fix: do not use square brackets error for unknown sender

If the sender of the message in protected group chat
is not a member of the chat, mark the sender name with `~`
as we do it in non-protected chats and set the error
instead of replacing the whole message with
"Unknown sender for this chat. See 'info' for more details."

To send a message to a protected group this way
the sender needs to know the group ID
and sign the message with the current verified key.
Usually this is just a late message
delivered shortly after the user has left
the group or was removed from it.

Replacing the message with a single error text part
as done before this change makes it impossible
to access anything other than text, such as attached images.

src/receive_imf.rs

@@ -636,14 +636,9 @@ async fn add_parts(
         if let Some(group_chat_id) = chat_id {
             if !chat::is_contact_in_chat(context, group_chat_id, from_id).await? {
                 let chat = Chat::load_from_db(context, group_chat_id).await?;
-                if chat.is_protected() {
+                if chat.is_protected() && chat.typ == Chattype::Single {
-                    if chat.typ == Chattype::Single {
+                    // Just assign the message to the 1:1 chat with the actual sender instead.
-                        // Just assign the message to the 1:1 chat with the actual sender instead.
+                    chat_id = None;
-                        chat_id = None;
-                    } else {
-                        let s = stock_str::unknown_sender_for_chat(context).await;
-                        mime_parser.replace_msg_by_error(&s);
-                    }
                 } else {
                     // In non-protected chats, just mark the sender as overridden. Therefore, the UI will prepend `~`
                     // to the sender's name, indicating to the user that he/she is not part of the group.
@@ -651,6 +646,12 @@ async fn add_parts(
                     let name: &str = from.display_name.as_ref().unwrap_or(&from.addr);
                     for part in &mut mime_parser.parts {
                         part.param.set(Param::OverrideSenderDisplayname, name);
+
+                        if chat.is_protected() {
+                            // In protected chat, also mark the message with an error.
+                            let s = stock_str::unknown_sender_for_chat(context).await;
+                            part.error = Some(s);
+                        }
                     }
                 }
             }

src/securejoin.rs

@@ -770,7 +770,7 @@ fn encrypted_and_signed(
 mod tests {
     use super::*;
     use crate::chat;
-    use crate::chat::ProtectionStatus;
+    use crate::chat::{remove_contact_from_chat, ProtectionStatus};
     use crate::chatlist::Chatlist;
     use crate::constants::Chattype;
     use crate::contact::ContactAddress;
@@ -1352,4 +1352,32 @@ First thread."#;
         assert!(get_securejoin_qr(&alice, Some(chat_id)).await.is_err());
         Ok(())
     }
+
+    #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+    async fn test_unknown_sender() -> Result<()> {
+        let mut tcm = TestContextManager::new();
+        let alice = tcm.alice().await;
+        let bob = tcm.bob().await;
+
+        tcm.execute_securejoin(&alice, &bob).await;
+
+        let alice_chat_id = alice
+            .create_group_with_members(ProtectionStatus::Protected, "Group with Bob", &[&bob])
+            .await;
+
+        let sent = alice.send_text(alice_chat_id, "Hi!").await;
+        let bob_chat_id = bob.recv_msg(&sent).await.chat_id;
+
+        let sent = bob.send_text(bob_chat_id, "Hi hi!").await;
+
+        let alice_bob_contact_id = Contact::create(&alice, "Bob", "bob@example.net").await?;
+        remove_contact_from_chat(&alice, alice_chat_id, alice_bob_contact_id).await?;
+
+        // The message from Bob is delivered late, Bob is already removed.
+        let msg = alice.recv_msg(&sent).await;
+        assert_eq!(msg.text, "Hi hi!");
+        assert_eq!(msg.error.unwrap(), "Unknown sender for this chat.");
+
+        Ok(())
+    }
 }

src/stock_str.rs

@@ -149,7 +149,7 @@ pub enum StockMessage {
                     however, of course, if they like, you may point them to 👉 https://get.delta.chat"))]
     WelcomeMessage = 71,
 
-    #[strum(props(fallback = "Unknown sender for this chat. See 'info' for more details."))]
+    #[strum(props(fallback = "Unknown sender for this chat."))]
     UnknownSenderForChat = 72,
 
     #[strum(props(fallback = "Message from %1$s"))]
@@ -930,7 +930,7 @@ pub(crate) async fn welcome_message(context: &Context) -> String {
     translated(context, StockMessage::WelcomeMessage).await
 }
 
-/// Stock string: `Unknown sender for this chat. See 'info' for more details.`.
+/// Stock string: `Unknown sender for this chat.`.
 pub(crate) async fn unknown_sender_for_chat(context: &Context) -> String {
     translated(context, StockMessage::UnknownSenderForChat).await
 }