Merge branch '21459-resolving-merge-conflicts-from-a-forked-repo-through-ui' into 'master'

Fix resolving conflicts on forks

## What does this MR do?

When we resolve conflicts, we create a merge commit in the source branch with parents `[source_branch_head, target_branch_head]`. But when the MR is from a fork, `target_branch_head` might not exist in the source repo at all, so we need to fetch it if it isn't there. We can do this locally so it should be fast.

## Are there points in the code the reviewer needs to double check?

The `TestEnv` changes are needed to reset the branch refs if we're reusing a git directory locally - otherwise, there might not be conflicts!

## Why was this MR needed?

It's a bug in a new feature!

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

Closes #21459.

See merge request !6082

1 files changed, 20 insertions, 1 deletions

diff --git a/app/services/merge_requests/resolve_service.rb b/app/services/merge_requests/resolve_service.rb
index adc71b0c2bc..19caa038c44 100644
--- a/app/services/merge_requests/resolve_service.rb
+++ b/app/services/merge_requests/resolve_service.rb
@@ -1,11 +1,14 @@
 module MergeRequests
   class ResolveService < MergeRequests::BaseService
-    attr_accessor :conflicts, :rugged, :merge_index
+    attr_accessor :conflicts, :rugged, :merge_index, :merge_request
 
     def execute(merge_request)
       @conflicts = merge_request.conflicts
       @rugged = project.repository.rugged
       @merge_index = conflicts.merge_index
+      @merge_request = merge_request
+
+      fetch_their_commit!
 
       conflicts.files.each do |file|
         write_resolved_file_to_index(file, params[:sections])
@@ -27,5 +30,21 @@ module MergeRequests
       merge_index.add(path: our_path, oid: rugged.write(new_file, :blob), mode: file.our_mode)
       merge_index.conflict_remove(our_path)
     end
+
+    # If their commit (in the target project) doesn't exist in the source project, it
+    # can't be a parent for the merge commit we're about to create. If that's the case,
+    # fetch the target branch ref into the source project so the commit exists in both.
+    #
+    def fetch_their_commit!
+      return if rugged.include?(conflicts.their_commit.oid)
+
+      random_string = SecureRandom.hex
+
+      project.repository.fetch_ref(
+        merge_request.target_project.repository.path_to_repo,
+        "refs/heads/#{merge_request.target_branch}",
+        "refs/tmp/#{random_string}/head"
+      )
+    end
   end
 end