Custom build flags choke on prefix workspaces

[ulysses4ever]

Description of the problem

I believe custom build flags don't play well with proper @namespace//...-like references.

Bugs: what's the simplest, easiest way to reproduce this bug?

I set up a repository with a repro -- check out the README. It follows closely the running example of drinks and flavors in the documentation. Except that it actually sets up a named workspace.

In the master branch of the repo, everything is fine: I'm able to pass the flag in the command line. In the bug branch I add the workspace prefix when referencing the flag target from the attribute of my rule. This breaks the setup: I pass the flag in the command-line as before but it has no effect. If I try to add the workspace prefix when referencing the flag from the command-line, I get "Unrecognized option".

What operating system are you running Bazel on?

Ubuntu 18.04

What's the output of bazel info release?

release 0.28.0- (@non-git)

If bazel info release returns "development version" or "(@non-git)", tell us how you built Bazel.

This is the version of Bazel imported by Nixpkgs version referenced in the current rules_haskell (master branch).

Have you found anything relevant by searching the web?

No. I only looked up the Issues here.

[ulysses4ever]

May I ping @juliexxia who did most of the job of implementing this exciting feature! And also @c-parsons who merged it.

[aiuto]

Thanks for the detailed example.
@juliexxia is away on a fellowship for a few months. We'll get the right people to look at it.

[gregestren]

This is a cutting edge feature so I'm not surprised this part doesn't work yet. I don't think anyone can take this on right now, but please re-ping whenever you feel to help us figure out a practical fix timeline.

Appreciate your patient guinea pigging!

[ulysses4ever]

@gregestren thanks for letting me know. I didn't quite get when you want me to re-ping you.

[gregestren]

There's no obvious person to work on this right now (#9177 (comment)). I don't want this to fall into oblivion but it might get lost in the issue load. So please offer us some leeway but feel free to re-ping if/when you feel it's sitting around for too long. :)

[ulysses4ever]

I see, thanks!

[aherrmann]

I've tested this issue again on the more recent Bazel version 2.1. The original issue is still present. I.e. defining the workspace name prefix on the flag causes the flag to be ignored, if it is defined locally. Meaning, this issue is still open.

However, what is working now is to prefix the workspace name if the flag is defined in an external workspace. This is the use-case that is interesting for rule authors, because it allows them to define configurations in their rules sets that users can then affect on the command-line.

I've forked and updated @ulysses4ever's reproduction here. This illustrates that if a rules set, say rules_example, defines a flag //:favorite_flavor, then users can change it in their project that uses rules_example as an external repository using the command-line flag like so:

bazel build //:my_drink --@rules_example//:favorite_flavor=True

[gregestren]

We recently prioritized this general theme in #11128 and added all related issues. I forgot to include this one. Apologies.

Thanks for the updated feedback.

[gregestren]

Let's consolidate these bugs into #11128.

When that one is fixed, it if doesn't address this we can re-open this.