Removing a git tag abandons commits in jj

[jacg]

It seems that removing a git tag causes jj to abandon any commits that were previously reachable from that tag, even if they are still reachable from visible heads in jj. Here's a quick demonstration:

# The following was observed in version 0.29.0
jj -version

# Create a fresh git-colocated repo
jj git init --colocate demo
cd demo

# Create some commits
for i in {1..3}; do
  echo line $i >> file && jj commit -m "line $i"
done

# Place a git tag on the last one
git tag the-tag

# Create more commits on top of the tagged one
for i in {1..3}; do
  echo line $i >> file && jj commit -m "line $i"
done

# Remove the tag ...
git tag -d the-tag
# ... this abandons all the commits that used to be reachable from the tag

These commits still are/should-be reachable from their visible, unabandoned commits in the jj repo, so they should not be abandoned. Abandoning them makes a complete mess of the repo, and gives the impression that data have been irretrievably lost!

However, if no commits are created on top of the tagged one, before the tag is removed, then nothing is abandoned.

# Create a fresh git-colocated repo
jj git init --colocate demo-2
cd demo-2

# Create some commits
for i in {1..6}; do
  echo line $i >> file && jj commit -m "line $i"
done

# Place a git tag on the last one
git tag the-tag @~~~

# Remove the tag before making any jj commits on top ...
git tag -d the-tag
# ... this time the commits are NOT abandoned

Bug or feature?

[yuja]

It's working as intended. The problem is that there's no native support for creation/deletion of tags. See #6325.

[jacg]

So, feature, it would seem. Can you help me understand why anyone would ever want this? [So far, jj has only ever surprised me in good ways (and it has done so many times), but this was a very nasty surprise, especially as the usually-reliable op restore is helpless against it!]

At least

[git]
abandon-unreachable-commits = false

exists, and seems to switch to (in my mind) sane behaviour.

(I need to look into the meaning of the word 'imported' as used in various places in #6325, and how importing things results in them being abandoned. I guess it's a bit like git add sometimes removing files.)

[bnjmnt4n]

This abandonment of BASE only happens if there is some WORK (i.e. when I'm likely to care about preserving BASE) but does not happen when there is no WORK (i.e. when I'm likely not to care about BASE).

For reference, this isn't true. Any commits referenced by a newly deleted tag which are now unreachable will be abandoned. The reason for this misconception is that in your second example, you're running the two git tag commands sequentially without any jj commands in between. Jujutsu will not know that there was any changes in the Git repository since the tag was added and removed before it imports from the repository again. If you add a jj command in between the two git tag commands, the behavior should be consistent (in that all unreachable commits will be abandoned).

[tim-janik]

I just ran into this without the tag being base, but keeping a sibling branch alive.
Deleting the tag deleted the sibling branch and much of the base of @.

Any commits referenced by a newly deleted tag which are now unreachable will be abandoned.

Yeah, exactly this. Paired with not considering @- reachable (which still boggles my mind) means an entire base branch that @ is based on can be lost.

To verify, here are my steps, create commits for base, twig, dev.
Then tag twig + delete tag and all of base is lost:

for i in {1..3}; do date >> file && jj commit -m "BASE-$i" ; done
for i in {1..3}; do date >> file && jj commit -m "TWIG-$i" ; done
jj new @---- # start development on last BASE
for i in {1..3}; do date >> file && jj commit -m "DEV-$i" ; done
git tag twig-3 $(jj log --no-graph -T commit_id -r 'description(TWIG-3)') && jj st
git tag -d twig-3 && jj st

Just to be sure, is this really the intended behavior of JJ (apart from undo not working), or is there some facet of a bug at play at least in this scenario?

[whitequark]

As a non-jj-user but someone who has been interested in it for some time, seeing people lose work here has essentially convinced me to avoid jj (until it is fixed).

[jennings]

@whitequark Good news, it's fixed! jj 0.35.0 includes a jj tag delete command which won't invoke the "became unreachable from git" logic.

[whitequark]

Oh nice, I've gotta try it then.