Fix error with GPG signature updater when commit was deleted

app/models/gpg_signature.rb

@@ -60,6 +60,8 @@ class GpgSignature < ActiveRecord::Base
   end
 
   def gpg_commit
+    return unless commit
+
     Gitlab::Gpg::Commit.new(commit)
   end
 end

lib/gitlab/gpg/invalid_gpg_signature_updater.rb

@@ -10,7 +10,7 @@ module Gitlab
           .select(:id, :commit_sha, :project_id)
           .where('gpg_key_id IS NULL OR verification_status <> ?', GpgSignature.verification_statuses[:verified])
           .where(gpg_key_primary_keyid: @gpg_key.keyids)
-          .find_each { |sig| sig.gpg_commit.update_signature!(sig) }
+          .find_each { |sig| sig.gpg_commit&.update_signature!(sig) }
       end
     end
   end

spec/models/gpg_signature_spec.rb

 require 'rails_helper'
 
 RSpec.describe GpgSignature do
+  let(:commit_sha) { '0beec7b5ea3f0fdbc95d0dd47f3c5bc275da8a33' }
+  let!(:project) { create(:project, :repository, path: 'sample-project') }
+  let!(:commit) { create(:commit, project: project, sha: commit_sha) }
+  let(:gpg_signature) { create(:gpg_signature, commit_sha: commit_sha) }
   let(:gpg_key) { create(:gpg_key) }
   let(:gpg_key_subkey) { create(:gpg_key_subkey) }
 
@@ -19,11 +23,6 @@ RSpec.describe GpgSignature do
 
   describe '#commit' do
     it 'fetches the commit through the project' do
-      commit_sha = '0beec7b5ea3f0fdbc95d0dd47f3c5bc275da8a33'
-      project = create :project, :repository
-      commit = create :commit, project: project
-      gpg_signature = create :gpg_signature, commit_sha: commit_sha
-
       expect_any_instance_of(Project).to receive(:commit).with(commit_sha).and_return(commit)
 
       gpg_signature.commit
@@ -44,11 +43,28 @@ RSpec.describe GpgSignature do
     end
 
     it 'clears gpg_key and gpg_key_subkey_id when passing nil' do
-      gpg_signature = create(:gpg_signature, gpg_key: gpg_key_subkey)
       gpg_signature.update_attribute(:gpg_key, nil)
 
       expect(gpg_signature.gpg_key_id).to be_nil
       expect(gpg_signature.gpg_key_subkey_id).to be_nil
     end
   end
+
+  describe '#gpg_commit' do
+    context 'when commit does not exist' do
+      it 'returns nil' do
+        allow(gpg_signature).to receive(:commit).and_return(nil)
+
+        expect(gpg_signature.gpg_commit).to be_nil
+      end
+    end
+
+    context 'when commit exists' do
+      it 'returns an instance of Gitlab::Gpg::Commit' do
+        allow(gpg_signature).to receive(:commit).and_return(commit)
+
+        expect(gpg_signature.gpg_commit).to be_an_instance_of(Gitlab::Gpg::Commit)
+      end
+    end
+  end
 end