Optimize commit_file_diffs by running xdiff::diff_unified in a differ…

…ent thread

Summary:
## This stack

Track and down methods that are holding up the reactor and optimize them.

## This diff

`xdiff::diff_unified` is synchronous and does a lot of work to format a diff. While we should optimize it, it is older code so that will realistically take a while. Running it on a separate, non-executor thread gets the job done.

Reviewed By: lmvasquezg

Differential Revision: D67025628

fbshipit-source-id: 1f946e8f7aabc0eb8f632a5724ed3a608ec6d877

eden/mononoke/mononoke_api/src/changeset_path_diff.rs

@@ -530,7 +530,9 @@ impl<R: MononokeRepo> ChangesetPathDiffContext<R> {
             copy_info,
         };
         // The base is the target, so we diff in the opposite direction.
-        let raw_diff = xdiff::diff_unified(other_file, base_file, opts);
+        let raw_diff =
+            tokio::task::spawn_blocking(move || xdiff::diff_unified(other_file, base_file, opts))
+                .await?;
         Ok(UnifiedDiff {
             raw_diff,
             is_binary,

eden/mononoke/mononoke_api/src/errors.rs

@@ -28,6 +28,7 @@ use mononoke_types::ChangesetId;
 use pushrebase::PushrebaseError;
 use repo_authorization::AuthorizationError;
 use thiserror::Error;
+use tokio::task::JoinError;
 
 #[derive(Clone, Debug)]
 pub struct InternalError(Arc<Error>);
@@ -213,3 +214,9 @@ impl From<CommitCloudError> for MononokeError {
         }
     }
 }
+
+impl From<JoinError> for MononokeError {
+    fn from(e: JoinError) -> Self {
+        MononokeError::from(anyhow::Error::from(e))
+    }
+}