fix: prefer last part in multipart/alternative

This is recommended by RFC 2046
and does not require a separate loop
looking for multipart subpart.

src/mimeparser.rs

@@ -1072,47 +1072,35 @@ impl MimeMessage {
         )?
         .0;
         match (mimetype.type_(), mimetype.subtype().as_str()) {
-            /* Most times, multipart/alternative contains true alternatives
-            as text/plain and text/html.  If we find a multipart/mixed
-            inside multipart/alternative, we use this (happens eg in
-            apple mail: "plaintext" as an alternative to "html+PDF attachment") */
             (mime::MULTIPART, "alternative") => {
-                for cur_data in &mail.subparts {
+                // multipart/alternative is described in
-                    let mime_type = get_mime_type(
+                // <https://datatracker.ietf.org/doc/html/rfc2046#section-5.1.4>.
+                // Specification says that last part should be preferred,
+                // so we iterate over parts in reverse order.
+
+                // Search for plain text or multipart part.
+                //
+                // If we find a multipart inside multipart/alternative
+                // and it has usable subparts, we only parse multipart.
+                // This happens e.g. in Apple Mail:
+                // "plaintext" as an alternative to "html+PDF attachment".
+                for cur_data in mail.subparts.iter().rev() {
+                    let (mime_type, _viewtype) = get_mime_type(
                         cur_data,
                         &get_attachment_filename(context, cur_data)?,
                         self.has_chat_version(),
-                    )?
+                    )?;
-                    .0;
+
-                    if mime_type == "multipart/mixed" || mime_type == "multipart/related" {
+                    if mime_type == mime::TEXT_PLAIN || mime_type.type_() == mime::MULTIPART {
                         any_part_added = self
                             .parse_mime_recursive(context, cur_data, is_related)
                             .await?;
                         break;
                     }
                 }
+
                 if !any_part_added {
-                    /* search for text/plain and add this */
+                    for cur_part in mail.subparts.iter().rev() {
-                    for cur_data in &mail.subparts {
-                        if get_mime_type(
-                            cur_data,
-                            &get_attachment_filename(context, cur_data)?,
-                            self.has_chat_version(),
-                        )?
-                        .0
-                        .type_()
-                            == mime::TEXT
-                        {
-                            any_part_added = self
-                                .parse_mime_recursive(context, cur_data, is_related)
-                                .await?;
-                            break;
-                        }
-                    }
-                }
-                if !any_part_added {
-                    /* `text/plain` not found - use the first part */
-                    for cur_part in &mail.subparts {
                         if self
                             .parse_mime_recursive(context, cur_part, is_related)
                             .await?

src/mimeparser/mimeparser_tests.rs

@@ -2084,3 +2084,48 @@ async fn test_4k_image_stays_image() -> Result<()> {
     assert_eq!(msg.param.get_int(Param::Height).unwrap_or_default(), 2160);
     Ok(())
 }
+
+/// Tests that if multiple alternatives are available in multipart/alternative,
+/// the last one is preferred.
+///
+/// RFC 2046 says the last supported alternative should be preferred:
+/// <https://datatracker.ietf.org/doc/html/rfc2046#section-5.1.4>
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn prefer_last_alternative() {
+    let mut tcm = TestContextManager::new();
+    let context = &tcm.alice().await;
+    let raw = br#"From: Bob <bob@example.net>
+To: Alice <alice@example.org>
+Subject: Alternatives
+Date: Tue, 5 May 2020 01:23:45 +0000
+MIME-Version: 1.0
+Chat-Version: 1.0
+Content-Type: multipart/alternative; boundary="boundary"
+
+This is a multipart message in MIME format.
+
+--boundary
+Content-Type: text/plain; charset="us-ascii"
+Content-Transfer-Encoding: 7bit
+
+First alternative.
+--boundary
+Content-Type: text/plain; charset="us-ascii"
+Content-Transfer-Encoding: 7bit
+
+Second alternative.
+--boundary
+Content-Type: text/plain; charset="us-ascii"
+Content-Transfer-Encoding: 7bit
+
+Third alternative.
+--boundary--
+"#;
+
+    let message = MimeMessage::from_bytes(context, &raw[..], None)
+        .await
+        .unwrap();
+    assert_eq!(message.parts.len(), 1);
+    assert_eq!(message.parts[0].typ, Viewtype::Text);
+    assert_eq!(message.parts[0].msg, "Third alternative.");
+}