Culling probably-unused Bazel flags

[Lukács T. Berki]

Hey there,

As discussed on GitHub, we are planning to remove flags from Bazel that are unused.

Given that we don't have a view of what flags are actually used by the community, we decided on the following approach:

1. Compute a list of unused flags in the Google monorepo
2. Eyeball the list and remove flags that we consider useful
3. Distribute the list of flags that are candidates for removal
4. Have a commenting period of reasonable length
5. Proceed with the removal.

We have completed (1) and (2), and this e-mail is (3). These are the flags we consider removing:

https://gist.github.com/lberki/e187abc04a59b6d3d1eb0e42e515fb60

Please chime in if you need a flag! We don't guarantee that any flag is kept, but we'll be considerate; we don't want to break anyone, but we do want to clean up our code base a little bit.

The commenting period will be two weeks long; if a flag doesn't get an objection or the objection is overruled, we'll proceed with the removal.

--

Lukács T. Berki | Software Engineer | lbe...@google.com | 

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[Fabian Meumertzheim]

I found these three flags that I would propose to keep:

incompatible_avoid_conflict_dlls: This is important on Windows (and only applies there, which is probably why it's unused at Google).

legacy_external_runfiles: The default is important for compatibility, the flipped state is an interesting performance optimization for sandboxed actions. Instead of removing this, could we revive the effort to flip it?

experimental_lazy_template_expansion: Gates https://bazel.build/rules/lib/builtins/actions#template_dict, which is used in built-in rules and should be stabilized, after which it can be removed.

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[Austin Schuh]

We use --experimental_multi_threaded_digest. It saves a ton of time
on our builds since computing the digest is mostly CPU bound for us
locally.

I filed a ticket years ago asking to make that not experimental.
https://github.com/bazelbuild/bazel/issues/6345

Austin

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[Alex Eagle]

I think `--[no]legacy_external_runfiles` needs to be kept. Even though no one is working on enabling that flag by default, it's needed to enable a performance optimization where the runfiles tree requires about half as many filesystem operations. In a project with tons of third-party dependency files like nodejs, that can make a substantial difference in test times.

[Tobias Werth]

Austin, experimental_multi_threaded_digest is a no-op since Bazel 5, see https://github.com/bazelbuild/bazel/issues/14034

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[Austin Schuh]

Oh, excellent!  I'll start removing it from our systems.

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