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.

2 files changed, 13 insertions, 3 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?