[PATCH proxmox] sendmail: conform more to mime Content-Disposition header and wrap lines

[Shannon Sterz <s.sterz@proxmox.com>]

the http and mime Content-Disposition headers function slightly
differently. for the http the following would be valid according to
rfc 6266:

Content-Disposition: attachment; filename="file.pdf";
    filename*=UTF-8''file.pdf

specifying multiple filename attributes like this isn't valid for mime
messages. according to rfc 2183 the above should be expressed like so
for mime messages (one variant, multiple are supported):

Content-Disposition: attachment;
    filename*0*=UTF-8''file.bin

this patch implements proper filename handling and also wraps
filenames in the content disposition and content type headers
appropriatelly to a mail being rejected due to having mime headers
that are too long.

Signed-off-by: Shannon Sterz <s.sterz@proxmox.com>
---

tested this with peat on the following muas:

* aerc
* thunderbird
* ios mail app
* canary mail app
* start mail web client
* gmail web client
* outlook web client

noticed this when peat suddenly struggled to send mails. thanks @ Stoiko
Ivanov for helping with analysing this issue.

 proxmox-sendmail/src/lib.rs | 131 +++++++++++++++++++++++++-----------
 1 file changed, 93 insertions(+), 38 deletions(-)

diff --git a/proxmox-sendmail/src/lib.rs b/proxmox-sendmail/src/lib.rs
index f5b04afd..4b686bd9 100644
--- a/proxmox-sendmail/src/lib.rs
+++ b/proxmox-sendmail/src/lib.rs
@@ -41,18 +41,19 @@ const RFC5987SET: &AsciiSet = &CONTROLS
     .add(b'{')
     .add(b'}');

+const DEFAULT_TEXT_WIDTH: usize = 72;
+
 /// base64 encode and hard-wrap the base64 encoded string every 72 characters. this improves
 /// compatibility.
-fn encode_base64_formatted<T: AsRef<[u8]>>(raw: T) -> String {
-    const TEXT_WIDTH: usize = 72;
-
+fn encode_base64_formatted<T: AsRef<[u8]>>(raw: T, text_width: Option<usize>) -> String {
     let encoded = proxmox_base64::encode(raw);
     let bytes = encoded.as_bytes();
+    let text_width = text_width.unwrap_or(DEFAULT_TEXT_WIDTH);

-    let lines = bytes.len().div_ceil(TEXT_WIDTH);
+    let lines = bytes.len().div_ceil(text_width);
     let mut out = Vec::with_capacity(bytes.len() + lines - 1); // account for 1 newline per line

-    for (line, chunk) in bytes.chunks(TEXT_WIDTH).enumerate() {
+    for (line, chunk) in bytes.chunks(text_width).enumerate() {
         out.extend_from_slice(chunk);
         if line + 1 != lines {
             // do not end last line with newline to give caller control over doing so.
@@ -64,6 +65,38 @@ fn encode_base64_formatted<T: AsRef<[u8]>>(raw: T) -> String {
     unsafe { String::from_utf8_unchecked(out) }
 }

+fn encode_filename_formatted(filename: &str) -> String {
+    let encoded = format!("{}", utf8_percent_encode(filename, RFC5987SET));
+    let bytes = encoded.as_bytes();
+
+    let prefix_len = "\tfilename*00*=".len();
+    // + 1 here to include a ";" per line
+    let lines = bytes.len().div_ceil(DEFAULT_TEXT_WIDTH - (prefix_len + 1));
+
+    // bytes + prefixes + plus ";\n" - 2 (the last line does not end in ";n")
+    let mut out = Vec::with_capacity(bytes.len() + prefix_len * lines + lines * 2 - 2);
+
+    for (line, chunk) in bytes
+        .chunks(DEFAULT_TEXT_WIDTH - (prefix_len + 1))
+        .enumerate()
+    {
+        out.extend_from_slice(format!("\tfilename*{line}*=").as_bytes());
+
+        if line == 0 {
+            out.extend_from_slice("UTF-8''".as_bytes());
+        }
+
+        out.extend_from_slice(chunk);
+        if line + 1 != lines {
+            out.push(b';');
+            out.push(b'\n')
+        }
+    }
+
+    // SAFETY: bytes are utf-8 since they are percent encoded.
+    unsafe { String::from_utf8_unchecked(out) }
+}
+
 struct Recipient {
     name: Option<String>,
     email: String,
@@ -104,28 +137,36 @@ impl Attachment<'_> {

         let mut attachment = String::new();

-        let encoded_filename = if self.filename.is_ascii() {
-            &self.filename
-        } else {
-            &format!("=?utf-8?B?{}?=", proxmox_base64::encode(&self.filename))
-        };
-
         let _ = writeln!(attachment, "\n--{file_boundary}");
-        let _ = writeln!(
-            attachment,
-            "Content-Type: {}; name=\"{encoded_filename}\"",
-            self.mime,
-        );

-        // both `filename` and `filename*` are included for additional compatibility
+        let _ = write!(attachment, "Content-Type: {};\n\tname=\"", self.mime);
+
+        if self.filename.is_ascii() {
+            let _ = write!(attachment, "{}", &self.filename);
+        } else {
+            let mut first = true;
+            for line in
+                encode_base64_formatted(&self.filename, Some(DEFAULT_TEXT_WIDTH - 14)).split('\n')
+            {
+                if first {
+                    let _ = write!(attachment, "=?utf-8?B?{line}?=");
+                    first = false;
+                } else {
+                    let _ = write!(attachment, "\n\t=?utf-8?B?{line}?=");
+                }
+            }
+        }
+
+        let _ = writeln!(attachment, "\"");
+
         let _ = writeln!(
             attachment,
-            "Content-Disposition: attachment; filename=\"{encoded_filename}\";\n\tfilename*=UTF-8''{}",
-            utf8_percent_encode(&self.filename, RFC5987SET)
+            "Content-Disposition: attachment;\n{}",
+            encode_filename_formatted(&self.filename)
         );

         attachment.push_str("Content-Transfer-Encoding: base64\n\n");
-        attachment.push_str(&encode_base64_formatted(self.content));
+        attachment.push_str(&encode_base64_formatted(self.content, None));

         attachment
     }
@@ -514,7 +555,7 @@ impl<'a> Mail<'a> {
             body.push_str(&self.body_txt);
         } else {
             body.push_str("Content-Transfer-Encoding: base64\n\n");
-            body.push_str(&encode_base64_formatted(&self.body_txt));
+            body.push_str(&encode_base64_formatted(&self.body_txt, None));
         }

         if let Some(html) = &self.body_html {
@@ -527,7 +568,7 @@ impl<'a> Mail<'a> {
                 body.push_str(html);
             } else {
                 body.push_str("Content-Transfer-Encoding: base64\n\n");
-                body.push_str(&encode_base64_formatted(html));
+                body.push_str(&encode_base64_formatted(html, None));
             }

             write!(body, "\n--{html_boundary}--")?;
@@ -809,9 +850,10 @@ Content-Transfer-Encoding: 7bit
 Lorem Ipsum Dolor Sit
 Amet
 ------_=_NextPart_001_1732806251
-Content-Type: application/octet-stream; name="deadbeef.bin"
-Content-Disposition: attachment; filename="deadbeef.bin";
-	filename*=UTF-8''deadbeef.bin
+Content-Type: application/octet-stream;
+	name="deadbeef.bin"
+Content-Disposition: attachment;
+	filename*0*=UTF-8''deadbeef.bin
 Content-Transfer-Encoding: base64

 3q2+796tvu/erb7v3q3erb7v3q2+796tvu/erd6tvu/erb7v3q2+796t3q2+796tvu/erb7v
@@ -838,7 +880,7 @@ Content-Transfer-Encoding: base64
         )
         .with_recipient_and_name("Receiver Name", "receiver@example.com")
         .with_attachment("deadbeef.bin", "application/octet-stream", &bin)
-        .with_attachment("🐄💀.bin", "image/bmp", &bin)
+        .with_attachment("🐄💀🐄💀🐄💀🐄💀🐄💀🐄💀🐄💀🐄💀🐄💀🐄💀🐄💀🐄💀🐄💀🐄💀🐄💀🐄💀.bin", "image/bmp", &bin)
         .with_html_alt("<html lang=\"de-at\"><head></head><body>\n\t<pre>\n\t\tLorem Ipsum Dolor Sit Amet\n\t</pre>\n</body></html>");

         let body = mail.format_mail(1732806251).expect("could not format mail");
@@ -879,17 +921,28 @@ Content-Transfer-Encoding: 7bit
 </body></html>
 ------_=_NextPart_002_1732806251--
 ------_=_NextPart_001_1732806251
-Content-Type: application/octet-stream; name="deadbeef.bin"
-Content-Disposition: attachment; filename="deadbeef.bin";
-	filename*=UTF-8''deadbeef.bin
+Content-Type: application/octet-stream;
+	name="deadbeef.bin"
+Content-Disposition: attachment;
+	filename*0*=UTF-8''deadbeef.bin
 Content-Transfer-Encoding: base64

 3q2+796tvu/erb7v3q3erb7v3q2+796tvu/erd6tvu/erb7v3q2+796t3q2+796tvu/erb7v
 3q2+796tvu8=
 ------_=_NextPart_001_1732806251
-Content-Type: image/bmp; name="=?utf-8?B?8J+QhPCfkoAuYmlu?="
-Content-Disposition: attachment; filename="=?utf-8?B?8J+QhPCfkoAuYmlu?=";
-	filename*=UTF-8''%F0%9F%90%84%F0%9F%92%80.bin
+Content-Type: image/bmp;
+	name="=?utf-8?B?8J+QhPCfkoDwn5CE8J+SgPCfkITwn5KA8J+QhPCfkoDwn5CE8J+SgPCfkI?=
+	=?utf-8?B?Twn5KA8J+QhPCfkoDwn5CE8J+SgPCfkITwn5KA8J+QhPCfkoDwn5CE8J+S?=
+	=?utf-8?B?gPCfkITwn5KA8J+QhPCfkoDwn5CE8J+SgPCfkITwn5KA8J+QhPCfkoAuYm?=
+	=?utf-8?B?lu?="
+Content-Disposition: attachment;
+	filename*0*=UTF-8''%F0%9F%90%84%F0%9F%92%80%F0%9F%90%84%F0%9F%92%80%F0%9F%90;
+	filename*1*=%84%F0%9F%92%80%F0%9F%90%84%F0%9F%92%80%F0%9F%90%84%F0%9F;
+	filename*2*=%92%80%F0%9F%90%84%F0%9F%92%80%F0%9F%90%84%F0%9F%92%80%F0;
+	filename*3*=%9F%90%84%F0%9F%92%80%F0%9F%90%84%F0%9F%92%80%F0%9F%90%84;
+	filename*4*=%F0%9F%92%80%F0%9F%90%84%F0%9F%92%80%F0%9F%90%84%F0%9F%92;
+	filename*5*=%80%F0%9F%90%84%F0%9F%92%80%F0%9F%90%84%F0%9F%92%80%F0%9F;
+	filename*6*=%90%84%F0%9F%92%80%F0%9F%90%84%F0%9F%92%80.bin
 Content-Transfer-Encoding: base64

 3q2+796tvu/erb7v3q3erb7v3q2+796tvu/erd6tvu/erb7v3q2+796t3q2+796tvu/erb7v
@@ -997,17 +1050,19 @@ PGh0bWwgbGFuZz0iZGUtYXQiPjxoZWFkPjwvaGVhZD48Ym9keT4KCTxwcmU+CgkJTG9yZW0g
 SXBzdW0gRMO2bG9yIFNpdCBBbWV0Cgk8L3ByZT4KPC9ib2R5PjwvaHRtbD4=
 ------_=_NextPart_002_1732806251--
 ------_=_NextPart_001_1732806251
-Content-Type: application/octet-stream; name="deadbeef.bin"
-Content-Disposition: attachment; filename="deadbeef.bin";
-	filename*=UTF-8''deadbeef.bin
+Content-Type: application/octet-stream;
+	name="deadbeef.bin"
+Content-Disposition: attachment;
+	filename*0*=UTF-8''deadbeef.bin
 Content-Transfer-Encoding: base64

 3q2+796tvu/erb7v3q3erb7v3q2+796tvu/erd6tvu/erb7v3q2+796t3q2+796tvu/erb7v
 3q2+796tvu8=
 ------_=_NextPart_001_1732806251
-Content-Type: image/bmp; name="=?utf-8?B?8J+QhPCfkoAuYmlu?="
-Content-Disposition: attachment; filename="=?utf-8?B?8J+QhPCfkoAuYmlu?=";
-	filename*=UTF-8''%F0%9F%90%84%F0%9F%92%80.bin
+Content-Type: image/bmp;
+	name="=?utf-8?B?8J+QhPCfkoAuYmlu?="
+Content-Disposition: attachment;
+	filename*0*=UTF-8''%F0%9F%90%84%F0%9F%92%80.bin
 Content-Transfer-Encoding: base64

 3q2+796tvu/erb7v3q3erb7v3q2+796tvu/erd6tvu/erb7v3q2+796t3q2+796tvu/erb7v
--
2.47.3