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.

1 files changed, 5 insertions, 0 deletions

diff --git a/changelogs/unreleased/sh-eliminate-gitaly-nplus-one-notes.yml b/changelogs/unreleased/sh-eliminate-gitaly-nplus-one-notes.yml
new file mode 100644
index 00000000000..00523f53dd7
--- /dev/null
+++ b/changelogs/unreleased/sh-eliminate-gitaly-nplus-one-notes.yml
@@ -0,0 +1,5 @@
+---
+title: Eliminate Gitaly N+1 queries with notes API
+merge_request: 32089
+author:
+type: performance