Support the use case of not using mergebase for --since

[SamChou19815]

Describe the feature you'd like to request

I am trying to integrate turborepo in my GitHub actions workflow.

I want to avoid checking out the entire history, so I tried to only check out two commits: the base commit and the merge commit of the pull request. Then I want to use --since=[base-commit] to only rebuild what has been affected by the request.

However, this will result in a fatal error due to these lines:

turborepo/cli/internal/scm/git.go

Lines 74 to 84 in 23cb6c1

	if fromCommit != "" {
	// Grab the diff from the merge-base to HEAD using ... syntax. This ensures we have just
	// the changes that have occurred on the current branch.
	command = []string{"diff", "--name-only", fromCommit + "...HEAD"}
	out, err = exec.Command("git", append(command, relSuffix...)...).CombinedOutput()
	if err != nil {
	log.Fatalf("unable to check diff vs. %s: %s", fromCommit, err)
	}
	committedChanges := strings.Split(string(out), "\n")
	files = append(files, committedChanges...)
	}

Example of failed run: https://github.com/SamChou19815/infra/pull/25

When using three dots, git will try to find the mergebase between the two commits, which usually does not exist when you only checkout two isolated commits. I would like to support the use case so I don't have to checkout the entire history just for an incremental build from turborepo.

Describe the solution you'd like

An option that runs git diff --name-only fromCommit..HEAD under the hood would be great.

Describe alternatives you've considered

Use build cache instead?

[attila]

I'm affected by this too. Trying to migrate from a lerna monorepo where PR builds run based on the diff between HEAD and the base ref – which are fetched individually prior to the build step.

If above code used something like git diff --name-only <fromCommit>..HEAD then it would work just fine. Not sure of other implications, of course.

[attila]

A potential workaround is to fetch enough history so that diffing via a merge-base becomes possible. It looks like https://github.com/rmacklin/fetch-through-merge-base is attempting to achieve exactly that.

It still is considered a kludge in my opinion, and support for an option to use ..-style diffs would be desirable.

[jaredpalmer]

Can you suggest how you'd like to specify ... vs. .. via command line flags?

[SamChou19815]

Maybe there can be a flag like --no-mergebase or a separate flag like --diff=[commit] that signals we only care about the diff between two commits.

[graup]

+1 on the issue. I had opened a discussion but seems is the same. #805

If we cannot fix it, at the very least we could improve the error message and documentation.

Could someone explain what the advantage of "..." is over ".." in this case?

[anthonyshew]

--since is deprecated so will close this as out of date!