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fix: prefer last part in multipart/alternative

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This is recommended by RFC 2046
and does not require a separate loop
looking for multipart subpart.
This commit is contained in:
link2xt
2025-10-01 05:56:09 +00:00
parent 33a127187b
commit af69756df0
2 changed files with 63 additions and 30 deletions
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48
src/mimeparser.rs
View File

@@ -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 {
let mime_type = get_mime_type(
// multipart/alternative is described in
// <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_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 {
for cur_part in mail.subparts.iter().rev() {
if self
.parse_mime_recursive(context, cur_part, is_related)
.await?

45
src/mimeparser/mimeparser_tests.rs
View File

@@ -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.");
}