Crash after GC in Git

[martinvonz]

Description

After importing a commit from Git, if that commit gets unreferenced and GC'd by Git, jj may crash.

Reported on Discord by @tp-woven.

Steps to Reproduce the Problem

1. Create a Git repo with two commits on branch main. Check out main~.
2. Initialize a jj repo backed by it
3. In the Git repo, rewind the main branch (git branch -f main) and force GC (git -c gc.reflogExpire=now gc --prune=now)
4. Go back to the jj repo and run e.g. jj log

Expected Behavior

jj log works fine and still sees the old location of the main branch (since we didn't re-import)

Actual Behavior

Crash:

Error: Unexpected error from store: Object not found

Specifications

• Version: 0.5.1 (and at least back to 0.4.0)

[martinvonz]

Here's a test script I've used for this (to be run in an empty temporary directory):

#!/bin/sh

rm -rf git jj

git init git
cd git
echo a > file
git add file
git commit -m a
echo b > file
git add file
git commit -m b
git checkout HEAD^
cd ..

jj init jj --git-repo=git

cd git
git branch -f main
git -c gc.reflogExpire=now gc --prune=now
cd ..

cd jj
jj log
cd ..