Add a chaos endpoint that triggers GC

This helps in providing a stable view on worker memory
during development and performance tests.

app/controllers/chaos_controller.rb

@@ -23,6 +23,15 @@ class ChaosController < ActionController::Base
     do_chaos :kill, Chaos::KillWorker
   end
 
+  def gc
+    gc_stat = Gitlab::Chaos.run_gc
+
+    render json: {
+      worker_id: Prometheus::PidProvider.worker_id,
+      gc_stat: gc_stat
+    }
+  end
+
   private
 
   def do_chaos(method, worker, *args)

config/routes.rb

@@ -158,6 +158,7 @@ Rails.application.routes.draw do
         get :db_spin
         get :sleep
         get :kill
+        post :gc
       end
     end
 

doc/development/chaos_endpoints.md

@@ -160,3 +160,58 @@ GET /-/chaos/kill?async=true
 curl "http://localhost:3000/-/chaos/kill" --header 'X-Chaos-Secret: secret'
 curl "http://localhost:3000/-/chaos/kill?token=secret"
 ```
+
+## Run garbage collector
+
+This endpoint triggers a GC run on the worker handling the request and returns its worker ID
+plus GC stats as JSON. This is mostly useful when running Puma in standalone mode, since
+otherwise the worker handling the request will not be known upfront.
+
+Endpoint:
+
+```plaintext
+POST /-/chaos/gc
+```
+
+Example request:
+
+```shell
+curl --request POST "http://localhost:3000/-/chaos/gc" --header 'X-Chaos-Secret: secret'
+curl --request POST "http://localhost:3000/-/chaos/gc?token=secret"
+```
+
+Example response:
+
+```json
+{
+  "worker_id": "puma_1",
+  "gc_stat": {
+    "count": 94,
+    "heap_allocated_pages": 9077,
+    "heap_sorted_length": 9077,
+    "heap_allocatable_pages": 0,
+    "heap_available_slots": 3699720,
+    "heap_live_slots": 2827510,
+    "heap_free_slots": 872210,
+    "heap_final_slots": 0,
+    "heap_marked_slots": 2827509,
+    "heap_eden_pages": 9077,
+    "heap_tomb_pages": 0,
+    "total_allocated_pages": 9077,
+    "total_freed_pages": 0,
+    "total_allocated_objects": 14229357,
+    "total_freed_objects": 11401847,
+    "malloc_increase_bytes": 8192,
+    "malloc_increase_bytes_limit": 30949538,
+    "minor_gc_count": 71,
+    "major_gc_count": 23,
+    "compact_count": 0,
+    "remembered_wb_unprotected_objects": 41685,
+    "remembered_wb_unprotected_objects_limit": 83370,
+    "old_objects": 2617806,
+    "old_objects_limit": 5235612,
+    "oldmalloc_increase_bytes": 8192,
+    "oldmalloc_increase_bytes_limit": 122713697
+  }
+}
+```

lib/gitlab/chaos.rb

@@ -47,5 +47,13 @@ module Gitlab
     def self.kill
       Process.kill("KILL", Process.pid)
     end
+
+    def self.run_gc
+      # Tenure any live objects from young-gen to old-gen
+      4.times { GC.start(full_mark: false) }
+      # Run a full mark-and-sweep collection
+      GC.start
+      GC.stat
+    end
   end
 end

spec/controllers/chaos_controller_spec.rb

@@ -124,4 +124,23 @@ RSpec.describe ChaosController do
       expect(response).to have_gitlab_http_status(:ok)
     end
   end
+
+  describe '#gc' do
+    let(:gc_stat) { GC.stat.stringify_keys }
+
+    it 'runs a full GC on the current web worker' do
+      expect(Prometheus::PidProvider).to receive(:worker_id).and_return('worker-0')
+      expect(Gitlab::Chaos).to receive(:run_gc).and_return(gc_stat)
+
+      post :gc
+
+      expect(response).to have_gitlab_http_status(:ok)
+      expect(response_json['worker_id']).to eq('worker-0')
+      expect(response_json['gc_stat']).to eq(gc_stat)
+    end
+  end
+
+  def response_json
+    Gitlab::Json.parse(response.body)
+  end
 end