Only search for MR revert commits on notes after MR was merged

If we search for notes before the MR was merged, we have to load every commit
that was ever part of the MR, or mentioned in a push. In extreme cases, this can
be tens of thousands of commits to load, but we know they can't revert the merge
commit, because they are from before the MR was merged.

In the (rare) case that we don't have a `merged_at` value for the MR, we can
still search all notes.

app/models/commit.rb

@@ -342,10 +342,11 @@ class Commit
     @merged_merge_request_hash[current_user]
   end
-  def has_been_reverted?(current_user, noteable = self)
+  def has_been_reverted?(current_user, notes_association = nil)
     ext = all_references(current_user)
-    noteable.notes_with_associations.system.each do |note|
+    notes_association ||= notes_with_associations
+    notes_association.system.each do |note|
       note.all_references(current_user, extractor: ext)
     end

app/models/merge_request.rb

@@ -982,7 +982,16 @@ class MergeRequest < ActiveRecord::Base
   end
   def can_be_reverted?(current_user)
-    merge_commit && !merge_commit.has_been_reverted?(current_user, self)
+    return false unless merge_commit
+
+    merged_at = metrics&.merged_at
+    notes_association = notes_with_associations
+
+    if merged_at
+      notes_association = notes_association.where('created_at > ?', merged_at)
+    end
+
+    !merge_commit.has_been_reverted?(current_user, notes_association)
   end
   def can_be_cherry_picked?

changelogs/unreleased/41807-15665-consistently-502s-because-it-fetches-every-commit.yml

+---
+title: Speed up loading merged merge requests when they contained a lot of commits
+  before merging
+merge_request: 16320
+author:
+type: performance

spec/models/commit_range_spec.rb

@@ -151,11 +151,11 @@ describe CommitRange do
         .with(commit1, user)
         .and_return(true)
-      expect(commit1.has_been_reverted?(user, issue)).to eq(true)
+      expect(commit1.has_been_reverted?(user, issue.notes_with_associations)).to eq(true)
     end
-    it 'returns false a commit has not been reverted' do
-      expect(commit1.has_been_reverted?(user, issue)).to eq(false)
+    it 'returns false if the commit has not been reverted' do
+      expect(commit1.has_been_reverted?(user, issue.notes_with_associations)).to eq(false)
     end
   end
 end

spec/models/merge_request_spec.rb

@@ -1035,6 +1035,83 @@ describe MergeRequest do
     end
   end
+  describe '#can_be_reverted?' do
+    context 'when there is no merged_at for the MR' do
+      before do
+        subject.metrics.update!(merged_at: nil)
+      end
+
+      it 'returns false' do
+        expect(subject.can_be_reverted?(nil)).to be_falsey
+      end
+    end
+
+    context 'when there is no merge_commit for the MR' do
+      before do
+        subject.metrics.update!(merged_at: Time.now.utc)
+      end
+
+      it 'returns false' do
+        expect(subject.can_be_reverted?(nil)).to be_falsey
+      end
+    end
+
+    context 'when the MR has been merged' do
+      before do
+        MergeRequests::MergeService
+          .new(subject.target_project, subject.author)
+          .execute(subject)
+      end
+
+      context 'when there is no revert commit' do
+        it 'returns true' do
+          expect(subject.can_be_reverted?(nil)).to be_truthy
+        end
+      end
+
+      context 'when there is a revert commit' do
+        let(:current_user) { subject.author }
+        let(:branch) { subject.target_branch }
+        let(:project) { subject.target_project }
+
+        let(:revert_commit_id) do
+          params = {
+            commit: subject.merge_commit,
+            branch_name: branch,
+            start_branch: branch
+          }
+
+          Commits::RevertService.new(project, current_user, params).execute[:result]
+        end
+
+        before do
+          project.add_master(current_user)
+
+          ProcessCommitWorker.new.perform(project.id,
+                                          current_user.id,
+                                          project.commit(revert_commit_id).to_hash,
+                                          project.default_branch == branch)
+        end
+
+        context 'when the revert commit is mentioned in a note after the MR was merged' do
+          it 'returns false' do
+            expect(subject.can_be_reverted?(current_user)).to be_falsey
+          end
+        end
+
+        context 'when the revert commit is mentioned in a note before the MR was merged' do
+          before do
+            subject.notes.last.update!(created_at: subject.metrics.merged_at - 1.second)
+          end
+
+          it 'returns true' do
+            expect(subject.can_be_reverted?(current_user)).to be_truthy
+          end
+        end
+      end
+    end
+  end
+
   describe '#participants' do
     let(:project) { create(:project, :public) }