Batchload blobs for diff generation

After installing a new gem, batch-loader, a construct can be used to
queue data to be fetched in bulk. The gem was also introduced in both
gitlab-org/gitlab-ce!14680 and gitlab-org/gitlab-ce!14846, but those mrs
are not merged yet.

For the generation of diffs, both the old blob and the new blob need to
be loaded. This for every file in the diff, too. Now we collect all
these so we do 1 fetch. Three `.allow_n_plus_1_calls` have been removed,
which I expect to be valid, but this needs to be confirmed by a full CI
run.

Possibly closes:
- https://gitlab.com/gitlab-org/gitlab-ce/issues/37445
- https://gitlab.com/gitlab-org/gitlab-ce/issues/37599
- https://gitlab.com/gitlab-org/gitlab-ce/issues/37431

2 files changed, 9 insertions, 14 deletions

diff --git a/lib/gitlab/diff/file.rb b/lib/gitlab/diff/file.rb
index ea5891a028a..d0cfe2386ca 100644
--- a/lib/gitlab/diff/file.rb
+++ b/lib/gitlab/diff/file.rb
@@ -25,6 +25,10 @@ module Gitlab
         @repository = repository
         @diff_refs = diff_refs
         @fallback_diff_refs = fallback_diff_refs
+
+        # Ensure items are collected in the the batch
+        new_blob
+        old_blob
       end
 
       def position(position_marker, position_type: :text)
@@ -95,21 +99,15 @@ module Gitlab
       end
 
       def new_blob
-        return @new_blob if defined?(@new_blob)
-
-        sha = new_content_sha
-        return @new_blob = nil unless sha
+        return unless new_content_sha
 
-        @new_blob = repository.blob_at(sha, file_path)
+        Blob.lazy(repository.project, new_content_sha, file_path)
       end
 
       def old_blob
-        return @old_blob if defined?(@old_blob)
-
-        sha = old_content_sha
-        return @old_blob = nil unless sha
+        return unless old_content_sha
 
-        @old_blob = repository.blob_at(sha, old_path)
+        Blob.lazy(repository.project, old_content_sha, old_path)
       end
 
       def content_sha
diff --git a/lib/gitlab/diff/file_collection/base.rb b/lib/gitlab/diff/file_collection/base.rb
index 88ae65cb468..a6007ebf531 100644
--- a/lib/gitlab/diff/file_collection/base.rb
+++ b/lib/gitlab/diff/file_collection/base.rb
@@ -22,10 +22,7 @@ module Gitlab
         end
 
         def diff_files
-          # n+1: https://gitlab.com/gitlab-org/gitlab-ce/issues/37445
-          Gitlab::GitalyClient.allow_n_plus_1_calls do
-            @diff_files ||= @diffs.decorate! { |diff| decorate_diff!(diff) }
-          end
+          @diff_files ||= @diffs.decorate! { |diff| decorate_diff!(diff) }
         end
 
         def diff_file_with_old_path(old_path)