Polish the code

- Reject user ids containing ANSI control characters when creating a new
  signing key.
- Prevent adding certificates twice when querying online sources if they
  contain multiple encryption keys.
- Dedup introducers if there are multiple authenticating paths.
- When listing locals, skip the signing key.
- Check if a mail can be encrypted at all due to available certificates.
  If not tell the MTA that the processing is done.
- Make `Introducer` comparable (`PartialEq`, `Eq` and `Hash`).
- minor polishing

src/mail.rs

@@ -19,19 +19,20 @@ use crate::types::errors::HuskError;
 use crate::types::recipient::Recipient;
 
 /// Parse the user id fields from a string.
-pub fn to_userid(rcpt: &CString) -> Option<UserID> {
-    match rcpt.to_str() {
-        Ok(s) => Some(UserID::from(s)),
-        _ => None
-    }
+pub fn to_userid(rcpt: &CString) -> Result<UserID> {
+    let rcpt = rcpt.to_str()?;
+    Ok(UserID::from(rcpt))
 }
 
 /// Get the email from the string representation of a user id.
-pub fn to_email(rcpt: &CString) -> Option<String> {
-    if let Some(u) = to_userid(rcpt) {
-       if let Ok(Some(e)) = u.email() { Some(e.to_string()) }
-       else { None }
-    } else { None }
+pub fn to_email(rcpt: &CString) -> Result<String> {
+    let u = to_userid(rcpt)?;
+
+    if let Some(e) = u.email()? {
+        Ok(e.to_string())
+    } else {
+        Err(HuskError::CannotConvertToEmail(u.to_string()).into())
+    }
 }
 
 /// Checks if the parameter has a valid email format.
@@ -78,7 +79,7 @@ pub async fn smtp_inject(from: String, to: &Vec<Recipient<'_>>, header: CHeader,
 
     // convert addresses
     let from_email = CString::new(from.clone())?;
-    let from_email = to_email(&from_email).unwrap(); // XXX
+    let from_email = to_email(&from_email)?;
     let from_email: Address = from_email.try_into()?;
 
     let mut rcpts: Vec<Address> = Vec::new();
@@ -87,7 +88,8 @@ pub async fn smtp_inject(from: String, to: &Vec<Recipient<'_>>, header: CHeader,
         rcpts.push(rcpt.try_into()?);
     }
 
-    let my_id = ClientId::Domain(defaults::HELO_REINJECT_ID.to_owned()); // TODO: add secret token?
+    // XXX: add secret token?
+    let my_id = ClientId::Domain(defaults::HELO_REINJECT_ID.to_owned());
     joinset.spawn(async move {
 
         // XXX