Labeler should never remove labels it has not configured

[silverwind]

Description:
The label addition is not atomic and there is a small time window after opening a PR where the action will unexpectedly remove user-added labels. Example from here:

I guess the API flow is like this:

• Labeler does a GET labels request
• User changes labels (which invalidates the labels seen in the GET)
• Labeler does a PUT labels request with previously seen labels, reverting the user's changes

Action version:
v5

Platform:

• All

Runner type:

• All

Repro steps:

1. Have a simple labeler action with this config
2. Open a PR
3. Immediatly set some labels on the PR
4. If the timing is right, the labeler will unexpectedly remove the user-added labels

Expected behavior:
The action should atomically update the labels or at least never removing labels it does not have in its config.

Actual behavior:
The action will remove user-set labels when set within a certain time window.

[HarithaVattikuti]

Hello @silverwind
Thank you for creating this issue. We will investigate it and get back to you as soon as we have some feedback.

[silverwind]

I reworded the PR because I think "atomic" is not the right term. Essentially it should:

• On first run of a PR, don't remove any labels, only add.
• On subsequent runs, never remove labels that are not in the configuration.

It will need to use a 1-shot API to achieve this, as 2-shot will always have the race condition.

[nzakas]

We are running into the same issue, where the labeler is removing a label that a different bot added.
eslint/eslint#18183