Speed up path generation with build artifacts

Previously it would take seconds to render the pipeline index JSON
(/gitlab-org/gitlab/pipelines.json) in the GitLab project. Apparently,
we render over 5000 paths for `BuildArtifactEntity`, and generation of
these paths dominated the request time.

The Rails path helpers are slow because they check the namespace and
project parameters against large regular expressions, which include
`TOP_LEVEL_ROUTES` and `PROJECT_WILDCARD_ROUTES`, respectively.

We can speed this up by generating the paths directly with
strings. While this isn't ideal, this results in a 10x improvement in
speed:

```
Calculating -------------------------------------
                orig    86.000  i/100ms
                 new     1.272k i/100ms
-------------------------------------------------
                orig      1.343k (±13.8%) i/s -      6.536k
                 new     13.756k (±15.3%) i/s -     67.416k

Comparison:
                 new:    13756.3 i/s
                orig:     1343.3 i/s - 10.24x slower
```

To prevent these helper methods from breaking if the routes ever change,
we add tests to ensure that the results are the same.

Closes https://gitlab.com/gitlab-org/gitlab/issues/121929

app/helpers/gitlab_routing_helper.rb

@@ -153,6 +153,29 @@ module GitlabRoutingHelper
 
   # Artifacts
 
+  # Rails path generators are slow because they need to do large regex comparisons
+  # against the arguments. We can speed this up 10x by generating the strings directly.
+
+  # /*namespace_id/:project_id/-/jobs/:job_id/artifacts/download(.:format)
+  def fast_download_project_job_artifacts_path(project, job)
+    expose_fast_artifacts_path(project, job, :download)
+  end
+
+  # /*namespace_id/:project_id/-/jobs/:job_id/artifacts/keep(.:format)
+  def fast_keep_project_job_artifacts_path(project, job)
+    expose_fast_artifacts_path(project, job, :keep)
+  end
+
+  #  /*namespace_id/:project_id/-/jobs/:job_id/artifacts/browse(/*path)
+  def fast_browse_project_job_artifacts_path(project, job)
+    expose_fast_artifacts_path(project, job, :browse)
+  end
+
+  def expose_fast_artifacts_path(project, job, action)
+    path = "#{project.full_path}/-/jobs/#{job.id}/artifacts/#{action}"
+    Gitlab::Utils.append_path(Gitlab.config.gitlab.relative_url_root, path)
+  end
+
   def artifacts_action_path(path, project, build)
     action, path_params = path.split('/', 2)
     args = [project, build, path_params]

app/serializers/build_artifact_entity.rb

@@ -2,6 +2,7 @@
 
 class BuildArtifactEntity < Grape::Entity
   include RequestAwareEntity
+  include GitlabRoutingHelper
 
   expose :name do |job|
     job.name
@@ -11,15 +12,15 @@ class BuildArtifactEntity < Grape::Entity
   expose :artifacts_expire_at, as: :expire_at
 
   expose :path do |job|
-    download_project_job_artifacts_path(project, job)
+    fast_download_project_job_artifacts_path(project, job)
   end
 
   expose :keep_path, if: -> (*) { job.has_expiring_artifacts? } do |job|
-    keep_project_job_artifacts_path(project, job)
+    fast_keep_project_job_artifacts_path(project, job)
   end
 
   expose :browse_path do |job|
-    browse_project_job_artifacts_path(project, job)
+    fast_browse_project_job_artifacts_path(project, job)
   end
 
   private

changelogs/unreleased/sh-speed-up-build-artifact-entry.yml

+---
+title: Speed up path generation with build artifacts
+merge_request: 22257
+author:
+type: performance

spec/helpers/gitlab_routing_helper_spec.rb

@@ -113,6 +113,29 @@ describe GitlabRoutingHelper do
     end
   end
 
+  context 'artifacts' do
+    let_it_be(:project) { create(:project) }
+    let_it_be(:job) { create(:ci_build, project: project, name: 'test:job', artifacts_expire_at: 1.hour.from_now) }
+
+    describe '#fast_download_project_job_artifacts_path' do
+      it 'matches the Rails download path' do
+        expect(fast_download_project_job_artifacts_path(project, job)).to eq(download_project_job_artifacts_path(project, job))
+      end
+    end
+
+    describe '#fast_keep_project_job_artifacts_path' do
+      it 'matches the Rails keep path' do
+        expect(fast_keep_project_job_artifacts_path(project, job)).to eq(keep_project_job_artifacts_path(project, job))
+      end
+    end
+
+    describe '#fast_browse_project_job_artifacts_path' do
+      it 'matches the Rails browse path' do
+        expect(fast_browse_project_job_artifacts_path(project, job)).to eq(browse_project_job_artifacts_path(project, job))
+      end
+    end
+  end
+
   context 'snippets' do
     let_it_be(:personal_snippet) { create(:personal_snippet) }
     let_it_be(:project_snippet) { create(:project_snippet) }