This script needs/uses a custom `BASHBREW_LIBRARY` directory, but it stores that value in the exported environment slightly too soon such that `generate-stackbrew-library.sh` picks it up (and shouldn't be).
We often use "`BASHBREW_LIBRARY` is unset (or empty)" as a conditional for whether to fall back to using "https://github.com/docker-library/official-images/raw/HEAD/library/" as an explicit prefix for querying "source of truth" values for things like supported parent architectures.
These two things collided in cc2dc88e04 (and similar commits) because the script saw `BASHBREW_LIBRARY` set, trusted it, but then fails to find the parent image.
This is the cleanest place to fix this such that `generate-stackbrew-library.sh` can take `BASHBREW_LIBRARY` from the provided environment instead of using our generated value.
I thought this was already the behavior, but I guess it was relaxed because previous iterations of this validation had to apply to the older format where we'd been less meticulous about enforcing this. Since those are all gone now, we can safely update the validation to enforce that commit hashes *must* be fully qualified.
It's been failing to upload for a while, and I don't think the failures are *completely* their fault (GitHub's got to share some of the blame, I think), but the end result is that we don't really have any good options for continuing to use the service (bad/unacceptable options include install an app with way too many privileges on the org/repo or add a "secret" with a personal access token ... and make that available to Pull Requests too 🙃).
This should be more reliable/correct than `uname -m` (because we're almost always looking for the *userspace* architecture, not the kernel architecture).
In my refactoring to use `go-git`'s `Tree` objects, I missed this edge case (that symlinks get resolved to be relative to the Git root, but our `Tree` object is a subdirectory).
This also finally adds `bashbrew context` as an explicit subcommand so that issues with this code are easier to test/debug (so we can generate the actual tarball and compare it to previous versions of it, versions generated by `git archive`, etc).
As-is, this currently generates verbatim identical checksums to 0cde8de57d/sources.sh (L90-L96) (by design). We'll wait to do any cache bust there until we implement `Dockerfile`/context filtering:
```console
$ bashbrew cat varnish:stable --format '{{ .TagEntry.GitCommit }} {{ .TagEntry.Directory }}'
0c295b528f28a98650fb2580eab6d34b30b165c4 stable/debian
$ git -C "$BASHBREW_CACHE/git" archive 0c295b528f28a98650fb2580eab6d34b30b165c4:stable/debian/ | ./tar-scrubber | sha256sum
3aef5ac859b23d65dfe5e9f2a47750e9a32852222829cfba762a870c1473fad6
$ bashbrew cat --format '{{ .ArchGitChecksum arch .TagEntry }}' varnish:stable
3aef5ac859b23d65dfe5e9f2a47750e9a32852222829cfba762a870c1473fad6
```
(Choosing `varnish:stable` there because it currently has [some 100% valid dangling symlinks](6b1c6ffedc/stable/debian/scripts) that tripped up my code beautifully 💕)
From a performance perspective (which was the original reason for looking into / implementing this), running the `meta-scripts/sources.sh` script against `--all` vs this, my local system gets ~18.5m vs ~4.5m (faster being this new pure-Go implementation).
There's some hopefully good code comments describing this better, but essentially this changes the algorithm to have the "first" tag/entry in the SharedTag group be the "owner" of all the shared tags (instead of the following tags clobbering that and them thus being "owned" by the final Windows entry).
Before (finding tags which have both Debian *and* Ubuntu in their parentage):
```console
$ comm -12 <(bashbrew children debian | sort) <(bashbrew children ubuntu | sort) | cut -d: -f1 | sort -u
$ # (empty list, but adding "--arch-filter" makes it work:)
$ comm -12 <(bashbrew children --arch-filter debian | sort) <(bashbrew children --arch-filter ubuntu | sort) | cut -d: -f1 | sort -u
clojure
maven
neo4j
```
After:
```console
$ comm -12 <(bashbrew children debian | sort) <(bashbrew children ubuntu | sort) | cut -d: -f1 | sort -u
clojure
maven
neo4j
```
Tianon Gravi