Add a spec that generates the metadata dynamically

spec/lib/gitlab/ci/build/artifacts/metadata_spec.rb

@@ -112,4 +112,34 @@ describe Gitlab::Ci::Build::Artifacts::Metadata do
       end
     end
   end
+
+  context 'generated metadata' do
+    let(:tmpfile) { Tempfile.new('test-metadata') }
+    let(:generator) { CiArtifactMetadataGenerator.new(tmpfile) }
+    let(:entry_count) { 5 }
+
+    before do
+      tmpfile.binmode
+
+      (1..entry_count).each do |index|
+        generator.add_entry("public/test-#{index}.txt")
+      end
+
+      generator.write
+    end
+
+    after do
+      File.unlink(tmpfile.path)
+    end
+
+    describe '#find_entries!' do
+      it 'reads expected number of entries' do
+        stream = File.open(tmpfile.path)
+
+        metadata = described_class.new(stream, 'public', { recursive: true })
+
+        expect(metadata.find_entries!.count).to eq entry_count
+      end
+    end
+  end
 end

spec/support/helpers/ci_artifact_metadata_generator.rb

+# frozen_sting_literal: true
+
+# This generates fake CI metadata .gz for testing
+# Based off https://gitlab.com/gitlab-org/gitlab-workhorse/blob/master/internal/zipartifacts/metadata.go
+class CiArtifactMetadataGenerator
+  attr_accessor :entries, :output
+
+  ARTIFACT_METADATA = "GitLab Build Artifacts Metadata 0.0.2\n".freeze
+
+  def initialize(stream)
+    @entries = {}
+    @output = Zlib::GzipWriter.new(stream)
+  end
+
+  def add_entry(filename)
+    @entries[filename] = { CRC: rand(0xfffffff), Comment: FFaker::Lorem.sentence(10) }
+  end
+
+  def write
+    write_version
+    write_errors
+    write_entries
+    output.close
+  end
+
+  private
+
+  def write_version
+    write_string(ARTIFACT_METADATA)
+  end
+
+  def write_errors
+    write_string('{}')
+  end
+
+  def write_entries
+    entries.each do |filename, metadata|
+      write_string(filename)
+      write_string(metadata.to_json + "\n")
+    end
+  end
+
+  def write_string(data)
+    bytes = [data.length].pack('L>')
+    output.write(bytes)
+    output.write(data)
+  end
+end