If the HTTP request to `/user/repo/pulls/N/merge` is cancelled by the user agent, don't stop work once we've passed validation and started to merge the PR. Go will automatically cancel the context if the user-agent disconnects, but that can leave Forgejo in an inconsistent state -- the `git` command can be cancelled at an arbitrary location, the `branch` database table update may not be completed, timers may not be stopped, cross-references may not be populated, etc.
Added test `TestMergeHTTPRequestCancellation` stress-tests the fix by cancelling merge requests, and then verifying that the in-database repository state and in-repository database state are consistent. I've verified that this test fails if the fix is removed -- the in-database commit and commit messages don't match the repository in all PRs.
This is a problem that likely affects other Forgejo endpoints. For example, even the PR merge API would be impacted. But this will be one of the most common real-world places for it to occur, so my thought is we'll see how well this fix works and what (if any) side-effects it has. We can apply a similar pattern in other areas if they are identified as problems.
## Checklist
The [contributor guide](https://forgejo.org/docs/next/contributor/) contains information that will be helpful to first time contributors. All work and communication must conform to Forgejo's [AI Agreement](https://codeberg.org/forgejo/governance/src/branch/main/AIAgreement.md). There also are a few [conditions for merging Pull Requests in Forgejo repositories](https://codeberg.org/forgejo/governance/src/branch/main/PullRequestsAgreement.md). You are also welcome to join the [Forgejo development chatroom](https://matrix.to/#/#forgejo-development:matrix.org).
### Tests for Go changes
- I added test coverage for Go changes...
- [ ] in their respective `*_test.go` for unit tests.
- [x] in the `tests/integration` directory if it involves interactions with a live Forgejo server.
- I ran...
- [ ] `make pr-go` before pushing
### Documentation
- [ ] I created a pull request [to the documentation](https://codeberg.org/forgejo/docs) to explain to Forgejo users how to use this change.
- [x] I did not document these changes and I do not expect someone else to do it.
### Release notes
- [x] This change will be noticed by a Forgejo user or admin (feature, bug fix, performance, etc.). I suggest to include a release note for this change.
- [ ] This change is not visible to a Forgejo user or admin (refactor, dependency upgrade, etc.). I think there is no need to add a release note for this change.
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/11821
Reviewed-by: Andreas Ahlenstorf <aahlenst@noreply.codeberg.org>
Co-authored-by: Mathieu Fenniak <mathieu@fenniak.net>
Co-committed-by: Mathieu Fenniak <mathieu@fenniak.net>
