feat: Assign message to ad-hoc group with matching name and members (#5385)

This should fix ad-hoc groups splitting when messages are fetched out of order from different
folders or otherwise reordered, or some messages are missing so that the messages reference chain is
broken, or a member was removed from the thread and readded later, etc. Even if this way two
different threads are merged, it looks acceptable, having many threads with the same name/subject
and members isn't a common use case.

src/receive_imf/tests.rs

@@ -204,6 +204,71 @@ async fn test_adhoc_group_show_all() {
     assert_eq!(chat::get_chat_contacts(&t, chat_id).await.unwrap().len(), 3);
 }
 
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn test_adhoc_groups_merge() -> Result<()> {
+    let mut tcm = TestContextManager::new();
+    let alice = &tcm.alice().await;
+    receive_imf(
+        alice,
+        b"From: bob@example.net\n\
+        To: alice@example.org, claire@example.com\n\
+        Message-ID: <1111@example.net>\n\
+        Date: Sun, 22 Mar 2020 22:37:57 +0000\n\
+        Subject: New thread\n\
+        \n\
+        The first of us should create a thread as discussed\n",
+        false,
+    )
+    .await?;
+    receive_imf(
+        alice,
+        b"From: alice@example.org\n\
+        To: bob@example.net, claire@example.com\n\
+        Message-ID: <2222@example.org>\n\
+        Date: Sun, 22 Mar 2020 22:37:58 +0000\n\
+        Subject: New thread\n\
+        \n\
+        The first of us should create a thread as discussed\n",
+        false,
+    )
+    .await?;
+    let chats = Chatlist::try_load(alice, 0, None, None).await?;
+    assert_eq!(chats.len(), 1);
+    let chat_id = chats.get_chat_id(0)?;
+    assert_eq!(chat_id.get_msg_cnt(alice).await?, 2);
+
+    // If member list doesn't match, threads aren't merged.
+    receive_imf(
+        alice,
+        b"From: bob@example.net\n\
+        To: alice@example.org, claire@example.com, fiona@example.net\n\
+        Message-ID: <3333@example.net>\n\
+        Date: Sun, 22 Mar 2020 22:37:57 +0000\n\
+        Subject: New thread\n\
+        \n\
+        This is another thread, with Fiona\n",
+        false,
+    )
+    .await?;
+    let chats = Chatlist::try_load(alice, 0, None, None).await?;
+    assert_eq!(chats.len(), 2);
+    receive_imf(
+        alice,
+        b"From: bob@example.net\n\
+        To: alice@example.org, fiona@example.net\n\
+        Message-ID: <4444@example.net>\n\
+        Date: Sun, 22 Mar 2020 22:37:57 +0000\n\
+        Subject: New thread\n\
+        \n\
+        This is yet another thread, with Fiona and 0 Claires\n",
+        false,
+    )
+    .await?;
+    let chats = Chatlist::try_load(alice, 0, None, None).await?;
+    assert_eq!(chats.len(), 3);
+    Ok(())
+}
+
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn test_read_receipt_and_unarchive() -> Result<()> {
     // create alice's account