feat: Don't ignore receive_imf_inner() errors, try adding partially downloaded message instead (#7196)

Ignoring `receive_imf_inner()` errors, i.e. silently skipping messages on failures, leads to bugs
never fixed. As for temporary I/O errors, ignoring them leads to lost messages, in this case it's
better to bubble up the error and get the IMAP loop stuck. However if there's some logic error, it's
better to show it to the user so that it's more likely reported, and continue receiving messages. To
distinguish these cases, on error, try adding the message as partially downloaded with the error set
to `msgs.error`, this way the user also can retry downloading the message to finally see it if the
problem is fixed.

deltachat-rpc-client/tests/test_something.py

@@ -331,6 +331,52 @@ def test_message(acfactory) -> None:
     assert reactions == snapshot.reactions
 
 
+def test_receive_imf_failure(acfactory) -> None:
+    alice, bob = acfactory.get_online_accounts(2)
+    alice_contact_bob = alice.create_contact(bob, "Bob")
+    alice_chat_bob = alice_contact_bob.create_chat()
+
+    bob.set_config("fail_on_receiving_full_msg", "1")
+    alice_chat_bob.send_text("Hello!")
+    event = bob.wait_for_incoming_msg_event()
+    chat_id = event.chat_id
+    msg_id = event.msg_id
+    message = bob.get_message_by_id(msg_id)
+    snapshot = message.get_snapshot()
+    assert snapshot.chat_id == chat_id
+    assert snapshot.download_state == DownloadState.AVAILABLE
+    assert snapshot.error is not None
+    assert snapshot.show_padlock
+
+    # The failed message doesn't break the IMAP loop.
+    bob.set_config("fail_on_receiving_full_msg", "0")
+    alice_chat_bob.send_text("Hello again!")
+    event = bob.wait_for_incoming_msg_event()
+    assert event.chat_id == chat_id
+    msg_id = event.msg_id
+    message1 = bob.get_message_by_id(msg_id)
+    snapshot = message1.get_snapshot()
+    assert snapshot.chat_id == chat_id
+    assert snapshot.download_state == DownloadState.DONE
+    assert snapshot.error is None
+
+    # The failed message can be re-downloaded later.
+    bob._rpc.download_full_message(bob.id, message.id)
+    event = bob.wait_for_event(EventType.MSGS_CHANGED)
+    message = bob.get_message_by_id(event.msg_id)
+    snapshot = message.get_snapshot()
+    assert snapshot.download_state == DownloadState.IN_PROGRESS
+    event = bob.wait_for_event(EventType.MSGS_CHANGED)
+    assert event.chat_id == chat_id
+    msg_id = event.msg_id
+    message = bob.get_message_by_id(msg_id)
+    snapshot = message.get_snapshot()
+    assert snapshot.chat_id == chat_id
+    assert snapshot.download_state == DownloadState.DONE
+    assert snapshot.error is None
+    assert snapshot.text == "Hello!"
+
+
 def test_selfavatar_sync(acfactory, data, log) -> None:
     alice = acfactory.get_online_account()