Disable project avatar validation if avatar has not changed

Every time a column in the projects table is changed, the Avatarable concern
would validate that the avatar file size was under 200K.  This not only delays
the database changes, but it also can lead to unrelated failures if the HTTP
request fails for some reason.

Closes #51053

app/models/concerns/avatarable.rb

@@ -9,7 +9,7 @@ module Avatarable
     include Gitlab::Utils::StrongMemoize
 
     validate :avatar_type, if: ->(user) { user.avatar.present? && user.avatar_changed? }
-    validates :avatar, file_size: { maximum: 200.kilobytes.to_i }
+    validates :avatar, file_size: { maximum: 200.kilobytes.to_i }, if: :avatar_changed?
 
     mount_uploader :avatar, AvatarUploader
 

changelogs/unreleased/sh-disable-unnecessary-avatar-revalidation.yml

+---
+title: Disable project avatar validation if avatar has not changed
+merge_request:
+author:
+type: performance

spec/models/concerns/avatarable_spec.rb

@@ -12,6 +12,26 @@ describe Avatarable do
     stub_config_setting(relative_url_root: relative_url_root)
   end
 
+  describe '#update' do
+    let(:validator) { project._validators[:avatar].detect { |v| v.is_a?(FileSizeValidator) } }
+
+    context 'when avatar changed' do
+      it 'validates the file size' do
+        expect(validator).to receive(:validate_each).and_call_original
+
+        project.update(avatar: 'uploads/avatar.png')
+      end
+    end
+
+    context 'when avatar was not changed' do
+      it 'skips validation of file size' do
+        expect(validator).not_to receive(:validate_each)
+
+        project.update(name: 'Hello world')
+      end
+    end
+  end
+
   describe '#avatar_path' do
     using RSpec::Parameterized::TableSyntax