Ghost: "Someone else is editing" errors when no-one else is editing

Issue Summary

We’ve had a number of reports where “Saving failed! Someone else is editing this post.” errors are shown when it’s not possible that anyone else was editing the post.

So far we have no reliable reproduction case although it does appear to occur more frequently on remotely hosted instances than local development instances.

My best guess at how this could happen:

1. First save request is fired by the admin client and the server handling/response is delayed for some reason
2. A second save request is fired (this could be through Cmd-S, an autosave, or editing a PSM field)
3. Server handles/responds to the first request, updating the updated_at field
4. Server handles/responds to the second request which at this point has an old updated_at value triggering the collision warning

The above scenario is much more likely to happen on systems that have occasional slow responses.

Reproduction steps

I’ve been able to use these steps to reproduce locally after tracing a production error and examining the logs for collision errors:

1. Open a post in the editor
2. Switch to “Slow 3G” throttling in Web Inspector
3. Make a change to the post body
4. Fairly quickly open the PSM then check the “feature” or “page” checkboxes
5. 🤞 You’ll get a collision error

This is happening because the save routines in the post settings menu bypass the sequential saves in the main editor controller and so are prone to triggering the “best guess” scenario mentioned above.

TODO

• evaluate debug logs
• define a new save task in the editor base controller mixin that is exposed to the PSM component and replace all instances where the PSM saves the post model directly with the save task (https://github.com/TryGhost/Ghost-Admin/pull/901)
  • the new save task will be in addition to the current save task because the current task has additional publishing workflow requirements that the PSM inputs skip
  • the new save task will be part of the saveTasks group so that all save requests are queued to prevent collision errors