Support bzlmod-based builds

[coryan]

Bazel is adding a new (better) way to handle dependencies:

https://bazel.build/external/overview#bzlmod

This automatically handles recursive dependencies, resolves diamond dependencies, etc. And supports multiple registries of packages:

https://bazel.build/external/registry

With a default "global registry":

https://github.com/bazelbuild/bazel-central-registry

We need to figure out a number of things, and implement a number of things. This list is probably not exhaustive, it needs to be filled out:

• We need to figure out how to have backup sources for our dependencies. Consider https://blog.aspect.build/configuring-bazels-downloader
• The curl module in BCR does not support HTTPs. Filed [Bug]: curl module provides non-functional http_only flag bazelbuild/bazel-central-registry#2332
• We need to fix renovate.sh to also update the MODULE.bazel file #14454
• We need to decide how to manage the googleapis module in BCR.
  • An override for development is fine, but does not work for downstream consumers.
  • Maybe we can create a new version of googleapis in BCR just before creating a new version of google-cloud-cpp in BCR.
• We would need at least one CI build using the bzlmod configuration
• We need to update the documentation for Bazel customers
• We need to decide if we recommend bzlmod in our quickstart files. Or more likely when we start doing so.
  - These cannot change until there is a BCR module for google-cloud-cpp.

[coryan]

Waiting for Bazel 7.0

[alevenberg]

I'll take a stab at some of the tasks this week.

[alevenberg]

The migration guide https://bazel.build/external/migration

[alevenberg]

I think we're gonna first just want to compile with bazel 7.0, and then we can tackle bazelmod.

Waiting to see if we can get that build working on Mac (bazelbuild/bazel#20513) and Windows.

[alevenberg]

FYI timeline for support for Bazel 6 is until December 2025

[coryan]

Yes, but the support policy for the libraries is only 6 months:

https://opensource.google/documentation/policies/cplusplus-support#support_criteria_3

I will argue that we should only start the clock on Bazel 6 once Bazel 7 works.

[scotthart]

Add this to 2024 planning.

[mering]

As there is a working MODULE.bazel file already, I would appreciate this being available in BCR. The first uploads could be manually while future ones could be more automated. Do you plan on adding it yourself or should I go ahead an propose a first upload to BCR?

[scotthart]

We're still waiting on several of our dependencies to be added to BCR. The Bazel team is aware of this and is working on it.