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, 6 insertions, 2 deletions

diff --git a/routers/private/hook_pre_receive.go b/routers/private/hook_pre_receive.go
index ccb6933787..8824d9cc39 100644
--- a/routers/private/hook_pre_receive.go
+++ b/routers/private/hook_pre_receive.go
@@ -45,6 +45,8 @@ type preReceiveContext struct {
 	env []string
 
 	opts *private.HookOptions
+
+	branchName string
 }
 
 // CanWriteCode returns true if pusher can write code
@@ -53,7 +55,7 @@ func (ctx *preReceiveContext) CanWriteCode() bool {
 		if !ctx.loadPusherAndPermission() {
 			return false
 		}
-		ctx.canWriteCode = ctx.userPerm.CanWrite(unit.TypeCode) || ctx.deployKeyAccessMode >= perm_model.AccessModeWrite
+		ctx.canWriteCode = ctx.userPerm.CanWriteToBranch(ctx.user, ctx.branchName) || ctx.deployKeyAccessMode >= perm_model.AccessModeWrite
 		ctx.checkedCanWriteCode = true
 	}
 	return ctx.canWriteCode
@@ -134,13 +136,15 @@ func HookPreReceive(ctx *gitea_context.PrivateContext) {
 }
 
 func preReceiveBranch(ctx *preReceiveContext, oldCommitID, newCommitID, refFullName string) {
+	branchName := strings.TrimPrefix(refFullName, git.BranchPrefix)
+	ctx.branchName = branchName
+
 	if !ctx.AssertCanWriteCode() {
 		return
 	}
 
 	repo := ctx.Repo.Repository
 	gitRepo := ctx.Repo.GitRepo
-	branchName := strings.TrimPrefix(refFullName, git.BranchPrefix)
 
 	if branchName == repo.DefaultBranch && newCommitID == git.EmptySHA {
 		log.Warn("Forbidden: Branch: %s is the default branch in %-v and cannot be deleted", branchName, repo)