python: handle NULL value returned from dc_get_msg

Returning None in this case and checked with mypy that callers can handle it.

CHANGELOG.md

@@ -9,6 +9,8 @@
 
 ## Fixes
 - Fix Securejoin for multiple devices on a joining side #3982
+- python: handle NULL value returned from `dc_get_msg()` #4020
+  Account.`get_message_by_id` may return `None` in this case.
 
 ## API-Changes
 

python/src/deltachat/account.py

@@ -20,7 +20,6 @@ from .cutil import (
     from_optional_dc_charpointer,
     iter_array,
 )
-from .events import EventThread, FFIEventLogger
 from .message import Message
 from .tracker import ConfigureTracker, ImexTracker
 
@@ -63,6 +62,8 @@ class Account(object):
     MissingCredentials = MissingCredentials
 
     def __init__(self, db_path, os_name=None, logging=True, closed=False) -> None:
+        from .events import EventThread
+
         """initialize account object.
 
         :param db_path: a path to the account database. The database
@@ -368,7 +369,7 @@ class Account(object):
     def get_fresh_messages(self) -> Generator[Message, None, None]:
         """yield all fresh messages from all chats."""
         dc_array = ffi.gc(lib.dc_get_fresh_msgs(self._dc_context), lib.dc_array_unref)
-        yield from iter_array(dc_array, lambda x: Message.from_db(self, x))
+        return (x for x in iter_array(dc_array, lambda x: Message.from_db(self, x)) if x is not None)
 
     def create_chat(self, obj) -> Chat:
         """Create a 1:1 chat with Account, Contact or e-mail address."""
@@ -413,7 +414,7 @@ class Account(object):
     def get_device_chat(self) -> Chat:
         return Contact(self, const.DC_CONTACT_ID_DEVICE).create_chat()
 
-    def get_message_by_id(self, msg_id: int) -> Message:
+    def get_message_by_id(self, msg_id: int) -> Optional[Message]:
         """return Message instance.
         :param msg_id: integer id of this message.
         :returns: :class:`deltachat.message.Message` instance.
@@ -596,6 +597,8 @@ class Account(object):
     #
 
     def run_account(self, addr=None, password=None, account_plugins=None, show_ffi=False):
+        from .events import FFIEventLogger
+
         """get the account running, configure it if necessary. add plugins if provided.
 
         :param addr: the email address of the account

python/src/deltachat/chat.py

@@ -282,12 +282,20 @@ class Chat(object):
         if msg.is_out_preparing():
             assert msg.id != 0
             # get a fresh copy of dc_msg, the core needs it
-            msg = Message.from_db(self.account, msg.id)
+            maybe_msg = Message.from_db(self.account, msg.id)
+            if maybe_msg is not None:
+                msg = maybe_msg
+            else:
+                raise ValueError("message does not exist")
+
         sent_id = lib.dc_send_msg(self.account._dc_context, self.id, msg._dc_msg)
         if sent_id == 0:
             raise ValueError("message could not be sent")
         # modify message in place to avoid bad state for the caller
-        msg._dc_msg = Message.from_db(self.account, sent_id)._dc_msg
+        sent_msg = Message.from_db(self.account, sent_id)
+        if sent_msg is None:
+            raise ValueError("cannot load just sent message from the database")
+        msg._dc_msg = sent_msg._dc_msg
         return msg
 
     def send_text(self, text):

python/src/deltachat/events.py

@@ -13,6 +13,7 @@ from .capi import ffi, lib
 from .cutil import from_optional_dc_charpointer
 from .hookspec import account_hookimpl
 from .message import map_system_message
+from .account import Account
 
 
 def get_dc_event_name(integer, _DC_EVENTNAME_MAP={}):
@@ -221,7 +222,7 @@ class EventThread(threading.Thread):
     With each Account init this callback thread is started.
     """
 
-    def __init__(self, account) -> None:
+    def __init__(self, account: Account) -> None:
         self.account = account
         super(EventThread, self).__init__(name="events")
         self.daemon = True
@@ -298,20 +299,22 @@ class EventThread(threading.Thread):
                 yield "ac_configure_completed", {"success": success, "comment": comment}
         elif name == "DC_EVENT_INCOMING_MSG":
             msg = account.get_message_by_id(ffi_event.data2)
-            yield map_system_message(msg) or ("ac_incoming_message", {"message": msg})
+            if msg is not None:
+                yield map_system_message(msg) or ("ac_incoming_message", {"message": msg})
         elif name == "DC_EVENT_MSGS_CHANGED":
             if ffi_event.data2 != 0:
                 msg = account.get_message_by_id(ffi_event.data2)
-                if msg.is_outgoing():
+                if msg is not None:
-                    res = map_system_message(msg)
+                    if msg.is_outgoing():
-                    if res and res[0].startswith("ac_member"):
+                        res = map_system_message(msg)
-                        yield res
+                        if res and res[0].startswith("ac_member"):
-                    yield "ac_outgoing_message", {"message": msg}
+                            yield res
-                elif msg.is_in_fresh():
+                        yield "ac_outgoing_message", {"message": msg}
-                    yield map_system_message(msg) or (
+                    elif msg.is_in_fresh():
-                        "ac_incoming_message",
+                        yield map_system_message(msg) or (
-                        {"message": msg},
+                            "ac_incoming_message",
-                    )
+                            {"message": msg},
+                        )
         elif name == "DC_EVENT_REACTIONS_CHANGED":
             assert ffi_event.data1 > 0
             msg = account.get_message_by_id(ffi_event.data2)

python/src/deltachat/message.py

@@ -42,9 +42,15 @@ class Message(object):
         )
 
     @classmethod
-    def from_db(cls, account, id):
+    def from_db(cls, account, id) -> Optional["Message"]:
+        """Attempt to load the message from the database given its ID.
+
+        None is returned if the message does not exist, i.e. deleted."""
         assert id > 0
-        return cls(account, ffi.gc(lib.dc_get_msg(account._dc_context, id), lib.dc_msg_unref))
+        res = lib.dc_get_msg(account._dc_context, id)
+        if res == ffi.NULL:
+            return None
+        return cls(account, ffi.gc(res, lib.dc_msg_unref))
 
     @classmethod
     def new_empty(cls, account, view_type):