[CI] skip docs-only jobs take #2
So we discovered CircleCI has a problem and `pipeline.git.base_revision` is unreliable - not always set - breaking the test. https://github.com/huggingface/transformers/pull/8826#issuecomment-735972196

We had a few PRs incorrectly skipping the jobs, as in this example: https://app.circleci.com/pipelines/github/huggingface/transformers/16541/workflows/17b20230-8d7c-4b36-813c-2681f2c8a977/jobs/128232

It's missing `<< pipeline.git.base_revision >>`  in

```
if git diff --name-only << pipeline.git.base_revision >>...<< pipeline.git.revision >> | egrep -qv '\.(md|rst)$'
```
resulting in:
```
if git diff --name-only ...5170e5381b9fccdfb9405d665ecee0515efc6453 | egrep -qv '\.(md|rst)$'
```
and hence fails the test. (it's missing the first hash before `...`).

This PR checks that the external variables `pipeline.git.base_revision` and `pipeline.git.revision` are set before we do the test. Should one of them be not set, the whole test is skipped and the job continues normally, regardless of whether it's docs only or not.

Meanwhile I filed a question about why `pipeline.git.base_revision` is not always set:
https://discuss.circleci.com/t/pipeline-git-base-revision-is-often-empty-which-reliable-variable-to-use/38301

Let's merge it at a time that one of us can monitor the next few PRs in case we need to back it out again.

If you have to back it out - you only need to comment out this line: `circleci step halt` and leave the invocations in place.

@sgugger, @LysandreJik 

Github actions decided to be down right at the moment where I wanted to monitor :disappointed: <|||||>Github actions? We are only doing this on circleCI - so far all seems to be working fine.<|||||>Hah, was so disappointed I let it blind me and think it was all CI. Will continue.<|||||>I'm pretty sure the CI should have run on the latest pipeline, as it did in the ones preceding it: https://app.circleci.com/pipelines/github/huggingface/transformers?branch=conda-ci

Changes were done to .yml files.<|||||>I believe the issue might come from the fact that it is looking at the build commit (d26ca66e2b12c5d5bc30be474d35f6e58dd21808) and comparing it to the previous build's commit (4780c8086a7aa95fc9b610cfe351ec5b226de669). It doesn't find any files, as the previous build commit (4780c8...) doesn't exist anymore, as I force pushed the branch, overwriting that commit's data.

Maybe checking for empty results in the diff would help in that regard?<|||||>OK, I found when we don't have `pipeline.git.base_revision` defined - it happens when PR is opened via github file edit. As I just did here:  https://github.com/huggingface/transformers/pull/8884
You can see then what happens: https://app.circleci.com/pipelines/github/huggingface/transformers/16617/workflows/fea80cc9-2093-4053-b3c3-f315632ab3a6/jobs/129069

```
#!/bin/bash -eo pipefail
# pipeline.git.base_revision is not always defined, so only proceed if all external vars are defined
if test -n "" && test -n "32f03035ce5b23abd8a1659f24f04b298319ae78"
then
    if git diff --name-only ...32f03035ce5b23abd8a1659f24f04b298319ae78 | egrep -qv '\.(md|rst)$'
    then
        echo "Non-docs were modified in this PR, proceeding normally"
    else
        echo "Only docs were modified in this PR, quitting this job"
        circleci step halt
    fi
else
    echo "Can't perform skipping check w/o base_revision defined, continuing the job"
fi

Can't perform skipping check w/o base_revision defined, continuing the job

CircleCI received exit code 0
```

So the workaround worked. The job continued normally.
<|||||>~Oh I always open PRs like that, so it was indeed the culprit for the two other times we saw that happen~.
That does not make sense since the I open the PR on GitHub but the CI runs on a commit. So forget I said anything!<|||||>> I believe the issue might come from the fact that it is looking at the build commit ([d26ca66](https://github.com/huggingface/transformers/commit/d26ca66e2b12c5d5bc30be474d35f6e58dd21808)) and comparing it to the previous build's commit ([4780c80](https://github.com/huggingface/transformers/commit/4780c8086a7aa95fc9b610cfe351ec5b226de669)). It doesn't find any files, as the previous build commit (4780c8...) doesn't exist anymore, as I force pushed the branch, overwriting that commit's data.
> 
> Maybe checking for empty results in the diff would help in that regard?

OK, so this is another edge case. So this pipeline thing is totally borked :( Why can't it give us a normal commit range of the PR.

So let's comment out `circleci step halt` and I will work on take #3 that will be much more elaborate. Should I do it or will you? I'm not sure if it's ok to commit directly.<|||||>You can comment it out! Thanks!<|||||>So the proposed logic for take 3 will be:

1. if pipeline.git.base_revision and pipeline.git.revision are defined
2. if git diff --name-only range returns anything
3. if what it returned in 2 is just docs
4. then skip
<|||||>


> You can comment it out! Thanks!

Done.<|||||>> ~Oh I always open PRs like that, so it was indeed the culprit for the two other times we saw that happen~.
> That does not make sense since the I open the PR on GitHub but the CI runs on a commit. So forget I said anything!

I'm not sure what you're saying - I think the point is that CircleCI can't find the branching point when the change is done via github file edit.

Note that `git diff --name-only $(git merge-base --fork-point master)` doesn't work on CirlceCI - otherwise we would have figured out the range ourselves.<|||||>Yes I don't do commit by editing files on GitHub, just the PR part, that's why I scratched what I was saying.<|||||>Thank you for clarifying that, @sgugger. Then this lack of `pipeline.git.base_revision` appears to be random then.