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Misc #16093

Updated by k0kubun (Takashi Kokubun) over 4 years ago

## Background 
 * When we migrated the canonical Ruby repository from Subversion to Git [Misc #14632], in that ticket nobody had objected to allowing a merge commit in the repository. 
 * At first, we decided to prohibit merge commits because: 
   * The first merge commit https://github.com/ruby/ruby/pull/2084 went well. Then we tried the second merge commit for https://github.com/ruby/ruby/pull/2079 but it failed. 
   * We struggled to find why only the first one succeeded. That was tricky because ruby-commit-hook's pre-receive hook was updated to a new revision *after* the first merge happened, and the new revision included a change that accidentally made a merge commit impossible. 
   * Because the merge commit made it harder to debug the issue in ruby-commit-hook, we decided to deliberately [prohibit](https://github.com/ruby/ruby-commit-hook/commit/d7759cca6282741ecc9c46053166a1f5f5779c10#diff-7e61afe505452158c45dfa32ea7d7a14) to push a merge commit to the master branch for a while to reduce the number of problems to be solved at least during the early stage of the migration to git. These days the ruby-commit-hook has worked stably. 
 * Then we also noticed that prohibiting merge commits makes it easier to efficiently list up revisions to be built by his [rubyfarmer](https://github.com/mame/rubyfarmer) without having a data store. 
 * If we do not make a merge commit, there's only one way to make a pull request "Merged" on GitHub ruby/ruby: 
   * Push commits in the pull requests to the master branch when their parent revision is the same as the master branch's one. 
 * Therefore, we have not been able to make a pull request "Merged" when a pull request's branch needs to be rebased before pushing it to the master branch. 
   * But force-pushing the rebased commits to their author's branch is a bad habit and we do not want to do so. 

 ## Problem 
 * Some contributors get confused when their pull request is committed to the master branch but it's marked as "Closed" on GitHub [Misc #16089]. 
 * Even though we clarified the situation in https://bugs.ruby-lang.org/projects/ruby/wiki/HowToContribute, a first-time contributor could be confused and the person might complain about it to the committer. 

 ## Solution 
 1. Improve the rubyfarmer's implementation to make it work even if we had merge commits. https://github.com/mame/rubyfarmer/pull/1 
 2. To maintain a consistent linear history in the git log even after allowing merge commits, implement a guard to prevent a ["foxtrot merge"](https://blog.developer.atlassian.com/stop-foxtrots-now/) in the pre-receive hook. 
   * Details: https://devblog.nestoria.com/post/98892582763/maintaining-a-consistent-linear-history-for-git 
 3. Fix bugs in [check-email.rb](https://github.com/ruby/ruby-commit-hook/blob/master/bin/check-email.rb) to correctly check merge commits. 
 4. Allow pushing merge commits to the master branch. 

 We'll implement 2, 3, and 4 shortly. Please let me know if you have any opinion about this.

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