Allow custom predicate to steal BG jobs

Modify the BackgroundMigration.steal method to accept an optional
predicate block that will only steal jobs matching the class name for
which the block returns true.

lib/gitlab/background_migration.rb

@@ -33,6 +33,7 @@ module Gitlab
 
           next unless job.queue == self.queue
           next unless migration_class == steal_class
+          next if block_given? && !(yield migration_args)
 
           begin
             perform(migration_class, migration_args) if job.delete

spec/lib/gitlab/background_migration_spec.rb

@@ -47,6 +47,25 @@ RSpec.describe Gitlab::BackgroundMigration do
 
           described_class.steal('Bar')
         end
+
+        context 'when a custom predicate is given' do
+          it 'steals jobs that match the predicate' do
+            expect(queue[0]).to receive(:delete).and_return(true)
+
+            expect(described_class).to receive(:perform)
+              .with('Foo', [10, 20])
+
+            described_class.steal('Foo') { |(arg1, arg2)| arg1 == 10 && arg2 == 20 }
+          end
+
+          it 'does not steal jobs that do not match the predicate' do
+            expect(described_class).not_to receive(:perform)
+
+            expect(queue[0]).not_to receive(:delete)
+
+            described_class.steal('Foo') { |(arg1, _)| arg1 == 5 }
+          end
+        end
       end
 
       context 'when one of the jobs raises an error' do