Add mail splitting

- If a mail has more then one recipient and not for all recipients an
  authenticated certificate can be obtained, the mail processing is
  split into 2 streams - one that encrypts and an other which sends the
  mail in plaintext.
- For both streams the header and the list of recipients is adjusted
  accordingly.
- As the milter interface only allows one stream, the second stream is
  reinjected into the MTA (via SMTP). The MTA is hardcoded to be
  localhost:25.

src/types/cheader.rs

@@ -5,6 +5,11 @@
 use std::ffi::CString;
 use std::fmt;
 
+use anyhow::Result;
+
+use crate::types::recipient::Recipient;
+
+#[derive(Debug, PartialEq, Clone)]
 pub struct CHeader {
     lines: Vec<(CString, CString)>
 }
@@ -52,6 +57,47 @@ impl CHeader {
     pub fn iter_lines(&self) -> std::slice::Iter<'_, (CString, CString)> {
         self.lines.iter()
     }
+
+    /// Remove a recipient from a value taken from a to, cc or bcc header
+    /// line. Only the email address is matched.
+    pub fn remove_recipient(header_value: &CString, to_remove: &Vec<Recipient>) -> Result<CString> {
+        let content = String::from_utf8_lossy(header_value.as_bytes());
+        let parts = content.split(",")
+            .filter(|part| {
+                let mut keep = true;
+                let p = part.to_ascii_lowercase();
+                for rcpt in to_remove {
+                    let rcpt = rcpt.email.to_ascii_lowercase();
+                    if p.contains(&rcpt) { keep = false; break; }
+                }
+                keep
+            })
+            .collect::<Vec<&str>>();
+        CString::new(parts.join(","))
+            .map_err(|e| e.into())
+    }
+
+    /// Returns a new header with `to_remove` removed from to, cc and bcc.
+    pub fn without_recipients(&self, to_remove: &Vec<Recipient>) -> CHeader {
+        let mut result = CHeader::new();
+
+        for (key, value) in self.iter_lines() {
+            let new_value = match key.as_bytes().to_ascii_lowercase().as_slice() {
+                b"to"
+                | b"cc"
+                | b"bcc" => Self::remove_recipient(value, to_remove),
+                _ => Ok(value.clone())
+            };
+            if let Ok(value) = new_value {
+                if ! value.is_empty() {
+                    result.append(key.clone(), value);
+                }
+            }
+        }
+
+        result
+    }
+
 }
 
 impl fmt::Display for CHeader {
@@ -75,10 +121,12 @@ impl fmt::Display for CHeader {
 #[cfg(test)]
 pub mod tests {
     use std::ffi::CString;
+    use crate::types::recipient::Recipient;
+
     use super::CHeader;
 
 
-#[test]
+    #[test]
     pub fn basic_functionality() {
         let cheader = CHeader::new()
             .add(CString::new("Content-Type").unwrap(), CString::new("text/plain").unwrap())
@@ -87,7 +135,7 @@ pub mod tests {
         assert_eq!("Content-Type: text/plain\r\nContent-Transfer-Encoding: 7bit\r\n", cheader.to_string());
     }
 
-#[test]
+    #[test]
     pub fn find() {
         let cheader = CHeader::new()
             .add(CString::new("Content-Type").unwrap(), CString::new("text/plain").unwrap())
@@ -103,7 +151,7 @@ pub mod tests {
         assert_eq!("text/plain", value);
     }
 
-#[test]
+    #[test]
     pub fn find_none() {
         let cheader = CHeader::new()
             .add(CString::new("Content-Type").unwrap(), CString::new("text/plain").unwrap())
@@ -114,7 +162,7 @@ pub mod tests {
         assert!(line.is_none());
     }
 
-#[test]
+    #[test]
     pub fn find_with_default() {
         let cheader = CHeader::new()
             .add(CString::new("Content-Type").unwrap(), CString::new("text/plain").unwrap())
@@ -124,5 +172,27 @@ pub mod tests {
 
         assert_eq!(value, "baaz");
     }
+
+    #[test]
+    pub fn remove_recipients() {
+        let header = CHeader::new()
+            .add(CString::new("To").unwrap(), CString::new("Alice <alice@example.com>, Bob <bob@other.org>").unwrap())
+            .add(CString::new("Cc").unwrap(), CString::new("carol@example.com").unwrap());
+
+        let modified_header = header.without_recipients(
+            &vec![ Recipient::from("bob@other.org".to_string()) ]
+        );
+        let expected_header = CHeader::new()
+            .add(CString::new("To").unwrap(), CString::new("Alice <alice@example.com>").unwrap())
+            .add(CString::new("Cc").unwrap(), CString::new("carol@example.com").unwrap());
+        assert_eq!(modified_header, expected_header);
+
+        let modified_header = header.without_recipients(
+            &vec![ Recipient::from("CaRol@exAmple.Com".to_string()) ]
+        );
+        let expected_header = CHeader::new()
+            .add(CString::new("To").unwrap(), CString::new("Alice <alice@example.com>, Bob <bob@other.org>").unwrap());
+        assert_eq!(modified_header, expected_header);
+    }
 }