Make use of local ref if it is reachablesh-avoid-fetching-temp-refs-within-project

1 files changed, 23 insertions, 9 deletions

diff --git a/lib/gitlab/git/repository.rb b/lib/gitlab/git/repository.rb
index d7f5720c795..c12cb6a6434 100644
--- a/lib/gitlab/git/repository.rb
+++ b/lib/gitlab/git/repository.rb
@@ -732,20 +732,27 @@ module Gitlab
       end
 
       def compare_source_branch(target_branch_name, source_repository, source_branch_name, straight:)
-        if source_repository == self
-          return Gitlab::Git::Compare.new(self, target_branch_name, source_branch_name, straight: straight)
-        end
+        reachable_ref =
+          if source_repository == self
+            source_branch_name
+          else
+            # If a tmp ref was created before for a separate repo comparison (forks),
+            # we're able to short-circuit the tmp ref re-creation:
+            # 1. Take the SHA from the source repo
+            # 2. Read that in the current "target" repo
+            # 3. If that SHA is still known (readable), it means GC hasn't
+            # cleaned it up yet, so we can use it instead re-writing the tmp ref.
+            source_commit_id = source_repository.commit(source_branch_name)&.sha
+            commit(source_commit_id)&.sha if source_commit_id
+          end
+
+        return compare(target_branch_name, reachable_ref, straight: straight) if reachable_ref
 
         tmp_ref = "refs/tmp/#{SecureRandom.hex}"
 
         return unless fetch_source_branch!(source_repository, source_branch_name, tmp_ref)
 
-        Gitlab::Git::Compare.new(
-          self,
-          target_branch_name,
-          tmp_ref,
-          straight: straight
-        )
+        compare(target_branch_name, tmp_ref, straight: straight)
       ensure
         delete_refs(tmp_ref) if tmp_ref
       end
@@ -1003,6 +1010,13 @@ module Gitlab
 
       private
 
+      def compare(base_ref, head_ref, straight:)
+        Gitlab::Git::Compare.new(self,
+                                 base_ref,
+                                 head_ref,
+                                 straight: straight)
+      end
+
       def empty_diff_stats
         Gitlab::Git::DiffStatsCollection.new([])
       end