diff: strange error when a ref (cmd argument) doesn't exist

[jorgeorpinel]

Note that this bug report is on the new (unreleased as of now) revision of dvc diff from #3229:

$ .venv/bin/dvc version
DVC version: 0.82.6+d864ca
Python version: 3.7.6
Platform: Darwin-19.0.0-x86_64-i386-64bit
Binary: False
Package: None
Cache: reflink - supported, hardlink - supported, symlink - supported

...

$ dvc diff -t small.txt HEAD^   # This is a newish repo and HEAD^ doesn't exist yet.
ERROR: unexpected error - Cmd('git') failed due to: exit code(128)      
  cmdline: git rev-parse --short HEAD^
  stderr: 'fatal: Needed a single revision'


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See #2999 (comment) for a fully reproducible script.

[dmpetrov]

@jorgeorpinel thank you for finding this!
I thought it was solved already. See cases 1 and 2 from #3229 (review)

But it seems like a special case of this bug.

@jorgeorpinel , I can't replicate the error, I tried with the following:

$ git init
$ dvc init
$ dvc diff 'HEAD^'

ERROR: failed to get diff - unknown Git revision 'HEAD^'

Could you please provide a reproduction script?

[shcheklein]

@MrOutis did you try the original repro script @jorgeorpinel put together in the comment above:

See #2999 (comment) for a fully reproducible script.

@shcheklein , I didn't see the script, thanks for pointing that out.

I tried it one more time with the original reproduction script and still couldn't reproduce it 🙁

@jorgeorpinel , in this version of dvc diff we deprecated the use of --target.

[dmpetrov]

I guess @jorgeorpinel was found the bug in the old version. It was a race condition between us 😄
It looks like it can be closed. @jorgeorpinel please reopen if I missed something.

[jorgeorpinel]

Thanks guys! I guess it was fixed before the feature was officially released. I was trying on the beta while writing the docs.