Very many epochs/* tags accumulating, can they be garbage collected?

[foolip]

#26122 introduced tags for pushes to the epochs/* branches, in order to have a record of what those branches had pointed to in the past. That's resulted in over 2000 such tags since then, which are pretty prominent in git log:

commit c2df3bee8330f8058bd15a59e51567d423ed3a81 (tag: merge_pr_28741, tag: epochs/twelve_hourly/2021-04-30_12H, tag: epochs/three_hourly/2021-04-30_12H, tag: epochs/six_hourly/2021-04-30_12H, origin/master, origin/epochs/twelve_hourly, origin/epochs/three_hourly, origin/epochs/six_hourly, origin/HEAD)
Author: Antonio Sartori <anton.sartori@gmail.com>
Date:   Fri Apr 30 13:12:21 2021 +0200

    Fix the Stash Lock to be unique (#28741)
    
    Co-authored-by: Antonio Sartori <antoniosartori@google.com>

It's not a big problem, but I wonder if these tags have helped debug issues, @clopez?

In the issue the challenge was identifying the commit that triggered the taskcluster run on epochs/daily for October 12, 2020.

The commit should be one listed by ./wpt rev-list --epoch 1d --max-count=365. Looking at./wpt rev-list --epoch 1d --max-count=365 | xargs git log --no-walk --pretty=fuller I think it was 1b2893a, and clicking through to https://community-tc.services.mozilla.com/tasks/MRVToMMNQjOG7npZAfF0jQ confirms it.

Being able to list the right commits after the fact was an intentional part of how ./wpt rev-list was implemented, but it's not documented how to figure this out anywhere.

@clopez did you recall if you looked for documentation about this anywhere? If you did, then maybe we can put documentation there, and stop creating the tags?

[clopez]

I knew about the command ./wpt rev-list, but for some reason I had difficulties finding the exact commit that caused #26097
Not sure if it could be related to different time-zones. Or maybe it was related because i didn't knew that it accepted a --max-count=365 parameter so I was trying to pass something like ./wpt rev-list --epoch 4d

It's not a big problem, but I wonder if these tags have helped debug issues, @clopez?

I didn't had a need to use them, since there wasn't any issue reported with the CI and WebKitGTK builds since then.

If no one else is finding this useful, then I agree it is a good idea to revert #26122 and document ./wpt rev-list

[clopez]

BTW; I'm quickly checking if all commits annotated via tags are reported by ./wpt rev-list and it seems a few are not.

# get list of commit hashes for each epoch tag
git tag|grep epochs/twelve_hourly|while read x; do git log -1 --pretty=%H $x; done >/tmp/twelve_hourly
git tag|grep epochs/six_hourly|while read x; do git log -1 --pretty=%H $x; done >/tmp/six_hourly
git tag|grep epochs/three_hourly|while read x; do git log -1 --pretty=%H $x; done >/tmp/three_hourly
git tag|grep epochs/daily|while read x; do git log -1 --pretty=%H $x; done >/tmp/daily
git tag|grep epochs/weekly|while read x; do git log -1 --pretty=%H $x; done >/tmp/weekly
# get list of commit hashes for each epoch tag from wpt rev-list
./wpt rev-list --epoch 1d --max-count=9999 >/tmp/daily.wpt
./wpt rev-list --epoch 1w --max-count=9999 >/tmp/weekly.wpt
./wpt rev-list --epoch 12h --max-count=9999 >/tmp/twelve_hourly.wpt
./wpt rev-list --epoch 6h --max-count=9999 >/tmp/six_hourly.wpt
./wpt rev-list --epoch 3h --max-count=9999 >/tmp/three_hourly.wpt
# compare to see if any tag is not reported by wpt rev-list
for epoch in daily weekly twelve_hourly six_hourly three_hourly; do cat /tmp/${epoch}| while read commit; do grep -q -o ${commit} /tmp/${epoch}.wpt || echo "${epoch} ${commit}"; done; done

Result:

daily b6fc05c48b13142e45ad0d409ed14bfc23262060
daily 2797f44946e26757effe22bac5573071a919b1cf
twelve_hourly b6fc05c48b13142e45ad0d409ed14bfc23262060
twelve_hourly 2797f44946e26757effe22bac5573071a919b1cf
six_hourly 9cdaa1607511824df0c9ae77f6b60d0b23f24d04
six_hourly 2797f44946e26757effe22bac5573071a919b1cf
three_hourly 030a772430d57102296d3d3f811cd4ff9f9bcd79
three_hourly b74014abe0da655dd4675b1ddce4364eabc0b3de

Those have an epoch tag but doesn't seem to appear in the wpt rev-list for such epoch

[foolip]

I've sent web-platform-tests/rfcs#117 to drop these tags.