Limit number of notes on a noteable

Initial limit is set at 5000. System notes are still allowed
to be created even when the noteable is over the limit.

app/models/concerns/noteable.rb

@@ -7,6 +7,8 @@ module Noteable
   # avoiding n+1 queries and improving performance.
   NoteableMeta = Struct.new(:user_notes_count)
 
+  MAX_NOTES_LIMIT = 5_000
+
   class_methods do
     # `Noteable` class names that support replying to individual notes.
     def replyable_types

app/models/note.rb

@@ -104,6 +104,8 @@ class Note < ApplicationRecord
     end
   end
 
+  validate :does_not_exceed_notes_limit?, on: :create, unless: [:system?, :importing?]
+
   # @deprecated attachments are handler by the MarkdownUploader
   mount_uploader :attachment, AttachmentUploader
 
@@ -525,6 +527,12 @@ class Note < ApplicationRecord
 
     system_note_metadata&.cross_reference_types&.include?(system_note_metadata&.action)
   end
+
+  def does_not_exceed_notes_limit?
+    return unless noteable
+
+    errors.add(:base, _('Maximum number of comments exceeded')) if noteable.notes.count >= Noteable::MAX_NOTES_LIMIT
+  end
 end
 
 Note.prepend_if_ee('EE::Note')

changelogs/unreleased/22388-limit-the-number-of-comments-on-a-noteable.yml

+---
+title: Limit the number of comments on an issue, MR, or commit
+merge_request: 18111
+author:
+type: added

doc/user/discussions/index.md

@@ -24,6 +24,9 @@ You can also reply to a comment notification email to reply to the comment if
 creates another standard comment. Replying to a threaded comment creates a reply in the thread. Email replies support
 [Markdown] and [quick actions], just as if you replied from the web.
 
+NOTE: **Note:**
+There is a limit of 5,000 comments for every object.
+
 ## Resolvable comments and threads
 
 > **Notes:**

locale/gitlab.pot

@@ -9932,6 +9932,9 @@ msgstr ""
 msgid "Maximum job timeout has a value which could not be accepted"
 msgstr ""
 
+msgid "Maximum number of comments exceeded"
+msgstr ""
+
 msgid "Maximum number of mirrors that can be synchronizing at the same time."
 msgstr ""
 

spec/models/note_spec.rb

@@ -70,6 +70,37 @@ describe Note do
         is_expected.to be_valid
       end
     end
+
+    describe 'max notes limit' do
+      let_it_be(:noteable) { create(:issue) }
+      let_it_be(:existing_note) { create(:note, project: noteable.project, noteable: noteable) }
+
+      before do
+        stub_const('Noteable::MAX_NOTES_LIMIT', 1)
+      end
+
+      context 'when creating a system note' do
+        subject { build(:system_note, project: noteable.project, noteable: noteable) }
+
+        it { is_expected.to be_valid }
+      end
+
+      context 'when creating a user note' do
+        subject { build(:note, project: noteable.project, noteable: noteable) }
+
+        it { is_expected.not_to be_valid }
+      end
+
+      context 'when updating an existing note on a noteable that already exceeds the limit' do
+        subject { existing_note }
+
+        before do
+          create(:system_note, project: noteable.project, noteable: noteable)
+        end
+
+        it { is_expected.to be_valid }
+      end
+    end
   end
 
   describe "Commit notes" do