Add Submodule Caching to Toolchain Generation

[TShapinsky]

Adds a job at the beginning of the build workflow which updates caches containing the different submodules required to build the different variants.

This job takes about 3 minutes to run the first time, and after that it takes about 4 seconds to check that the cache is still up to date. This could also be moved to a different trigger which runs less often.

This greatly reduces the amount of resources used to clone submodules (just for the build variants for now). The clones that do happen are shallow (except for uclibc-ng) which also shrinks the git overhead by at least 1GB.

This action only caches the .git/modules directory as the rest of the working directory can be rebuilt from those without needing to download anything else. This allows us to choose which submodules we want to manifest into our build environment. The directory sizes for GCC and LLVM are 1.1GB and 1.9GB respectively, so being able to pick which we want in our environment saves a lot of disk space.

An additional note on github caches. They are scoped to branches, but any branch can restore a cache that was saved by master. Repos are limited to 10GB of total cache storage, so restricting cache generation to master may also be a good move to prevent using all of the cache storage capacity.

[TShapinsky]

Workflow can be seen here

[TommyMurphyTM1234]

Minor comments

[TommyMurphyTM1234]

Typo? "is" should be "if"?

Very minor cosmetic (pedantic?!) issue - why the capitalisation of words in comments (here and elsewhere)?

[TShapinsky]

Thanks for catching the typo. I was using title case because they appear in the CI as the titles of each step. But if the standard here is all lower case or sentence case I'll happily switch to that.

[TommyMurphyTM1234]

Thanks for catching the typo. I was using title case because they appear in the CI as the titles of each step. But if the standard here is all lower case or sentence case I'll happily switch to that.

Ah - they're names of the steps/tasks. I mistakenly thought that they were comments. If that's the convention then it makes sense to adhere to it. But should every word start with a capital letter? Not all do - e.g. "Check if Cache Exists for Exact Submodule Configuration", "If no Cache Hit, Update Cache", "Clone submodules", etc. Maybe prepositions are exempt from the rule? :-)

[TShapinsky]

Yeah, maybe just the first word should be capital. Title case is what I was going for, link, which does not capitalize prepositions or conjunctions.

[TommyMurphyTM1234]

Is this PR still needed or is it made redundant by this one which has been merged?

• Update build workflow #1608

[cmuellner]

Thanks for the contribution!
Closing this as superseded by #1608 and the current reusable CI workflow.
The current build workflow already has submodule caching keyed by the
submodule hash, shallow submodule checkout on cache miss, and cache
restore in the build jobs.