Fix #2429 (message was downloaded multiple times a second), add test (#2430)

The problem was:

If a message has the \Deleted flag set, we ignore it. But we forgot to
update last_uid, so that uid_next was not updated at 47e639b777/src/imap.rs (L730) and the same message was
fetched over and over again.

Fix #2429

python/tests/test_account.py

@@ -1970,6 +1970,47 @@ class TestOnlineAccount:
         assert msg_back.chat == chat
         assert chat.get_profile_image() is None
 
+    def test_fetch_deleted_msg(self, acfactory, lp):
+        """This is a regression test: Messages with \\Deleted flag were downloaded again and again,
+        hundreds of times, because uid_next was not updated.
+
+        See https://github.com/deltachat/deltachat-core-rust/issues/2429.
+        """
+        ac1 = acfactory.get_one_online_account()
+        ac1.stop_io()
+
+        ac1.direct_imap.append("INBOX", """
+            From: alice <alice@example.org>
+            Subject: subj
+            To: bob@example.com
+            Chat-Version: 1.0
+            Message-ID: <aepiors@example.org>
+            Content-Type: text/plain; charset=utf-8
+
+            Deleted message
+        """)
+        ac1.direct_imap.delete("1:*", expunge=False)
+        ac1.start_io()
+
+        for ev in ac1._evtracker.iter_events():
+            if ev.name == "DC_EVENT_MSGS_CHANGED":
+                pytest.fail("A deleted message was shown to the user")
+
+            if ev.name == "DC_EVENT_INFO" and "1 mails read from" in ev.data2:
+                break
+
+        # The message was downloaded once, now check that it's not downloaded again
+
+        for ev in ac1._evtracker.iter_events():
+            if ev.name == "DC_EVENT_INFO" and "1 mails read from" in ev.data2:
+                pytest.fail("The same email was read twice")
+
+            if ev.name == "DC_EVENT_MSGS_CHANGED":
+                pytest.fail("A deleted message was shown to the user")
+
+            if ev.name == "DC_EVENT_INFO" and "INBOX: Idle entering wait-on-remote state" in ev.data2:
+                break  # DC is done with reading messages
+
     def test_send_receive_locations(self, acfactory, lp):
         now = datetime.utcnow()
         ac1, ac2 = acfactory.get_two_online_accounts()

src/imap.rs

@@ -925,7 +925,11 @@ impl Imap {
 
                 let is_deleted = msg.flags().any(|flag| flag == Flag::Deleted);
                 if is_deleted || msg.body().is_none() {
-                    // No need to process these.
+                    info!(
+                        context,
+                        "Not processing deleted or empty msg {}", server_uid
+                    );
+                    last_uid = Some(server_uid);
                     continue;
                 }