Support multiple --bazelrc on command line

[wisechengyi]

Address #7489

Motivation

Multiple --bazelrc on CLI would be useful for us for various reasons:

Mostly importantly, we will have very long argument lists to pass to bazel query, so they have to be in a bazelrc file. import/try import in bazelrc would still work but very awkwardly. For example, if there are multiple bazelrc files to import, say A and B,

import A needs to be added into $WORKSPACE/.bazelrc
import B needs to be added into A
meaning the former bazelrc file needs to know what comes next.

Therefore allowing multiple --bazelrc would greatly improve the ergonomics, so the caller can create and append the new bazelrc without modifying the previous rc files.

Note

Options passed on CLI will still overwrite any options specified in any bazelrcs.

Result

bazel --bazelrc=x.rc --bazelrc=y.rc ... now works.

  --bazelrc (a string; default: see description)
    The location of the user .bazelrc file containing default values of Bazel 
    options. This option can also be chained together.
    E.g. `--bazelrc=x.rc --bazelrc=y.rc` so options in both RCs will be read.
    Note: `--bazelrc x.rc y.rc` is illegal, and each bazelrc file needs to be 
    accompanied by --bazelrc flag before it.
    If unspecified, Bazel uses the first .bazelrc file it finds in the 
    following two locations: the workspace directory, then the user's home 
    directory. Use /dev/null to disable the search for a user rc file, e.g. in 
    release builds.

[bierbaum]

Hi Yi, I left some style nits to get this change in line with what is going on in this codebase.

You also might want to add coverage in src/test/cpp/startup_options_test.cc.

[wisechengyi]

You also might want to add coverage in src/test/cpp/startup_options_test.cc.

In terms of bazelrcs chaining together, it was already supported, but just not from command line. Hence the scope of this change merely adds the bazelrcs passed via CLI to the final list of bazelrcs that are going to be parsed. I don't see anything specific about CLI related tests in startup_options_test.cc. Happy to add more if @michajlo thinks it is necessary and can kindly offer some guidance.

[aiuto]

cc: @juliexxia
This should have been discussed as an issue first so we could discuss the merits. It's debatable if we want to add any new complexity to arg parsing, since a few of us have recently noticed that it is a mess in many ways and is ripe for a complete rebuilding.

[wisechengyi]

Hi @michajlo I have made the changes as discussed. Please kindly give it another go when you get a chance. Thanks!

[wisechengyi]

Comment addressed. Thanks!

[michajlo]

I had overlooked that there's some documentation that needs to be updated. Can you update https://github.com/bazelbuild/bazel/blob/master/site/docs/guide.md and https://github.com/bazelbuild/bazel/blob/master/site/docs/user-manual.html, and do a quick skim to see if there's anything else we missed?

[wisechengyi]

Hi @michajlo, happy to do that. However logistically are we going to run into issues like #12194 where we cannot directly update doc from github PR? If so, would you like me to send a separate PR, and then you can modify the internal repo accordingly?

[michajlo]

I think this instance is fine to change.

[wisechengyi]

Doc updated. https://github.com/bazelbuild/bazel/blob/master/site/docs/user-manual.html should be still accurate on --bazelrc

[wisechengyi]

Thank you @michajlo !