ctexplain: report a build's trimmability

[gregestren]

Example:

Collecting configured targets for //testex:split... done in 1.00 s.

Configurations: 3
Targets: 12
Configured targets: 22 (+83.3% vs. targets)
Targets with multiple configs: 10
Configured targets with optimal trimming: 12 (+0.0% vs. targets)
Trimming impact on configured target graph size: -45.5%

Note the Copybara imports are messed up as checked in. I'll have to figure out (in another PR) how to stop them from doing that.

Testing: $ bazel test //tools/ctexplain:all
Work towards #10613

[nikhilkalige]

@gregestren I was playing around with this change. It fails when you have multiple transitions. I will try to create an example, but to put it in words

All my transitions are on string_flag with single value
Cquery gives me 4 config values ex: [1xxx, 2xxx, 3xxx, 4xxx]

cquery --show_config_fragments.. runs
//:mainA (1xxx) [flagA, flagB, .... ]
.....

bazel config 1xxx
name: user-defined
options: {
  flagA: flagValue
}

With the above data, in _get_required_fragments ,req loops over all fragments (flagA, flagB), but only flagA is defined in the config.

Also should this be changed to

for req in ct.transitive_fragments:
        if (req in ct.config.fragments or req.startswith("--") or req == "CoreOptions"):

user_defined = ct.config.options.get('user-defined', dict)
for req in ct.transitive_fragments:
        if (req in ct.config.fragments or req.startswith("--") or req in user_defined or req == "CoreOptions"):

[gregestren]

I really need to land the 1.0 of ctexplain, which I believe is this and another pending PR. From there we can properly examine what does and doesn't work.

I'll admit it's hard for me to prioritize this at the moment. If anyone wanted to assist that could help.

[nikhilkalige]

I can help out. Let me know how I can be useful.

[gregestren]

@nikhilkalige still any interest? I realize it's been a long time, so no worries whatever the case.

[nikhilkalige]

@gregestren Sorry.. I missed that.. But I am interested in helping out

[gregestren]

We just decided to revive this with @sdtwigg . Taking note of your interest. We'll have to coordinate to agree on next steps, but it'll be basically getting this code properly merged.

[keertk]

Hi there! Thank you for contributing to the Bazel repository. We appreciate your time and effort. We're doing a clean up of old PRs and will be closing this one since it seems to have stalled. Please feel free to reopen if you’re still interested in pursuing this or if you'd like to discuss anything further. We’ll respond as soon as we have the bandwidth/resources to do so.

[fmeum]

@gregestren I would still be interested in this functionality. Are you planning to work on it again at some point?

[gregestren]

I'd love to properly land this but I need reviewer support

I would consider trying to push this through in the new year.

Let's talk about this as one of our subjects at our bi-weekly syncs I'm still responsible for setting up. :)