Return unloaded commit if it couldn't be enriched

When a commit could not be enriched, we want to return the original
commit.

When calling `Commit.lazy` we get a `BatchLoader` instance that
behaves like `nil`. So now we explicitly trigger the loading in batch
and replace the commit with the original commit if it could not be
loaded.

When a commit could not be enriched, we want to return the original
commit.

When calling `Commit.lazy` we get a `BatchLoader` instance that
behaves like `nil`. So now we explicitly trigger the loading in batch
and replace the commit with the original commit if it could not be
loaded.

This makes sure the merge request view can still be loaded for merge
requests for which the source branch became unavailable.

app/models/commit_collection.rb

@@ -72,8 +72,15 @@ class CommitCollection
     end.compact]
 
     # Replace the commits, keeping the same order
-    @commits = @commits.map do |c|
-      replacements.fetch(c.id, c)
+    @commits = @commits.map do |original_commit|
+      # Return the original instance: if it didn't need to be batchloaded, it was
+      # already enriched.
+      batch_loaded_commit = replacements.fetch(original_commit.id, original_commit)
+
+      # If batch loading the commit failed, fall back to the original commit.
+      # We need to explicitly check `.nil?` since otherwise a `BatchLoader` instance
+      # that looks like `nil` is returned.
+      batch_loaded_commit.nil? ? original_commit : batch_loaded_commit
     end
 
     self

changelogs/unreleased/bvl-fix-view-mr-deleted-repo.yml

+---
+title: Fix viewing merge reqeust from a fork that's being deleted
+merge_request: 17894
+author:
+type: fixed

spec/features/merge_request/user_views_merge_request_from_deleted_fork_spec.rb

+# frozen_string_literal: true
+require 'spec_helper'
+
+# This is a feature spec because the problems arrise when rendering the view for
+# an actual project for which the repository is removed but the cached not
+# updated.
+# This can occur when the fork a merge request is created from is in the process
+# of being destroyed.
+describe 'User views merged merge request from deleted fork' do
+  include ProjectForksHelper
+
+  let(:project) { create(:project, :repository) }
+  let(:source_project) { fork_project(project, nil, repository: true) }
+  let(:user) { project.owner }
+  let!(:merge_request) { create(:merge_request, :merged, source_project: source_project, target_project: project) }
+
+  before do
+    sign_in user
+
+    fork_owner = source_project.namespace.owners.first
+    # Place the source_project in the weird in between state
+    source_project.update_attribute(:pending_delete, true)
+    Projects::DestroyService.new(source_project, fork_owner, {}).__send__(:trash_repositories!)
+  end
+
+  it 'correctly shows the merge request' do
+    visit(merge_request_path(merge_request))
+
+    expect(page).to have_content(merge_request.title)
+  end
+end

spec/models/commit_collection_spec.rb

@@ -149,6 +149,17 @@ describe CommitCollection do
 
         collection.enrich!
       end
+
+      it 'returns the original commit if the commit could not be lazy loaded' do
+        collection = described_class.new(project, [hash_commit])
+        unexisting_lazy_commit = Commit.lazy(project, Gitlab::Git::BLANK_SHA)
+
+        expect(Commit).to receive(:lazy).with(project, hash_commit.id).and_return(unexisting_lazy_commit)
+
+        collection.enrich!
+
+        expect(collection.commits).to contain_exactly(hash_commit)
+      end
     end
   end