Fix handling of annotated tags when Gitaly is not in use

Attempting to view an annotated tag in the TreeController would result
in `NoMethodError: undefined method 'tree'` when Rugged was in use.
`Blob#find_by_rugged` assumes that the ref is a true. Using
the commit ID ensures that the right ref is being used.

Note that in 11.1, `Blob#find` no longer uses Rugged, so this is only
a bug in 11.0.

Closes gitlab-org/gitlab-ce#47797

changelogs/unreleased/sh-fix-issue-47797-ce.yml

+---
+title: Fix handling of annotated tags when Gitaly is not in use
+merge_request:
+author:
+type: fixed

lib/extracts_path.rb

@@ -139,6 +139,11 @@ module ExtractsPath
 
   def lfs_blob_ids
     blob_ids = tree.blobs.map(&:id)
+
+    # When current endpoint is a Blob then `tree.blobs` will be empty, it means we need to analyze
+    # the current Blob in order to determine if it's a LFS object
+    blob_ids = Array.wrap(@repo.blob_at(@commit.id, @path)&.id) if blob_ids.empty? # rubocop:disable Gitlab/ModuleWithInstanceVariables
+
     @lfs_blob_ids = Gitlab::Git::Blob.batch_lfs_pointers(@project.repository, blob_ids).map(&:id) # rubocop:disable Gitlab/ModuleWithInstanceVariables
   end
 

spec/lib/extracts_path_spec.rb

@@ -203,4 +203,30 @@ describe ExtractsPath do
       expect(extract_ref_without_atom('foo.atom')).to eq(nil)
     end
   end
+
+  describe '#lfs_blob_ids' do
+    shared_examples '#lfs_blob_ids' do
+      let(:tag) { @project.repository.add_tag(@project.owner, 'my-annotated-tag', 'master', 'test tag') }
+      let(:ref) { tag.target }
+      let(:params) { { ref: ref, path: 'README.md' } }
+
+      before do
+        @project = create(:project, :repository)
+      end
+
+      it 'handles annotated tags' do
+        assign_ref_vars
+
+        expect(lfs_blob_ids).to eq([])
+      end
+    end
+
+    context 'when gitaly is enabled' do
+      it_behaves_like '#lfs_blob_ids'
+    end
+
+    context 'when gitaly is disabled', :skip_gitaly_mock do
+      it_behaves_like '#lfs_blob_ids'
+    end
+  end
 end