Look at notes created just before merge when deciding if an MR can be reverted

On MySQL, at least, `Note#created_at` doesn't seem to store fractional seconds,
while `MergeRequest::Metrics#merged_at` does. This breaks the optimization
assumption that we only need to search for notes created *after* the MR has
been merged.

Unsynchronized system clocks also make this a dangerous assumption to make.

Adding a minute of leeway still optimizes away most notes, but allows both
cases to be handled more gracefully. If the system clocks are more than a
minute out, we'll still be broken, of course.

1 files changed, 7 insertions, 1 deletions

diff --git a/app/models/merge_request.rb b/app/models/merge_request.rb
index 8028ff3875b..4accb08eaf9 100644
--- a/app/models/merge_request.rb
+++ b/app/models/merge_request.rb
@@ -989,8 +989,14 @@ class MergeRequest < ActiveRecord::Base
     merged_at = metrics&.merged_at
     notes_association = notes_with_associations
 
+    # It is not guaranteed that Note#created_at will be strictly later than
+    # MergeRequestMetric#merged_at. Nanoseconds on MySQL may break this
+    # comparison, as will a HA environment if clocks are not *precisely*
+    # synchronized. Add a minute's leeway to compensate for both possibilities
+    cutoff = merged_at - 1.minute
+
     if merged_at
-      notes_association = notes_association.where('created_at > ?', merged_at)
+      notes_association = notes_association.where('created_at >= ?', cutoff)
     end
 
     !merge_commit.has_been_reverted?(current_user, notes_association)