mimeparser: wrap try_decrypt() into block_in_place()

try_decrypt() is a CPU-bound task.
When called from async function,
it should be wrapped in tokio::task::spawn_blocking().
Using tokio::task::spawn_blocking() is difficult here
because of &mail, &private_keyring and &public_keyring borrows,
so we should at least use tokio::task::block_in_place()
to avoid blocking the executor.

CHANGELOG.md

@@ -6,6 +6,7 @@
 - Make smeared timestamp generation non-async. #4075
 
 ### Fixes
+- Do not block async task executor while decrypting the messages. #4079
 
 ### API-Changes
 

src/mimeparser.rs

@@ -256,8 +256,9 @@ impl MimeMessage {
         hop_info += &decryption_info.dkim_results.to_string();
 
         let public_keyring = keyring_from_peerstate(decryption_info.peerstate.as_ref());
-        let (mail, mut signatures, encrypted) =
+        let (mail, mut signatures, encrypted) = match tokio::task::block_in_place(|| {
-            match try_decrypt(context, &mail, &private_keyring, &public_keyring) {
+            try_decrypt(context, &mail, &private_keyring, &public_keyring)
+        }) {
             Ok(Some((raw, signatures))) => {
                 mail_raw = raw;
                 let decrypted_mail = mailparse::parse_mail(&mail_raw)?;