feat: do not mark message as failed for which we got a read receipt before

this PR avoids marking a message as being failed if we already received
an read receipt for it.

background: esp. with multi relay it will easily happen that we get some
NDN for some relays. this does not mean the message at a whole was
failing, esp. if we got a read receipt (MDN) before

with https://github.com/chatmail/core/pull/8722 we do no longer process
NDN in groups, so the change of this PR affects one-to-one chats only

src/message.rs

@@ -1448,10 +1448,7 @@ impl MessageState {
     /// Returns true if the message can transition to `OutFailed` state from the current state.
     pub fn can_fail(self) -> bool {
         use MessageState::*;
-        matches!(
-            self,
-            OutPending | OutDelivered | OutMdnRcvd // OutMdnRcvd can still fail because it could be a group message and only some recipients failed.
-        )
+        matches!(self, OutPending | OutDelivered)
     }
 
     /// Returns true for any outgoing message states.

src/message/message_tests.rs

@@ -760,3 +760,23 @@ async fn test_get_existing_msg_ids() -> Result<()> {
 
     Ok(())
 }
+
+#[test]
+fn test_can_fail() -> Result<()> {
+    use MessageState::*;
+
+    // states that are not allowed to transition to OutFailed
+    assert!(!Undefined.can_fail());
+    assert!(!InFresh.can_fail());
+    assert!(!InNoticed.can_fail());
+    assert!(!InSeen.can_fail());
+    assert!(!OutDraft.can_fail());
+    assert!(!OutFailed.can_fail());
+    assert!(!OutMdnRcvd.can_fail());
+
+    // states that are allowed to transition to OutFailed
+    assert!(OutPending.can_fail());
+    assert!(OutDelivered.can_fail());
+
+    Ok(())
+}