fix: trash early MDNs that reference no message

A report referencing no message can never be applied to one,
so it must not create a contact, a chat or a `last_seen` update
on its way to the trash.

src/receive_imf.rs

@@ -506,6 +506,18 @@ pub(crate) async fn receive_imf_inner(
         Ok(mime_parser) => mime_parser,
     };
 
+    if !mime_parser.mdn_reports.is_empty()
+        && mime_parser.mdn_reports.iter().all(|report| {
+            report.original_message_id.is_none() && report.additional_message_ids.is_empty()
+        })
+    {
+        // A report naming no message can never be applied to one,
+        // and nothing else should come out of it: no contact, no chat,
+        // and no `last_seen` update lighting up an online dot.
+        info!(context, "Report without message reference (TRASH).");
+        return trash().await;
+    }
+
     if !mime_parser.was_encrypted()
         && mime_parser.get_header(HeaderDef::SecureJoin).is_none()
         && context.get_config_bool(Config::ForceEncryption).await?

src/receive_imf/receive_imf_tests.rs

@@ -271,6 +271,33 @@ async fn test_mdn_and_alias() -> Result<()> {
     Ok(())
 }
 
+/// Tests that an MDN referencing no message is trashed early:
+/// there is nothing it could ever be applied to,
+/// so it must not create a contact or a chat on the way.
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn test_mdn_without_message_reference() -> Result<()> {
+    let alice = TestContext::new_alice().await;
+    alice
+        .set_config_bool(Config::ForceEncryption, false)
+        .await?;
+    let contacts = Contact::get_real_cnt(&alice).await?;
+    let chatlist_len = Chatlist::try_load(&alice, 0, None, None).await?.len();
+
+    receive_imf(
+        &alice,
+        include_bytes!("../../test-data/message/mdn_without_message_reference.eml"),
+        false,
+    )
+    .await?;
+
+    assert_eq!(Contact::get_real_cnt(&alice).await?, contacts);
+    assert_eq!(
+        Chatlist::try_load(&alice, 0, None, None).await?.len(),
+        chatlist_len
+    );
+    Ok(())
+}
+
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn test_no_from() {
     // if there is no from given, from_id stays 0 which is just fine. These messages

test-data/message/mdn_without_message_reference.eml

@@ -0,0 +1,23 @@
+From: bob@example.net
+To: alice@example.org
+Subject: message opened
+Date: Sun, 22 Mar 2020 23:37:57 +0000
+Message-ID: <aranudiaerudiaduiaertd@example.com>
+Content-Type: multipart/report; report-type=disposition-notification; boundary="SNIPP"
+
+
+--SNIPP
+Content-Type: text/plain; charset=utf-8
+
+Read receipts do not guarantee sth. was read.
+
+
+--SNIPP
+Content-Type: message/disposition-notification
+
+Original-Recipient: rfc822;bob@example.net
+Final-Recipient: rfc822;bob@example.net
+Disposition: automatic-action/MDN-sent-automatically; processed
+
+
+--SNIPP--