Only search for MR revert commits on notes after MR was merged41807-15665-consistently-502s-because-it-fetches-every-commit

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.

5 files changed, 99 insertions, 6 deletions

diff --git a/app/models/commit.rb b/app/models/commit.rb
index 39d7f5b159d..ede8ad301e4 100644
--- a/app/models/commit.rb
+++ b/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)
+    notes_association ||= notes_with_associations
 
-    noteable.notes_with_associations.system.each do |note|
+    notes_association.system.each do |note|
       note.all_references(current_user, extractor: ext)
     end
 
diff --git a/app/models/merge_request.rb b/app/models/merge_request.rb
index ef58816937c..8efe7d41f37 100644
--- a/app/models/merge_request.rb
+++ b/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?
diff --git a/changelogs/unreleased/41807-15665-consistently-502s-because-it-fetches-every-commit.yml b/changelogs/unreleased/41807-15665-consistently-502s-because-it-fetches-every-commit.yml
new file mode 100644
index 00000000000..146ae12afbd
--- /dev/null
+++ b/changelogs/unreleased/41807-15665-consistently-502s-because-it-fetches-every-commit.yml
@@ -0,0 +1,6 @@
+---
+title: Speed up loading merged merge requests when they contained a lot of commits
+  before merging
+merge_request: 16320
+author:
+type: performance
diff --git a/spec/models/commit_range_spec.rb b/spec/models/commit_range_spec.rb
index 38829773599..f2efcd9d0e9 100644
--- a/spec/models/commit_range_spec.rb
+++ b/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
diff --git a/spec/models/merge_request_spec.rb b/spec/models/merge_request_spec.rb
index 07b3e1c1758..02ccef9becc 100644
--- a/spec/models/merge_request_spec.rb
+++ b/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) }