Guard against nil dereferenced_target

changelogs/unreleased/jc-guard-against-empty-dereferenced_target.yml

+---
+title: Guard against nil dereferenced_target
+merge_request: 27192
+author:
+type: fixed

lib/gitlab/git/repository.rb

@@ -466,7 +466,7 @@ module Gitlab
         @refs_hash = Hash.new { |h, k| h[k] = [] }
 
         (tags + branches).each do |ref|
-          next unless ref.target && ref.name
+          next unless ref.target && ref.name && ref.dereferenced_target&.id
 
           @refs_hash[ref.dereferenced_target.id] << ref.name
         end

spec/lib/gitlab/git/repository_spec.rb

@@ -531,6 +531,13 @@ describe Gitlab::Git::Repository, :seed_helper do
     it 'has valid commit ids as keys' do
       expect(subject.keys).to all( match(Commit::COMMIT_SHA_PATTERN) )
     end
+
+    it 'does not error when dereferenced_target is nil' do
+      blob_id = repository.blob_at('master', 'README.md').id
+      repository_rugged.tags.create("refs/tags/blob-tag", blob_id)
+
+      expect { subject }.not_to raise_error
+    end
   end
 
   describe '#fetch_repository_as_mirror' do