jest-changed-files fails with missing merge base

[jdanil]

💥 Regression Report

When an environment does not have the full commit history, running jest with the --changedSince option fails with an error.

A repository may find itself in this state when checking out a single commit rather than cloning the whole repo and history.

The regression was introduced in #10155.

Last working version

Worked up to version: 26.1.0

Stopped working in version: 26.2.0

To Reproduce

Steps to reproduce the behavior:

1. Checkout a single commit from a feature branch.
2. Run jest --changedSince=master

Expected behavior

jest-changed-files should detect changed files without erroring with a non-zero exit code.

Link to repl or repo (highly encouraged)

https://github.com/jdanil/jest-changed-files-repro

Logs from CI for any pull request.

yarn run v1.22.5
$ yarn test:ci --changedSince="master"
$ yarn test --ci --changedSince=master
$ jest --ci --changedSince=master


  ● Test suite failed to run

    fatal: master...HEAD: no merge base

Error: Process completed with exit code 1.

Run npx envinfo --preset jest

Paste the results here:

System:
  OS: Windows 10 10.0.18363
  CPU: (8) x64 Intel(R) Core(TM) i7-7820HQ CPU @ 2.90GHz
Binaries:
  Node: 14.16.0 - C:\Program Files\nodejs\node.EXE
  Yarn: 2.4.1 - C:\Program Files (x86)\Yarn\bin\yarn.CMD
  npm: 6.14.11 - C:\Program Files\nodejs\npm.CMD

[jdanil]

Updated the description after debugging a bit more. The issue seems to be due to a missing merge base rather than being in a detached HEAD state.

[SimenB]

It sorta makes sense to me to fail in this case as the commit is missing. What should the behaviour be? We should not just ignore the flag as we wanna pick up typos (I think). Should we just document you need some other fetch depth for the flag to work?

I guess we could print a warning if we cannot resolve the base and just run all tests

[jdanil]

Hey @SimenB, thanks for reviewing this issue so quickly.

I think even if the merge base is missing, if we have another branch to compare against, we can still run a diff on it. I was experimenting with running something like git diff --name-only ${changedSince} rather than git diff --name-only ${changedSince}...HEAD which seems to work in this case for me. When I ran the e2e tests though, I found that this script was catching two extra files as "changed" even though they were modified on the master branch only (the last expectation in this test case gets changed files for git).

Even if it would be preferred to not modify the current git diff command, I'm wondering whether it would be possible to put a try-catch around it and fall back to the proposed command or previous git log command in cases where there is a missing merge base?

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