mailparsing: preserve supplemental Content-Type parameters on rebuild

When performing a rebuild, we'd build completely new RFC 2045/RFC 2047
mime headers.

However, sometimes there are "load bearing" additional parameters in
the source part that are important to preserve; for example, calendar
invites include a `method=REQUEST` parameter that is important to
pass through in order for the invitation to correctly interface with
the recipients calendar.

This commit adjusts the rebuild logic to merge in any parameters
that we didn't set in the basic rebuild.

crates/mailparsing/src/mimepart.rs

@@ -428,14 +428,59 @@ impl<'a> MimePart<'a> {
         };
 
         for hdr in self.headers.iter() {
-            // Skip rfc2045 associated headers; we already rebuilt
-            // those above
             let name = hdr.get_name();
-            if name.eq_ignore_ascii_case("Content-Type")
-                || name.eq_ignore_ascii_case("Content-Transfer-Encoding")
-                || name.eq_ignore_ascii_case("Content-Disposition")
-                || name.eq_ignore_ascii_case("Content-ID")
-            {
+            if name.eq_ignore_ascii_case("Content-ID") {
+                continue;
+            }
+
+            // Merge in any MimeParameters that we might otherwise have lost
+            // in the rebuild
+            if name.eq_ignore_ascii_case("Content-Type") {
+                if let Ok(params) = hdr.as_content_type() {
+                    let Some(mut dest) = rebuilt.headers_mut().content_type()? else {
+                        continue;
+                    };
+
+                    for (k, v) in params.parameter_map() {
+                        if dest.get(&k).is_none() {
+                            dest.set(&k, &v);
+                        }
+                    }
+
+                    rebuilt.headers_mut().set_content_type(dest)?;
+                }
+                continue;
+            }
+            if name.eq_ignore_ascii_case("Content-Transfer-Encoding") {
+                if let Ok(params) = hdr.as_content_transfer_encoding() {
+                    let Some(mut dest) = rebuilt.headers_mut().content_transfer_encoding()? else {
+                        continue;
+                    };
+
+                    for (k, v) in params.parameter_map() {
+                        if dest.get(&k).is_none() {
+                            dest.set(&k, &v);
+                        }
+                    }
+
+                    rebuilt.headers_mut().set_content_transfer_encoding(dest)?;
+                }
+                continue;
+            }
+            if name.eq_ignore_ascii_case("Content-Disposition") {
+                if let Ok(params) = hdr.as_content_disposition() {
+                    let Some(mut dest) = rebuilt.headers_mut().content_disposition()? else {
+                        continue;
+                    };
+
+                    for (k, v) in params.parameter_map() {
+                        if dest.get(&k).is_none() {
+                            dest.set(&k, &v);
+                        }
+                    }
+
+                    rebuilt.headers_mut().set_content_disposition(dest)?;
+                }
                 continue;
             }
 
@@ -1519,4 +1564,89 @@ Ok(
 
         assert_eq!(body, DecodedBody::Binary(expect.to_vec()));
     }
+
+    /// Validate that we don't lose supplemental mime parameters like:
+    /// `Content-Type: text/calendar; method=REQUEST`
+    #[test]
+    fn rebuild_invitation() {
+        let message = concat!(
+            "Subject: Test for events 2\r\n",
+            "Content-Type: multipart/mixed;\r\n",
+            " boundary=8a54d64d7ad7c04a084478052b36cbe1609b33bf3a41203aaee8dd642cd3\r\n",
+            "\r\n",
+            "--8a54d64d7ad7c04a084478052b36cbe1609b33bf3a41203aaee8dd642cd3\r\n",
+            "Content-Type: multipart/alternative;\r\n",
+            " boundary=a4e0aff9e05c7d94e2e13bd5590302f7802daac1e952c065207790d15a9f\r\n",
+            "\r\n",
+            "--a4e0aff9e05c7d94e2e13bd5590302f7802daac1e952c065207790d15a9f\r\n",
+            "Content-Transfer-Encoding: quoted-printable\r\n",
+            "Content-Type: text/plain; charset=UTF-8\r\n",
+            "\r\n",
+            "This is a test for calendar event invitation\r\n",
+            "--a4e0aff9e05c7d94e2e13bd5590302f7802daac1e952c065207790d15a9f\r\n",
+            "Content-Transfer-Encoding: quoted-printable\r\n",
+            "Content-Type: text/html; charset=UTF-8\r\n",
+            "\r\n",
+            "<p>This is a test for calendar event invitation</p>\r\n",
+            "--a4e0aff9e05c7d94e2e13bd5590302f7802daac1e952c065207790d15a9f--\r\n",
+            "\r\n",
+            "--8a54d64d7ad7c04a084478052b36cbe1609b33bf3a41203aaee8dd642cd3\r\n",
+            "Content-Disposition: inline; name=\"Invitation.ics\"\r\n",
+            "Content-Type: text/calendar; method=REQUEST; name=\"Invitation.ics\"\r\n",
+            "\r\n",
+            "Invitation\r\n",
+            "--8a54d64d7ad7c04a084478052b36cbe1609b33bf3a41203aaee8dd642cd3\r\n",
+            "Content-Disposition: attachment; filename=\"event.ics\"\r\n",
+            "Content-Type: application/ics\r\n",
+            "\r\n",
+            "Event\r\n",
+            "--8a54d64d7ad7c04a084478052b36cbe1609b33bf3a41203aaee8dd642cd3--\r\n",
+            "\r\n"
+        );
+
+        let part = MimePart::parse(message).unwrap();
+        let rebuilt = part.rebuild().unwrap();
+
+        k9::snapshot!(
+            rebuilt.to_message_string(),
+            r#"
+Content-Type: multipart/mixed;\r
+\tboundary="8a54d64d7ad7c04a084478052b36cbe1609b33bf3a41203aaee8dd642cd3"\r
+Subject: Test for events 2\r
+\r
+--8a54d64d7ad7c04a084478052b36cbe1609b33bf3a41203aaee8dd642cd3\r
+Content-Type: multipart/alternative;\r
+\tboundary="a4e0aff9e05c7d94e2e13bd5590302f7802daac1e952c065207790d15a9f"\r
+\r
+--a4e0aff9e05c7d94e2e13bd5590302f7802daac1e952c065207790d15a9f\r
+Content-Type: text/plain;\r
+\tcharset="us-ascii"\r
+\r
+This is a test for calendar event invitation\r
+--a4e0aff9e05c7d94e2e13bd5590302f7802daac1e952c065207790d15a9f\r
+Content-Type: text/html;\r
+\tcharset="us-ascii"\r
+\r
+<p>This is a test for calendar event invitation</p>\r
+--a4e0aff9e05c7d94e2e13bd5590302f7802daac1e952c065207790d15a9f--\r
+--8a54d64d7ad7c04a084478052b36cbe1609b33bf3a41203aaee8dd642cd3\r
+Content-Type: text/calendar;\r
+\tcharset="us-ascii";\r
+\tmethod="REQUEST";\r
+\tname="Invitation.ics"\r
+\r
+Invitation\r
+--8a54d64d7ad7c04a084478052b36cbe1609b33bf3a41203aaee8dd642cd3\r
+Content-Disposition: attachment;\r
+\tfilename="event.ics"\r
+Content-Type: application/ics;\r
+\tname="event.ics"\r
+Content-Transfer-Encoding: base64\r
+\r
+RXZlbnQNCg==\r
+--8a54d64d7ad7c04a084478052b36cbe1609b33bf3a41203aaee8dd642cd3--\r
+
+"#
+        );
+    }
 }