Create CI task to fail PR if merge commit exists

[nrmancuso]

Noticed at #12256 (comment), even though we have a bunch of checks in place for commit message, stale PR branch, etc., we do not fail CI for merge commits in PR branches.

I see github action https://github.com/greenled/no-merge-commits-check, maybe we can re-use the script in this repo and create a CI task to fail build if merge commits exist, and provide a message with link to https://checkstyle.org/beginning_development.html#Starting_Development .

[romani]

We need to recheck why our commit message validation is not catching this.

[rnveach]

checkstyle/src/test/java/com/puppycrawl/tools/checkstyle/internal/CommitValidationTest.java

Line 260 in 70bbcd8

if (isMergeCommit(headCommit)) {

checkstyle/src/test/java/com/puppycrawl/tools/checkstyle/internal/CommitValidationTest.java

Line 372 in 70bbcd8

private static final class OmitMergeCommitsIterator implements Iterator<RevCommit> {

Merge commits will fail CI as travis always works as a merge commit even if there is none in PR.

[nrmancuso]

Sounds like shell script is the best way to handle this case then.

[romani]

What is we made update in code and allow skip of merged only if some system variable is defined. And set it in Travis. All others will run without allowance of merge commit.

[rnveach]

system variable is defined. And set it in Travis

I remember we had issues with different CIs and looking at git history similar to PR view. Travis was just the one I remember clearly.

[romani]

At time we created this Junit validation Travis probably was the only CI we had.

[nrmancuso]

I am on it