subordinate trees ? we have %G-space, we should add %K-space

hi git people,

managing .configs, over time, and across work-trees, is an ongoing burden. I wonder if/how git could help.

I imagine a subordinate/secondary tree, which basically has only 1 tracked file ( .config ) in it. Auto-commit on .config ( perhaps as 1st step of make ) would mean no changes are missed

A tertiary tree could hold all of ./builds/* or really any set of files that are ignored by the primary tree's .gitignore file its purpose would be to track build products; all *.o etc its kind-of a superset of what might be found in your .ccache, (it also includes *.cmd.o etc, and all other intermediate outputs) but it is attributable to a TooChainCompilationTransferFn(.config, HEAD)

Since HEAD already has a sha*, we just need one for .config, the secondary tree keeps that for us.

the tertiary tree doesnt REALLY need to keep *all* the build products, it just needs to keep track of them (just the sha*, since they are reproducible), but every commit (to either source, or to .config) should result in 1 or more build products changing. It might be useful for QA purposes to predict and verify what products should change.

Autocommit here means that make *anything* has known inputs, so if results are useful/reportable, they have known inputs (no -dirty)

Also, it might be handy to be able to diff any 2 objdumps/readelfs of foo.o, which would be easier with a cryptographically known DB

Heres how I think it could get leveraged for linux itself

We have %G space, expressed in the kernel name:
  5.14.0-rc1-lm1-00003-g4a7356173ef9-dirty

Lets add %K space, to extend that:
  5.14.0-rc1-lm1-00003-g4a7356173ef9-dirty-k6173ef96173

now posit a kverstr("%D-%G-%K") producing the 2nd name above.
    %T - tag :  "5.14.0-rc1"
    %L - localversion : "-lm1"
    %C - count : "00003", %C{5} default
    %G - HEAD-sha : "-g4a7356173ef9-dirty"
    %K - sha(.config) : "-k6c173efc961c", obviously 'c' is taken.
    %D - desc : "%T-%L-%C"

%L is seductive, but ultimately a nuisance; mine above is a super-imaginitive "-localmod-1" abbreviation.

In contrast, %K would give:

- updates every time the .config is tweaked.

- each arch make defconfig yields single %Kval
  or a few (1 per salt-val)

- normalization done by make oldconfig narrows %K-space
  some %Kvals are "impossible" (aka: hand altered or fuzzed .configs)

- many .configs will get "well-known" %Kvals
  bots will report probs with %Gs against specific %Ks

- all well-known configs: defconfig, allmodconfig, etc...
  get unique %Ks, maybe become well-known
  esp as %K{5} which is short enough to recognize
  %Ks could be annotated
  %Ks change slowly as **/KBuild.* composition changes
  traceable to commits in %G

With (imagined) Git extensions:

1. subordinate trees, in same src/work-dir:

a configs subtree would have only .config file checked in, and probably should auto-commit on changes.

ls ./.gitinclude.*

These actively define inclusions of files in the named subtree, rather than ignores. They dont see files that are not explicitly ignored by primary tree.

so ./.gitinclude.1.configs: # with .config

could always, actively, track and commit changes to .config, `history|grep config | tail -n5`, automatically added to commit-log, would help (a little) explain why. A string serialization of the CONFIG_* changes, would completely characterize the .config diff, and

and ./.gitinclude.3.builds # with ./builds/

captures all build product files written under ./builds/* into a single subordinate tree. This tree can only contain files ignored by .gitignore (ie the primary/source tree)

obviously this will include all of ./**/*.o, which is essentially a %G+%K versioned (and clean) ccache result set.

./builds/{*}   # distinct build-product (%G+%K dependent) sub-trees

./builds/foo # foo ./builds/bar

not really sure how the 2ndary tree (.config tracker) should play with worktrees, or even with multiple ./builds/ subdirs, each with a .config those subdirs are our %K-space-explored, ie those configs which we have at least tried

make randconfig is a rather low-quality way to expand the tested %K-space but with %K handled a bit better, we could at least track how much of the space gets tested this way.

ok, now Im meandering.