If I want to upload a file to an endpoint (like a package to an artifactory) in a custom plugin, is using

Process

with

curl

the best way? Or is there another, preferred way to do it using the Rules API?

I'm not aware of any method to do that using the rules API, and it seems like a good use-case for a process, as rule evaluation should try to stay cheap. Otherwise you could of course use something like urllib/requests.

How does the publish built-in goal publish wheels? Does it internally use Process as well?

I believe Pants uses https://pypi.org/project/twine/ to publish wheels.

Oh I see. Thank you!

To be exact, it builds a pex with twine, and then executes that in a process.

I see. But what about the

publish

goal? How does that publish artifacts to a artifactory?

It depends on the "implementation" of the goal for a specific target/language. For Python, it's as I described. For my OCI plugin, I use skopeo. Other backends will use their own tool.

pants publish

isn't a specific codepath, it finds rules that accept the specified target(s) and delegates to that rule to do the lifting. See this code here, which is porcelain side of the equation: https://github.com/pantsbuild/pants/blob/main/src/python/pants/core/goals/publish.py#L219-L273 - it gathers a number of process-objects (i.e., not launched) for all possible targets, then handles one at a time to do the actual publishing. The plumbing side is here: https://github.com/pantsbuild/pants/blob/main/src/python/pants/backend/python/goals/publish.py#L134. You can see it returns a specific "PublishProcess", which is mostly a process with metadata.