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Updated documentation of CE to EE merges 

This updates the documentation of automatic CE to EE merges to cover the
new automatic merging setup.
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doc/development/automatic_ce_ee_merge.md
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# Automatic CE->EE merge
GitLab Community Edition is merged automatically every 3 hours into the
Enterprise Edition (look for the [`CE Upstream` merge requests]).
This merge is done automatically in a
[scheduled pipeline](https://gitlab.com/gitlab-org/release-tools/-/jobs/43201679).
## What to do if you are pinged in a `CE Upstream` merge request to resolve a conflict?
1. Please resolve the conflict as soon as possible or ask someone else to do it
- It's ok to resolve more conflicts than the one that you are asked to resolve.
In that case, it's a good habit to ask for a double-check on your resolution
by someone who is familiar with the code you touched.
1. Once you have resolved your conflicts, push to the branch (no force-push)
1. Assign the merge request to the next person that has to resolve a conflict
1. If all conflicts are resolved after your resolution is pushed, keep the merge
request assigned to you: **you are now responsible for the merge request to be
green**
1. If you are the last person to resolve the conflicts, the pipeline is green,
and you have merge rights, merge the MR, but **do not** choose to squash.
Otherwise, assign the MR to someone that can merge.
1. If you need any help, you can ping the current [release managers], or ask in
the `#ce-to-ee` Slack channel
A few notes about the automatic CE->EE merge job:
- If a merge is already in progress, the job
[doesn't create a new one](https://gitlab.com/gitlab-org/release-tools/-/jobs/43157687).
- If there is nothing to merge (i.e. EE is up-to-date with CE), the job doesn't
create a new one
- The job posts messages to the `#ce-to-ee` Slack channel to inform what's the
current CE->EE merge status (e.g. "A new MR has been created", "A MR is still pending")
[`CE Upstream` merge requests]: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests?label_name%5B%5D=CE+upstream
[release managers]: https://about.gitlab.com/release-managers/
Whenever a commit is pushed to the CE `master` branch, it is automatically
merged into the EE `master` branch. If the commit produces any conflicts, it is
instead reverted from CE `master`. When this happens, a merge request will be
set up automatically that can be used to reinstate the changes. This merge
request will be assigned to the author of the conflicting commit, or the merge
request author if the commit author could not be associated with a GitLab user.
If no author could be found, the merge request is assigned to a random member of
the Delivery team. It is then up to this team member to figure out who to assign
the merge request to.
Because some commits can not be reverted if new commits depend on them, we also
run a job periodically that processes a range of commits and tries to merge or
revert them. This should ensure that all commits are either merged into EE
`master`, or reverted, instead of just being left behind in CE.
## Always merge EE merge requests before their CE counterparts
**In order to avoid conflicts in the CE->EE merge, you should always merge the
EE version of your CE merge request first, if present.**
The rationale for this is that as CE->EE merges are done automatically every few
hours, it can happen that:
The rationale for this is that as CE->EE merges are done automatically, it can
happen that:
1. A CE merge request that needs EE-specific changes is merged
1. The automatic CE->EE merge happens
1. A CE merge request that needs EE-specific changes is merged.
1. The automatic CE->EE merge happens.
1. Conflicts due to the CE merge request occur since its EE merge request isn't
merged yet
1. The automatic merge bot will ping someone to resolve the conflict **that are
already resolved in the EE merge request that isn't merged yet**
That's a waste of time, and that's why you should merge EE merge request before
their CE counterpart.
merged yet.
1. The CE changes are reverted.
## Avoiding CE->EE merge conflicts beforehand
......@@ -69,136 +45,89 @@ detect if the current branch's changes will conflict during the CE->EE merge.
The job reports what files are conflicting and how to set up a merge request
against EE.
#### How the job works
1. Generates the diff between your branch and current CE `master`
1. Tries to apply it to current EE `master`
1. If it applies cleanly, the job succeeds, otherwise...
1. Detects a branch with the `ee-` prefix or `-ee` suffix in EE
1. If it exists, generate the diff between this branch and current EE `master`
1. Tries to apply it to current EE `master`
1. If it applies cleanly, the job succeeds
In the case where the job fails, it means you should create an `ee-<ce_branch>`
or `<ce_branch>-ee` branch, push it to EE and open a merge request against EE
`master`.
At this point if you retry the failing job in your CE merge request, it should
now pass.
Notes:
- This task is not a silver-bullet, its current goal is to bring awareness to
developers that their work needs to be ported to EE.
- Community contributors shouldn't be required to submit merge requests against
EE, but reviewers should take actions by either creating such EE merge request
or asking a GitLab developer to do it **before the merge request is merged**.
- If you branch is too far behind `master`, the job will fail. In that case you
should rebase your branch upon latest `master`.
- Code reviews for merge requests often consist of multiple iterations of
feedback and fixes. There is no need to update your EE MR after each
iteration. Instead, create an EE MR as soon as you see the
`ee_compat_check` job failing. After you receive the final approval
from a Maintainer (but **before the CE MR is merged**) update the EE MR.
This helps to identify significant conflicts sooner, but also reduces the
number of times you have to resolve conflicts.
- Please remember to
[always have your EE merge request merged before the CE version](#always-merge-ee-merge-requests-before-their-ce-counterparts).
- You can use [`git rerere`](https://git-scm.com/docs/git-rerere)
to avoid resolving the same conflicts multiple times.
### Cherry-picking from CE to EE
For avoiding merge conflicts, we use a method of creating equivalent branches
for CE and EE. If the `ee-compat-check` job fails, this process is required.
This method only requires that you have cloned both CE and EE into your computer.
If you don't have them yet, please go ahead and clone them:
- Clone CE repo: `git clone git@gitlab.com:gitlab-org/gitlab-ce.git`
- Clone EE repo: `git clone git@gitlab.com:gitlab-org/gitlab-ee.git`
And the only additional setup we need is to add CE as remote of EE and vice-versa:
- Open two terminal windows, one in CE, and another one in EE:
- In EE: `git remote add ce git@gitlab.com:gitlab-org/gitlab-ce.git`
- In CE: `git remote add ee git@gitlab.com:gitlab-org/gitlab-ee.git`
That's all setup we need, so that we can cherry-pick a commit from CE to EE, and
from EE to CE.
Now, every time you create an MR for CE and EE:
1. Open two terminal windows, one in CE, and another one in EE
1. In the CE terminal:
1. Create the CE branch, e.g., `branch-example`
1. Make your changes and push a commit (commit A)
1. Create the CE merge request in GitLab
1. In the EE terminal:
1. Create the EE-equivalent branch ending with `-ee`, e.g.,
`git checkout -b branch-example-ee`
1. Fetch the CE branch: `git fetch ce branch-example`
1. Cherry-pick the commit A: `git cherry-pick commit-A-SHA`
1. If Git prompts you to fix the conflicts, do a `git status`
to check which files contain conflicts, fix them, save the files
1. Add the changes with `git add .` but **DO NOT commit** them
1. Continue cherry-picking: `git cherry-pick --continue`
1. Push to EE: `git push origin branch-example-ee`
1. Create the EE-equivalent MR and link to the CE MR from the
description "Ports [CE-MR-LINK] to EE"
1. Once all the jobs are passing in both CE and EE, you've addressed the
feedback from your own team, and got them approved, the merge requests can be merged.
1. When both MRs are ready, the EE merge request will be merged first, and the
CE-equivalent will be merged next.
**Important notes:**
- The commit SHA can be easily found from the GitLab UI. From a merge request,
open the tab **Commits** and click the copy icon to copy the commit SHA.
- To cherry-pick a **commit range**, such as [A > B > C > D] use:
```shell
git cherry-pick "oldest-commit-SHA^..newest-commit-SHA"
```
For example, suppose the commit A is the oldest, and its SHA is `4f5e4018c09ed797fdf446b3752f82e46f5af502`,
and the commit D is the newest, and its SHA is `80e1c9e56783bd57bd7129828ec20b252ebc0538`.
The cherry-pick command will be:
```shell
git cherry-pick "4f5e4018c09ed797fdf446b3752f82e46f5af502^..80e1c9e56783bd57bd7129828ec20b252ebc0538"
```
- To cherry-pick a **merge commit**, use the flag `-m 1`. For example, suppose that the
merge commit SHA is `138f5e2f20289bb376caffa0303adb0cac859ce1`:
```shell
git cherry-pick -m 1 138f5e2f20289bb376caffa0303adb0cac859ce1
```
- To cherry-pick multiple commits, such as B and D in a range [A > B > C > D], use:
```shell
git cherry-pick commmit-B-SHA commit-D-SHA
```
For example, suppose commit B SHA = `4f5e4018c09ed797fdf446b3752f82e46f5af502`,
and the commit D SHA = `80e1c9e56783bd57bd7129828ec20b252ebc0538`.
The cherry-pick command will be:
```shell
git cherry-pick 4f5e4018c09ed797fdf446b3752f82e46f5af502 80e1c9e56783bd57bd7129828ec20b252ebc0538
```
This case is particularly useful when you have a merge commit in a sequence of
commits and you want to cherry-pick all but the merge commit.
- If you push more commits to the CE branch, you can safely repeat the procedure
to cherry-pick them to the EE-equivalent branch. You can do that as many times as
necessary, using the same CE and EE branches.
- If you submitted the merge request to the CE repo and the `ee-compat-check` job passed,
you are not required to submit the EE-equivalent MR, but it's still recommended. If the
job failed, you are required to submit the EE MR so that you can fix the conflicts in EE
before merging your changes into CE.
---
[Return to Development documentation](README.md)
## How to reinstate changes
When a commit is reverted, the corresponding merge request to reinstate the
changes will include all the details necessary to ensure the changes make it
back into CE and EE. However, you still need to manually set up an EE merge
request that resolves the conflicts.
Each merge request used to reinstate changes will have the "reverted" label
applied. Please do not remove this label, as it will be used to determine how
many times commits are reverted and how long it takes to reinstate the changes.
An example merge request can be found in [CE merge request
23280](https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23280).
## How it works
The automatic merging is performed using a project called [Merge
Train](https://gitlab.com/gitlab-org/merge-train/). For every commit to merge or
revert, we generate patches using `git format-patch` which we then try to apply
using `git am --3way`. If this succeeds we push the changes to EE, if this fails
we decide what to do based on the failure reason:
1. If the patch could not be applied because it was already applied, we just
skip it.
1. If the patch caused conflicts, we revert the source commits.
Commits are reverted in reverse order, ensuring that if commit B depends on A,
and both conflict, we first revert B followed by reverting A.
## FAQ
### Why?
We want to work towards being able to deploy continuously, but this requires
that `master` is always stable and has all the changes we need. If CE `master`
can not be merged into EE `master` due to merge conflicts, this prevents _any_
change from CE making its way into EE. Since GitLab.com runs on EE, this
effectively prevents us from deploying changes.
Past experiences and data have shown that periodic CE to EE merge requests do
not scale, and often take a very long time to complete. For example, [in this
comment](https://gitlab.com/gitlab-org/release/framework/issues/49#note_114614619)
we determined that the average time to close an upstream merge request is around
5 hours, with peaks up to several days. Periodic merge requests are also
frustrating to work with, because they often include many changes unrelated to
your own changes.
Automatically merging or reverting commits allows us to keep merging changes
from CE into EE, as we never have to wait hours for somebody to resolve a set of
merge conflicts.
### Does the CE to EE merge take into account merge commits?
No. When merging CE changes into EE, merge commits are ignored.
### My changes are reverted, but I set up an EE MR to resolve conflicts
Most likely the automatic merge job ran before the EE merge request was merged.
If this keeps happening, consider reporting a bug in the [Merge Train issue
tracker](https://gitlab.com/gitlab-org/merge-train/issues).
### My changes keep getting reverted, and this is really annoying!
This is understandable, but the solution to this is fairly straightforward:
simply set up an EE merge request for every CE merge request, and resolve your
conflicts before the changes are reverted.
### Will we allow certain people to still merge changes, even if they conflict?
No.
### Some files I work with often conflict, how can I best deal with this?
If you find you keep running into merge conflicts, consider refactoring the file
so that the EE specific changes are not intertwined with CE code. For Ruby code
you can do this by moving the EE code to a separate module, which can then be
injected into the appropriate classes or modules. See [Guidelines for
implementing Enterprise Edition features](ee_features.md) for more information.
### Will changelog entries be reverted automatically?
Only if the changelog was added in the commit that was reverted. If a changelog
entry was added in a separate commit, it is possible for it to be left behind.
Since changelog entries are related to the changes in question, there is no real
reason to commit the changelog separately, and as such this should not be a big
problem.
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