Eliminate Gitaly N+1 queries with notes APIsh-eliminate-gitaly-nplus-one-notes

Similar to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31834,
we see that in https://gitlab.com/gitlab-org/gitlab-ce/issues/65957
there can be hundreds, even thousands, of Gitaly requests in the
`/api/:version/projects/:id/merge_requests/:noteable_id/notes` endpoint.

Previously, the API to retrieve notes generated hundreds of Gitaly calls
to determine whether a system note should be shown to the user. It did
this by:

1. Rendering the Markdown
2. Extracting cross-references from the Markdown
3. Issuing a Gitaly `FindCommit` RPC for every reference to validate
that the commit exists.

The last step is unnecessary because we don't need to display a commit
if the user doesn't have access to the project in the first place.
`RendersNotes#prepare_notes_for_rendering` is already used in
`MergeRequestsController`, which is why we don't see N+1 Gitaly calls
there. We use it here to optimize the note redaction process.

2 files changed, 9 insertions, 6 deletions

diff --git a/lib/api/helpers/notes_helpers.rb b/lib/api/helpers/notes_helpers.rb
index 6bf9057fad7..b2bf6bf7417 100644
--- a/lib/api/helpers/notes_helpers.rb
+++ b/lib/api/helpers/notes_helpers.rb
@@ -3,6 +3,8 @@
 module API
   module Helpers
     module NotesHelpers
+      include ::RendersNotes
+
       def self.noteable_types
         # This is a method instead of a constant, allowing EE to more easily
         # extend it.
diff --git a/lib/api/notes.rb b/lib/api/notes.rb
index 9381f045144..84563d66ee8 100644
--- a/lib/api/notes.rb
+++ b/lib/api/notes.rb
@@ -36,12 +36,13 @@ module API
           # page can have less elements than :per_page even if
           # there's more than one page.
           raw_notes = noteable.notes.with_metadata.reorder(order_options_with_tie_breaker)
-          notes =
-            # paginate() only works with a relation. This could lead to a
-            # mismatch between the pagination headers info and the actual notes
-            # array returned, but this is really a edge-case.
-            paginate(raw_notes)
-            .reject { |n| n.cross_reference_not_visible_for?(current_user) }
+
+          # paginate() only works with a relation. This could lead to a
+          # mismatch between the pagination headers info and the actual notes
+          # array returned, but this is really a edge-case.
+          notes = paginate(raw_notes)
+          notes = prepare_notes_for_rendering(notes)
+          notes = notes.reject { |n| n.cross_reference_not_visible_for?(current_user) }
           present notes, with: Entities::Note
         end
         # rubocop: enable CodeReuse/ActiveRecord