fix: Don't show wrong system message on Bob's second device

Before this, Bob's second device showed a system message
"⚠️ It seems you are using Delta Chat on multiple devices that cannot
decrypt each other's outgoing messages..."

src/chat.rs

@@ -2948,7 +2948,7 @@ async fn prepare_send_msg(
                 .get_bool(Param::ForcePlaintext)
                 .unwrap_or_default()
                 // V2 securejoin messages are symmetrically encrypted, no need for the public key:
-                || msg.securejoin_step() == Some("vb-request-with-auth")
+                || msg.is_vb_request_with_auth()
         }
         _ => false,
     };
@@ -3052,6 +3052,9 @@ pub(crate) async fn create_send_msg_jobs(context: &Context, msg: &mut Message) -
     if (context.get_config_bool(Config::BccSelf).await?
         || msg.param.get_cmd() == SystemMessage::AutocryptSetupMessage)
         && (context.get_config_delete_server_after().await? != Some(0) || !recipients.is_empty())
+        // `vb-request-with-auth` messages are symmetrically encrypted
+        // with a secret which the other device doesn't have:
+        && !msg.is_vb_request_with_auth()
     {
         recipients.push(from);
     }

src/chat/chat_tests.rs

@@ -3130,14 +3130,19 @@ async fn test_leave_broadcast_multidevice() -> Result<()> {
     let qr = get_securejoin_qr(alice, Some(alice_chat_id)).await.unwrap();
     join_securejoin(bob0, &qr).await.unwrap();
     let request = bob0.pop_sent_msg().await;
+
+    // Bob must send the message only to Alice, not to Self,
+    // because otherwise, his second device would show a device message
+    // "⚠️ It seems you are using Delta Chat on multiple devices that cannot decrypt each other's outgoing messages.
+    // To fix this, on the older device use \"Settings / Add Second Device\" and follow the instructions."
+    assert_eq!(request.recipients, "alice@example.org");
+
     alice.recv_msg_trash(&request).await;
     let answer = alice.pop_sent_msg().await;
     bob0.recv_msg(&answer).await;
 
     // Sync Bob's verification of Alice:
     sync(bob0, bob1).await;
-    // TODO uncommenting the next line creates a message "Can't decrypt outgoing messages, probably you're using DC on multiple devices without transferring your key"
-    // bob1.recv_msg(&request).await;
     bob1.recv_msg(&answer).await;
 
     // The 1:1 chat should not be visible to the user on any of the devices.

src/message.rs

@@ -1386,15 +1386,16 @@ impl Message {
         self.error.clone()
     }
 
-    // TODO this function could be used a lot more
-    /// If this is a secure-join message,
-    /// returns the current step,
-    /// which is put into the `Secure-Join` header.
-    pub(crate) fn securejoin_step(&self) -> Option<&str> {
+    /// Returns `true` if this message is a `vb-request-with-auth` SecureJoin message.
+    pub(crate) fn is_vb_request_with_auth(&self) -> bool {
         if self.param.get_cmd() == SystemMessage::SecurejoinMessage {
-            self.param.get(Param::Arg)
+            // CAVE: You can't check in the same way whether the message
+            // is a `v{g|b}-member-added` message,
+            // because for these messages,
+            // `param.get_cmd()` returns `SystemMessage::MemberAddedToGroup`.
+            self.param.get(Param::Arg) == Some("vb-request-with-auth")
         } else {
-            None
+            false
         }
     }
 }

src/mimefactory.rs

@@ -1916,8 +1916,7 @@ fn hidden_recipients() -> Address<'static> {
 }
 
 fn should_encrypt_with_auth_token(msg: &Message) -> bool {
-    msg.param.get_cmd() == SystemMessage::SecurejoinMessage
-        && msg.param.get(Param::Arg).unwrap_or_default() == "vb-request-with-auth"
+    msg.is_vb_request_with_auth()
 }
 
 fn should_encrypt_with_broadcast_secret(msg: &Message, chat: &Chat) -> bool {