Add back Rugged support for retrieving a commit tree entry

This brings back some of the changes in
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20176/diffs.

We discovered another N+1 that hits Gitaly `TreeEntry` via the
`RelativeLinkFilter`:
https://gitlab.com/gitlab-org/gitlab-ce/issues/58657. When a blob is
loaded with many relative links, `TreeEntry` is called for each link to
scan the URI type.

There are multiple paths that hit Gitaly `TreeEntry`, and
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25706 did not
cover all cases. This commit covers another common use case.

For users using Gitaly on top of NFS, accessing the Git data directly
via Rugged may be faster than going through than Gitaly. This merge
request introduces the feature flag `rugged_commit_tree_entry` to
activate the Rugged method.

changelogs/unreleased/sh-rugged-commit-tree-entry.yml

+---
+title: Bring back Rugged implementation of commit_tree_entry
+merge_request: 25896
+author:
+type: other

lib/gitlab/git/commit.rb

@@ -314,11 +314,16 @@ module Gitlab
       def tree_entry(path)
         return unless path.present?
 
+        commit_tree_entry(path)
+      end
+
+      def commit_tree_entry(path)
         # We're only interested in metadata, so limit actual data to 1 byte
         # since Gitaly doesn't support "send no data" option.
         entry = @repository.gitaly_commit_client.tree_entry(id, path, 1)
         return unless entry
 
+        # To be compatible with the rugged format
         entry = entry.to_h
         entry.delete(:data)
         entry[:name] = File.basename(path)

lib/gitlab/git/rugged_impl/commit.rb

@@ -43,6 +43,30 @@ module Gitlab
           end
         end
 
+        override :commit_tree_entry
+        def commit_tree_entry(path)
+          if Feature.enabled?(:rugged_commit_tree_entry)
+            rugged_tree_entry(path)
+          else
+            super
+          end
+        end
+
+        # Is this the same as Blob.find_entry_by_path ?
+        def rugged_tree_entry(path)
+          rugged_commit.tree.path(path)
+        rescue Rugged::TreeError
+          nil
+        end
+
+        def rugged_commit
+          @rugged_commit ||= if raw_commit.is_a?(Rugged::Commit)
+                               raw_commit
+                             else
+                               @repository.rev_parse_target(id)
+                             end
+        end
+
         def init_from_rugged(commit)
           author = commit.author
           committer = commit.committer

lib/gitlab/git/rugged_impl/repository.rb

@@ -12,7 +12,7 @@ module Gitlab
       module Repository
         extend ::Gitlab::Utils::Override
 
-        FEATURE_FLAGS = %i(rugged_find_commit rugged_tree_entries rugged_tree_entry rugged_commit_is_ancestor).freeze
+        FEATURE_FLAGS = %i(rugged_find_commit rugged_tree_entries rugged_tree_entry rugged_commit_is_ancestor rugged_commit_tree_entry).freeze
 
         def alternate_object_directories
           relative_object_directories.map { |d| File.join(path, d) }

spec/models/commit_spec.rb

@@ -542,7 +542,7 @@ eos
     end
   end
 
-  describe '#uri_type' do
+  shared_examples '#uri_type' do
     it 'returns the URI type at the given path' do
       expect(commit.uri_type('files/html')).to be(:tree)
       expect(commit.uri_type('files/images/logo-black.png')).to be(:raw)
@@ -561,6 +561,20 @@ eos
     end
   end
 
+  describe '#uri_type with Gitaly enabled' do
+    it_behaves_like "#uri_type"
+  end
+
+  describe '#uri_type with Rugged enabled', :enable_rugged do
+    it 'calls out to the Rugged implementation' do
+      allow_any_instance_of(Rugged::Tree).to receive(:path).with('files/html').and_call_original
+
+      commit.uri_type('files/html')
+    end
+
+    it_behaves_like '#uri_type'
+  end
+
   describe '.from_hash' do
     let(:new_commit) { described_class.from_hash(commit.to_hash, project) }