error = "trim"

[wlandau]

Prework

• I understand and agree to help guide.
• I understand and agree to contributing guide.
• New features take time and effort to create, and they take even more effort to maintain. So if the purpose of the feature is to resolve a struggle you are encountering personally, please consider first posting a "trouble" or "other" issue so we can discuss your use case and search for existing solutions first.

Proposal

Suggested by @hadley in #1310 (comment): if a target fails, then

1. Don't start anything downstream of that target.
2. Cancel dynamic branches with the same parent.
3. Let everything else run.

For (2), there is the question of what we mean by "cancel". When one branch throws an error, some of its siblings may not have started yet, whereas others may already be running. I would prefer to cancel the former and let the latter keep running. Reasons:

1. It seems friendlier for users and easier for them to understand.
2. In distributed computing scenarios, it won't require Support for mirai cancellation shikokuchuo/mirai#112 or WISH: Add method for interrupting / terminating a future HenrikBengtsson/future#93. (NB clustermq doesn't support cancellation either AFAIK.)
3. It's much easier to implement and maintain.
4. It's more consistent with the rest of targets: error = "trim" will treat currently running targets the exact same way as error = "abridge"

Overall, I think error = "trim" will be a nice addition to targets.

[wlandau]

Added in #1332