Apply picks up stale plans

[giuli007]

In Atlantis plans are stored in pr-and-workspace-specific directories.

When using non-default workspaces plans for different projects of the same PR can be stored on a different copy of the repository on disk (if they use different workspaces from each other).
NB This is usually the case if the end user wants to enable parallel plan/applies (in fact it is common to generate a different atlantis workspace for each project in order to allow them to be run in parallel from the same PR, eg https://github.com/transcend-io/terragrunt-atlantis-config does that with the --create-workspace parameter).

If a PR that originally triggers n number of plans in Atlantis is subsequently updated it might happen that some of the plans are no longer relevant with regard to the latest changes.
However because they live in different directories Atlantis will only discard and update the plans for workspaces that are still affected by the latest changes, while plans for workspaces that are no longer affected by the changes in the PR will remain on disk.

This is a problem because when an atlantis apply is triggered it will pick up all the plans it can find for a specific PR and try to apply them, which can include things that were not meant to be applied by the end user (since the PR was updated to not include certain projects).

Here are a couple of screenshots to show the issue:

• original change

- amended change

- atlantis interaction

This is quite a surprising behaviour and it can cause real issues as there is no clue for an end user looking at the PR to guess that the old stale plans will be applied.

[giuli007]

I just found out that this is practically a duplicate of #1122.
both would be fixed by either #1633 or #1704 (alternative to each other)

[giuli007]

I think this issue should still be open since #1704 was reverted
#1633 is the most recent up-to-date fix for it
cc @jamengual