feat: Make it possible to leave broadcast channels (#6984)

Part of #6884.
The channel owner will not be notified in any way that you left, they
will only see that there is one member less.

For the translated stock strings, this is what we agreed on in the
group:
- Add a new "Leave Channel" stock string (will need to be done in UIs)
- Reword the existing "Are you sure you want to leave this group?"
string to "Are you sure you want to leave?" (the options are "Cancel"
and "Leave Group" / "Leave Channel", so it's clear what you are leaving)
(will need to be done in the deltachat-android repo, other UIs will pick
it up automatically)
- Reword the existing "You left the group." string to "You left". (done
here, I will adapt the strings in deltachat-android, too)

I adapted DC Android by pushing
6df2740884
to https://github.com/deltachat/deltachat-android/pull/3783.

---------

Co-authored-by: l <link2xt@testrun.org>

src/chat/chat_tests.rs

@@ -5,7 +5,7 @@ use crate::ephemeral::Timer;
 use crate::headerdef::HeaderDef;
 use crate::imex::{ImexMode, has_backup, imex};
 use crate::message::{MessengerMessage, delete_msgs};
-use crate::mimeparser;
+use crate::mimeparser::{self, MimeMessage};
 use crate::receive_imf::receive_imf;
 use crate::test_utils::{
     AVATAR_64x64_BYTES, AVATAR_64x64_DEDUPLICATED, TestContext, TestContextManager,
@@ -374,7 +374,10 @@ async fn test_member_add_remove() -> Result<()> {
     // Alice leaves the chat.
     remove_contact_from_chat(&alice, alice_chat_id, ContactId::SELF).await?;
     let sent = alice.pop_sent_msg().await;
-    assert_eq!(sent.load_from_db().await.get_text(), "You left the group.");
+    assert_eq!(
+        sent.load_from_db().await.get_text(),
+        stock_str::msg_group_left_local(&alice, ContactId::SELF).await
+    );
 
     Ok(())
 }
@@ -2931,6 +2934,108 @@ async fn test_broadcast_channel_protected_listid() -> Result<()> {
     Ok(())
 }
 
+/// Test that if Bob leaves a broadcast channel,
+/// Alice (the channel owner) won't see him as a member anymore,
+/// but won't be notified about this in any way.
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn test_leave_broadcast() -> Result<()> {
+    let mut tcm = TestContextManager::new();
+    let alice = &tcm.alice().await;
+    let bob = &tcm.bob().await;
+
+    tcm.section("Alice creates broadcast channel with Bob.");
+    let alice_chat_id = create_broadcast(alice, "foo".to_string()).await?;
+    let bob_contact = alice.add_or_lookup_contact(bob).await.id;
+    add_contact_to_chat(alice, alice_chat_id, bob_contact).await?;
+
+    tcm.section("Alice sends first message to broadcast.");
+    let sent_msg = alice.send_text(alice_chat_id, "Hello!").await;
+    let bob_msg = bob.recv_msg(&sent_msg).await;
+
+    assert_eq!(get_chat_contacts(alice, alice_chat_id).await?.len(), 1);
+
+    // Clear events so that we can later check
+    // that the 'Broadcast channel left' message didn't trigger IncomingMsg:
+    alice.evtracker.clear_events();
+
+    // Shift the time so that we can later check the "Broadcast channel left" message's timestamp:
+    SystemTime::shift(Duration::from_secs(60));
+
+    tcm.section("Bob leaves the broadcast channel.");
+    let bob_chat_id = bob_msg.chat_id;
+    bob_chat_id.accept(bob).await?;
+    remove_contact_from_chat(bob, bob_chat_id, ContactId::SELF).await?;
+
+    let leave_msg = bob.pop_sent_msg().await;
+    alice.recv_msg_trash(&leave_msg).await;
+
+    assert_eq!(get_chat_contacts(alice, alice_chat_id).await?.len(), 0);
+
+    alice.emit_event(EventType::Test);
+    alice
+        .evtracker
+        .get_matching(|ev| match ev {
+            EventType::Test => true,
+            EventType::IncomingMsg { .. } => {
+                panic!("'Broadcast channel left' message should be silent")
+            }
+            EventType::MsgsNoticed(..) => {
+                panic!("'Broadcast channel left' message shouldn't clear notifications")
+            }
+            EventType::MsgsChanged { .. } => {
+                panic!("Broadcast channels should be left silently, without any message");
+            }
+            _ => false,
+        })
+        .await;
+
+    Ok(())
+}
+
+/// Tests that if Bob leaves a broadcast channel with one device,
+/// the other device shows a correct info message "You left.".
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn test_leave_broadcast_multidevice() -> Result<()> {
+    let mut tcm = TestContextManager::new();
+    let alice = &tcm.alice().await;
+    let bob0 = &tcm.bob().await;
+    let bob1 = &tcm.bob().await;
+
+    tcm.section("Alice creates broadcast channel with Bob.");
+    let alice_chat_id = create_broadcast(alice, "foo".to_string()).await?;
+    let bob_contact = alice.add_or_lookup_contact(bob0).await.id;
+    add_contact_to_chat(alice, alice_chat_id, bob_contact).await?;
+
+    tcm.section("Alice sends first message to broadcast.");
+    let sent_msg = alice.send_text(alice_chat_id, "Hello!").await;
+    let bob0_hello = bob0.recv_msg(&sent_msg).await;
+    let bob1_hello = bob1.recv_msg(&sent_msg).await;
+
+    tcm.section("Bob leaves the broadcast channel with his first device.");
+    let bob_chat_id = bob0_hello.chat_id;
+    bob_chat_id.accept(bob0).await?;
+    remove_contact_from_chat(bob0, bob_chat_id, ContactId::SELF).await?;
+
+    let leave_msg = bob0.pop_sent_msg().await;
+    let parsed = MimeMessage::from_bytes(bob1, leave_msg.payload().as_bytes(), None).await?;
+    assert_eq!(
+        parsed.parts[0].msg,
+        stock_str::msg_group_left_remote(bob0).await
+    );
+
+    let rcvd = bob1.recv_msg(&leave_msg).await;
+
+    assert_eq!(rcvd.chat_id, bob1_hello.chat_id);
+    assert!(rcvd.is_info());
+    assert_eq!(rcvd.get_info_type(), SystemMessage::MemberRemovedFromGroup);
+    assert_eq!(
+        rcvd.text,
+        stock_str::msg_group_left_local(bob1, ContactId::SELF).await
+    );
+
+    Ok(())
+}
+
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn test_create_for_contact_with_blocked() -> Result<()> {
     let t = TestContext::new().await;