merge 引用及用途
10.3 Git 内部原理 - Git 引用[1] 及 10.5 Git 内部原理 - 引用规范[2] 中介绍了 Git 的引用（references，或简写为 refs）及其规范，可在本地 Git 仓库的 .git/refs 路径下查看已经 fetch 到本地的引用文件。
当我们在 GitLab 中创建一个 MergeRequest 时，remote 仓库中会自动创建 refs/merge-requests/iid/head 和 refs/merge-requests/iid/merge 两个引用，分别对应 MR 源分支内容，以及 将 MR 源分支内容合并至目标分支后的内容。
当我们对这个 merge 引用进行持续集成时，就可以实现未雨绸缪的效果：
即使尚未合并 MR，持续集成检查和验证的，却相当于是将 MR 合并至目标分支之后的效果。
这样在出现问题时，只需继续在 MR 中进行修改即可，而不是将已经合并至目标分支的内容回滚之后再重新处理。
本地获取 merge 引用
增加 fetch 引用规范
.git/config 配置文件中可以指定多个用于获取操作的引用规范。
添加 merge 引用的方式为增加 refs/merge-requests/*/merge:refs/remotes/origin/merge-requests/*/merge，如：
[remote "origin"]
url = https://gitlab.com/AlphaHinex/merge-refs-test.git
fetch = refs/heads/*:refs/remotes/origin/*
fetch = refs/merge-requests/*/merge:refs/remotes/origin/merge-requests/*/merge
在 GitLab 上创建 MergeRequest 之后，可使用 fetch 命令获取 merge 引用，如：
$ git fetch origin
remote: Enumerating objects: 1, done.
remote: Counting objects: 100% (1/1), done.
remote: Total 1 (delta 0), reused 0 (delta 0), pack-reused 0 (from 0)
展开对象中: 100% (1/1), 248 字节 | 248.00 KiB/s, 完成.
来自 https://gitlab.com/AlphaHinex/merge-refs-test
* [新引用] refs/merge-requests/1/merge -> origin/merge-requests/1/merge
show-ref 查看引用列表
之后可通过 show-ref 查看引用及对应的 commit id：
$ git show-ref
dcaeb64e20196eda19b7f71b86287566d987ea56 refs/heads/main
8f8976a512437c13784586460556dffeef53c645 refs/heads/mr
dcaeb64e20196eda19b7f71b86287566d987ea56 refs/remotes/origin/HEAD
dcaeb64e20196eda19b7f71b86287566d987ea56 refs/remotes/origin/main
f03da17f11a905fcc4766c97f05de2fa8c78ecda refs/remotes/origin/merge-requests/1/merge
8f8976a512437c13784586460556dffeef53c645 refs/remotes/origin/mr
也可以将 fetch 配置为 refs/merge-requests/*:refs/remotes/origin/merge-requests/*，同时获得 MR 的 head 和 merge 分支：
$ git fetch
来自 https://gitlab.com/AlphaHinex/merge-refs-test
* [新引用] refs/merge-requests/1/head -> origin/merge-requests/1/head
$ git show-ref
dcaeb64e20196eda19b7f71b86287566d987ea56 refs/heads/main
8f8976a512437c13784586460556dffeef53c645 refs/heads/mr
dcaeb64e20196eda19b7f71b86287566d987ea56 refs/remotes/origin/HEAD
dcaeb64e20196eda19b7f71b86287566d987ea56 refs/remotes/origin/main
8f8976a512437c13784586460556dffeef53c645 refs/remotes/origin/merge-requests/1/head
f03da17f11a905fcc4766c97f05de2fa8c78ecda refs/remotes/origin/merge-requests/1/merge
8f8976a512437c13784586460556dffeef53c645 refs/remotes/origin/mr
可以看到，共有三个不同的 commit id：
dcaeb64e20196eda19b7f71b86287566d987ea56：对应本地及远程的main分支及HEAD引用8f8976a512437c13784586460556dffeef53c645：对应本地及远程的mr分支，以及merge-requests/1/head引用f03da17f11a905fcc4766c97f05de2fa8c78ecda：对应远程的merge-requests/1/merge引用
左侧未显示完整的即为
merge-requests/1/merge引用。
从本地和 GitLab 上的 main 分支及 mr 分支提交记录里可以找到前两个 commit id，而第三个 commit id 则是 GitLab 在远程仓库的 merge-requests/1/merge 分支上将 mr 分支合并至目标分支 main 自动创建的 commit 对应的 id。
为 merge 引用创建分支
代码语言：javascript复制$ git branch -a
* main
mr
remotes/origin/HEAD -> origin/main
remotes/origin/main
remotes/origin/merge-requests/1/head
remotes/origin/merge-requests/1/merge
remotes/origin/mr
$ git checkout merge-requests/1/merge
分支 'merge-requests/1/merge' 设置为跟踪来自 'origin' 的远程分支 'merge-requests/1/merge'。
切换到一个新分支 'merge-requests/1/merge'
$ git branch -a
main
* merge-requests/1/merge
mr
remotes/origin/HEAD -> origin/main
remotes/origin/main
remotes/origin/merge-requests/1/head
remotes/origin/merge-requests/1/merge
remotes/origin/mr
$ git status
位于分支 merge-requests/1/merge
您的分支与上游分支 'origin/merge-requests/1/merge' 一致。
无文件要提交，干净的工作区
此时即可直接在本地的 merge-requests/1/merge 分支上审查 MR 合并后的代码状态。
merge 引用行为验证
以下内容在 GitLab Community Edition v16.8.1 版本，及在线版 GitLab Enterprise Edition 17.3.0-pre 453542d17ca[3] 中验证。
创建 MR 之前，是否会在远程库中出现 head/merge 分支
不会。
在 MR 分支上提交新 commit 时，merge 分支内容是否会同步变化？
会。但可能稍微有个延迟。
代码语言：javascript复制$ git checkout mr
切换到分支 'mr'
您的分支与上游分支 'origin/mr' 一致。
$ touch new-file-in-mr-branch
$ git add .
$ git commit -am "New commit in mr branch"
[mr 3436570] New commit in mr branch
1 file changed, 0 insertions( ), 0 deletions(-)
create mode 100644 new-file-in-mr-branch
$ git push origin mr
枚举对象中: 3, 完成.
对象计数中: 100% (3/3), 完成.
使用 8 个线程进行压缩
压缩对象中: 100% (2/2), 完成.
写入对象中: 100% (2/2), 306 字节 | 306.00 KiB/s, 完成.
总共 2（差异 0），复用 0（差异 0），包复用 0
remote:
remote: View merge request for mr:
remote: https://gitlab.com/AlphaHinex/merge-refs-test/-/merge_requests/1
remote:
To https://gitlab.com/AlphaHinex/merge-refs-test.git
8f8976a..3436570 mr -> mr
拉取远程库的 merge 引用：
$ git fetch origin
来自 https://gitlab.com/AlphaHinex/merge-refs-test
8f8976a..3436570 refs/merge-requests/1/head -> origin/merge-requests/1/head
如果拉取时没有 merge 引用的变化，可稍等一会之后再试：
代码语言：javascript复制$ git fetch
remote: Enumerating objects: 1, done.
remote: Counting objects: 100% (1/1), done.
remote: Total 1 (delta 0), reused 0 (delta 0), pack-reused 0 (from 0)
展开对象中: 100% (1/1), 248 字节 | 248.00 KiB/s, 完成.
来自 https://gitlab.com/AlphaHinex/merge-refs-test
f03da17...b20a111 refs/merge-requests/1/merge -> origin/merge-requests/1/merge (强制更新)
在目标分支上提交新 commit 时，merge 分支内容是否会同步变化？
也会，同样可能有个延迟。
代码语言：javascript复制$ git checkout main
$ touch new-file-in-main-branch
$ git add .
$ git commit -am "New commit in main branch"
[main 8a98bc0] New commit in main branch
1 file changed, 0 insertions( ), 0 deletions(-)
create mode 100644 new-file-in-main-branch
$ git push origin main
枚举对象中: 4, 完成.
对象计数中: 100% (4/4), 完成.
使用 8 个线程进行压缩
压缩对象中: 100% (2/2), 完成.
写入对象中: 100% (3/3), 299 字节 | 299.00 KiB/s, 完成.
总共 3（差异 0），复用 0（差异 0），包复用 0
To https://gitlab.com/AlphaHinex/merge-refs-test.git
dcaeb64..8a98bc0 main -> main
$ git fetch origin
remote: Enumerating objects: 4, done.
remote: Counting objects: 100% (4/4), done.
remote: Compressing objects: 100% (2/2), done.
remote: Total 2 (delta 1), reused 0 (delta 0), pack-reused 0 (from 0)
展开对象中: 100% (2/2), 293 字节 | 146.00 KiB/s, 完成.
来自 https://gitlab.com/AlphaHinex/merge-refs-test
b20a111...2978ae6 refs/merge-requests/1/merge -> origin/merge-requests/1/merge (强制更新)
MR 分支新 commit 与目标分支新 commit 存在冲突时，merge 分支内容是否会同步变化？
此时 merge 分支不再同步变化，后续 MR 分支上的新提交 commit 也不会同步到 merge 分支上，需要处理冲突之后，才能恢复同步。
MR 关闭之后，是否还会同步更新 merge 分支内容？
不会了，重新开启 MR 后会继续同步更新。
参考资料
[1]
10.3 Git 内部原理 - Git 引用: https://git-scm.com/book/zh/v2/Git-内部原理-Git-引用
[2]
10.5 Git 内部原理 - 引用规范: https://git-scm.com/book/zh/v2/Git-内部原理-引用规范
[3]
453542d17ca: https://gitlab.com/gitlab-org/gitlab/-/commits/453542d17ca
[4]
[Commit Status Publisher] Allow refs/merge-requests/* in gitlab support: https://youtrack.jetbrains.com/issue/TW-45099/Commit-Status-Publisher-Allow-refs-merge-requests-in-gitlab-support
[5]
TeamCity Trigger on Pull Request vs Trigger on Merge: https://stackoverflow.com/questions/38619518/teamcity-trigger-on-pull-request-vs-trigger-on-merge/49388182#49388182
[6]
6.3 GitHub - Maintaining a Project: https://git-scm.com/book/id/v2/GitHub-Maintaining-a-Project
[7]
Git refs merge vs head in pull request: https://stackoverflow.com/questions/63594658/git-refs-merge-vs-head-in-pull-request
[8]
Create merge refs for MRs (merge and squash methods): https://gitlab.com/gitlab-org/gitlab-foss/-/issues/47110
[9]
Consider using the merge URL on PR builds: https://github.com/readthedocs/readthedocs.org/issues/6958
[10]
Difference of refs/pull-requests//merge and refs/pull-requests//from: https://community.atlassian.com/t5/Bitbucket-questions/Difference-of-refs-pull-requests-lt-ID-gt-merge-and-refs-pull/qaq-p/772142
[11]
Use merge request HEAD ref for detached merge request pipelines: https://gitlab.com/gitlab-org/gitlab-foss/-/merge_requests/25504
[12]
Able to push to /ref/merge-requests/*/head: https://gitlab.com/gitlab-org/gitlab/-/issues/215432
[13]
Merge requests API: https://docs.gitlab.com/ee/api/merge_requests.html
[14]
Gitlab 中文文档 - Merge requests API: https://www.bookstack.cn/read/gitlab-doc-zh/docs-337.md
[15]
Merge request pipelines: https://docs.gitlab.com/ee/ci/pipelines/merge_request_pipelines.html
[16]
Using Git refs to check out GitLab Merge Requests, from your local repo: https://www.jvt.me/posts/2019/01/19/git-ref-gitlab-merge-requests/
