`plots diff` & `exp show`: unexpected error - Empty git repo

[mattseddon]

Bug Report

Description

After initialising a repository with both Git and DVC both plots diff and exp show throw ERROR: unexpected error - Empty git repo.

Reproduce

1. mkdir new
2. cd new
3. git init
4. dvc init
5. dvc exp show => throws error
6. dvc plots diff => throws error

Expected

Neither of these commands should throws an error under these circumstances but they should return an empty dict for the --json option.

Environment information

Output of dvc doctor:

$ dvc doctor
DVC version: 2.18.1 (pip)
---------------------------------
Platform: Python 3.9.13 on macOS-12.5.1-arm64-arm-64bit
Supports:
	http (aiohttp = 3.8.1, aiohttp-retry = 2.8.3),
	https (aiohttp = 3.8.1, aiohttp-retry = 2.8.3),
	webhdfs (fsspec = 2022.7.1)
Cache types: <https://error.dvc.org/no-dvc-cache>
Caches: local
Remotes: None
Workspace directory: apfs on /dev/disk3s1s1
Repo: dvc, git

Additional Information (if any):

This was brought up in the cross team meeting and is part of a larger discussion on error handling.

The requested behaviour is exhibited under the same circumstances for the following commands:

/new master +3 ❯ dvc list . --dvc-only --recursive --json
[]
/new master +3 ❯ dvc status --json
{}
/new master +3 ❯ dvc diff --json
{}
/new master +3 ❯ dvc data status --json
{}

[dberenbaum]

Thanks @mattseddon! Could you describe how it impacts VS Code and how important it is?

I see that it's inconsistent with other commands, but both behaviors seem initially reasonable to me, and since I have never seen a user complaint about it, I would only prioritize it based on your needs.

[mattseddon]

This broke the workflow of getting setup with a new project/repository. That is something that we want to cover with the extension so it is fairly important to us.

Here are a couple of thoughts that I've had:

1. If the error is expected (which I think is reasonable) then we should not get an "unexpected error"
2. If we have to have a commit for some commands to work then maybe dvc init should generate that commit.

[daavoo]

If the error is expected (which I think is reasonable) then we should not get an "unexpected error"

Before the extraction and creation of https://github.com/iterative/scmrepo (dvc<=2.8.3), dvc exp show didn't raise any error but showed an empty table / returned this with --json flag:

{                                                                     
  "": {
    "baseline": {
      "error": {
        "type": "InvalidSpecError",
        "msg": ": failed to parse revision specifier - Invalid pattern ''"
      }
    }
  }
}

plots diff failed but with dvc.scm.base.RevError instead of unexpected error

[daavoo]

I think we just need to do the same as dvc diff / exp diff / metrics diff / params diff:

dvc/dvc/repo/diff.py

Lines 23 to 24 in 460d080

	if self.scm.no_commits:
	return {}

[pmrowla]

dvc/dvc/scm.py

Line 163 in 460d080

if not rev.startswith("refs/"):

This should also be

if not (rev == "HEAD" or rev.startswith("refs/")):

(This does not resolve the issue, but it makes DVC generate a proper error message rather than unexpected error.)

ERROR: failed to show experiments - unknown Git revision 'HEAD'

[daavoo]

@mattseddon Could you try #8177 to see if it matches what you expected

[mattseddon]

Will test after the weekend 🙏🏻.