Support get_latest_build_number in the GCS Artifact Registry driver (#9)

Enable referencing the latest build with `latest` when using the `build`
flag when using the GCS artifact registry.

Reviewers: rpere, rfaugeroux, joshk

lib/kube_deploy_tools/artifact_registry/driver_gcs.rb

@@ -29,12 +29,21 @@ def get_local_artifact_path(name:, flavor:, local_dir:)
       local_artifact_path
     end
 
+    def get_registry_build_path(project:)
+      # NOTE: If the naming format changes, it represents a breaking change
+      # where all past clients will not be able to list/download new builds
+      # and new clients will not be able to list/download old builds. Change
+      # with caution.
+      #
+      "#{@bucket}/project/#{project}/build"
+    end
+
     def get_registry_artifact_path(name:, flavor:, project:, build_number:)
       # NOTE(joshk): If the naming format changes, it represents a breaking
       # change where all past clients will not be able to download new builds and
       # new clients will not be able to download old builds. Change with caution.
       #
-      "#{@bucket}/project/#{project}/build/#{build_number}/artifact/#{get_artifact_name(name: name, flavor: flavor)}"
+      "#{get_registry_build_path(project: project)}/#{build_number}/artifact/#{get_artifact_name(name: name, flavor: flavor)}"
     end
 
     def get_artifact_name(name:, flavor:)
@@ -84,6 +93,18 @@ def download_artifact(input_path, output_dir_path)
       output_path
     end
 
+    def get_latest_build_number(project)
+      out = Shellrunner.check_call('gsutil', 'ls', get_registry_build_path(project: project))
+
+      # pick out the build numbers from the list
+      build_regex = /([0-9]+)\/$/
+      build_entries = out.scan(build_regex)
+
+      build_entries.
+      map { |x| x[0].to_s.to_i }.
+      max.to_s
+    end
+
     def upload(local_dir:, name:, flavor:, project:, build_number:)
       # Pack up contents of each flavor_dir to a correctly named artifact.
       flavor_dir = File.join(local_dir, "#{name}_#{flavor}")