decode("utf8")
         try:
-            data2 = ffi.string(ffi.cast('char*', data2)).decode("utf8")
+            if isinstance(data2, bytes):
+                data2 = data2.decode("utf8")
         except UnicodeDecodeError:
-            # XXX ignoring this error is not quite correct but for now
+            # XXX ignoring the decode error is not quite correct but for now
             # i don't want to hunt down encoding problems in the c lib
-            data2 = ffi.string(ffi.cast('char*', data2))
-
+            pass
     try:
         ret = callback(ctx, evt_name, data1, data2)
         if ret is None:

python/src/deltachat/cutil.py

@@ -16,4 +16,4 @@ def iter_array(dc_array_t, constructor):
 
 
 def from_dc_charpointer(obj):
-    return ffi.string(obj).decode("utf8")
+    return ffi.string(ffi.gc(obj, lib.dc_str_unref)).decode("utf8")

python/src/deltachat/message.py

@@ -142,7 +142,7 @@ class Message(object):
         import email.parser
         mime_headers = lib.dc_get_mime_headers(self._dc_context, self.id)
         if mime_headers:
-            s = ffi.string(mime_headers)
+            s = ffi.string(ffi.gc(mime_headers, lib.dc_str_unref))
             if isinstance(s, bytes):
                 s = s.decode("ascii")
             return email.message_from_string(s)