Possible Race Condition when editing a post

bug
1

Problem

Concurrent edits to one wiki page can lead to unexpected behavior. In my case, the quoted_posts relationship between two post becomes wrong.

This may leads to other problems when quoted_posts is used, like in User#refresh_avatar. The missing quoted_posts makes it unable to rebake all quoted posts.

Possible Cause

The warning introduced by this commit is not enough to safeguard concurrent edit to a post, as the check is not atomic. This is a rare situation but it still can happen.

PostRevisor#revise! calls QuotedPost.extract_from(@post) , which delete all quoted_posts if there are no any quotes in a post. When two edit calls PostRevisor#revise! simultaneously, with one edit to add a quoted post and another to add a quoted post, the post content could end up with the edit that has quotes and the quoted_post relationship destroyed by the edit without any quotes.

Possible Fix

Maybe we should consider acquiring a Redis lock when editing a post.

3 Likes
2

This is possible I guess and I am not strongly against adding a distributed lock here. The tricky thing is that I don’t want to risk any deadlocks so we would have to be extremely careful.

If concurrent editing is really your thing why not try shared edits?

3 Likes