Add "Allow edits from maintainer" feature (#18002)

Adds a feature [like GitHub has](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/creating-a-pull-request-from-a-fork) (step 7).
If you create a new PR from a forked repo, you can select (and change later, but only if you are the PR creator/poster) the "Allow edits from maintainers" option.
Then users with write access to the base branch get more permissions on this branch:
* use the update pull request button
* push directly from the command line (`git push`)
* edit/delete/upload files via web UI
* use related API endpoints

You can't merge PRs to this branch with this enabled, you'll need "full" code write permissions.

This feature has a pretty big impact on the permission system. I might forgot changing some things or didn't find security vulnerabilities. In this case, please leave a review or comment on this PR.

Closes #17728

Co-authored-by: 6543 <6543@obermui.de>

1 files changed, 10 insertions, 0 deletions

diff --git a/modules/context/permission.go b/modules/context/permission.go
index 142b86faea..8dc3b3cd46 100644
--- a/modules/context/permission.go
+++ b/modules/context/permission.go
@@ -29,6 +29,16 @@ func RequireRepoWriter(unitType unit.Type) func(ctx *Context) {
 	}
 }
 
+// CanEnableEditor checks if the user is allowed to write to the branch of the repo
+func CanEnableEditor() func(ctx *Context) {
+	return func(ctx *Context) {
+		if !ctx.Repo.Permission.CanWriteToBranch(ctx.Doer, ctx.Repo.BranchName) {
+			ctx.NotFound("CanWriteToBranch denies permission", nil)
+			return
+		}
+	}
+}
+
 // RequireRepoWriterOr returns a middleware for requiring repository write to one of the unit permission
 func RequireRepoWriterOr(unitTypes ...unit.Type) func(ctx *Context) {
 	return func(ctx *Context) {