DEV: add debugging scripts for memory leaks

These scripts are somewhat rough but I needed them to help debug a memory
leak we have noticed in rails 6.

The biggest object script finds all the biggest objects we have in memory
after boot.

The test memory leak runs a very simple iteration through all multisites
and observed memory.

script/biggest_objects.rb

@@ -0,0 +1,36 @@
+# frozen_string_literal: true
+
+# simple script to measure largest objects in memory post boot
+
+if ENV['RAILS_ENV'] != "production"
+  exec "RAILS_ENV=production ruby #{__FILE__}"
+end
+
+require 'objspace'
+
+ObjectSpace.trace_object_allocations do
+
+  require File.expand_path("../../config/environment", __FILE__)
+
+  Rails.application.routes.recognize_path('abc') rescue nil
+
+  # load up the yaml for the localization bits, in master process
+  I18n.t(:posts)
+
+  RailsMultisite::ConnectionManagement.each_connection do
+    (ActiveRecord::Base.connection.tables - %w[schema_migrations versions]).each do |table|
+      table.classify.constantize.first rescue nil
+    end
+  end
+
+end
+
+5.times do
+  GC.start(full_mark: true, immediate_sweep: true)
+end
+
+[String, Array, Hash].each do |klass|
+  ObjectSpace.each_object(klass).sort { |a, b| b.length <=> a.length }.first(50).each do |obj|
+    puts "#{klass} size: #{obj.length} #{ObjectSpace.allocation_sourcefile(obj)} #{ObjectSpace.allocation_sourceline(obj)}"
+  end
+end

script/test_memory_leak.rb

@@ -0,0 +1,108 @@
+# frozen_string_literal: true
+
+# simple test to check for memory leaks
+#
+# this performs a trivial operation walking all multisites and grabbing first topic / localizing
+# the expectation is that RSS will remain static no matter how many iterations run
+
+if ENV['RAILS_ENV'] != "production"
+  exec "RAILS_ENV=production ruby #{__FILE__}"
+end
+
+if !ENV['LD_PRELOAD']
+  exec "LD_PRELOAD=/usr/lib/libjemalloc.so.1 ruby #{__FILE__}"
+end
+
+if ENV['LD_PRELOAD'].include?("jemalloc")
+  # for 3.6.0 we need a patch jemal 1.1.0 gem (1.1.1 does not support 3.6.0)
+  # however ffi is a problem so we need to patch the gem
+  require 'jemal'
+
+  $jemalloc = true
+end
+
+if ENV['LD_PRELOAD'].include?("mwrap")
+  $mwrap = true
+  require 'mwrap'
+end
+
+def bin_diff(current)
+  $baseline[:arenas].each_with_index do |arena, i|
+    next if !arena || !arena[:bins]
+    arena[:bins].each do |size, stat|
+      allocated = (current.dig(:arenas, i, :bins, size, :allocated) || 0)
+      diff = allocated - stat[:allocated]
+      puts "bin #{size} delta #{diff}"
+    end
+  end
+end
+
+require File.expand_path("../../config/environment", __FILE__)
+
+Rails.application.routes.recognize_path('abc') rescue nil
+I18n.t(:posts)
+
+def rss
+  `ps -o rss -p #{$$}`.chomp.split("\n").last.to_i
+end
+
+def loop_sites
+  RailsMultisite::ConnectionManagement.each_connection do
+    yield
+  end
+end
+
+def biggest_klass(klass)
+  ObjectSpace.each_object(klass).max { |a, b| a.length <=> b.length }
+end
+
+def iter(warmup: false)
+  loop_sites { Topic.first; I18n.t('too_late_to_edit') }
+  if !warmup
+    GC.start(full_mark: true, immediate_sweep: true)
+
+    if $jemalloc
+      jemal_stats = Jemal.stats
+      jedelta = "(jdelta #{jemal_stats[:active] - $baseline_jemalloc_active})"
+    end
+
+    if $mwrap
+      mwrap_delta = (Mwrap.total_bytes_allocated - Mwrap.total_bytes_freed) - $mwrap_baseline
+      mwrap_delta = "(mwrap delta #{mwrap_delta})"
+    end
+
+    rss_delta = rss - $baseline_rss
+    array_delta = biggest_klass(Array).length - $biggest_array_length
+    puts "rss: #{rss} (#{rss_delta}) #{mwrap_delta}#{jedelta} heap_delta: #{GC.stat[:heap_live_slots] - $baseline_slots} array_delta: #{array_delta}"
+
+    if $jemalloc
+      bin_diff(jemal_stats)
+    end
+  end
+
+end
+
+iter(warmup: true)
+4.times do
+  GC.start(full_mark: true, immediate_sweep: true)
+end
+
+if $jemalloc
+  $baseline = Jemal.stats
+  $baseline_jemalloc_active = $baseline[:active]
+  4.times do
+    GC.start(full_mark: true, immediate_sweep: true)
+  end
+end
+
+if $mwrap
+  $mwrap_baseline = Mwrap.total_bytes_allocated - Mwrap.total_bytes_freed
+end
+
+$baseline_slots = GC.stat[:heap_live_slots]
+$baseline_rss = rss
+$biggest_array_length = biggest_klass(Array).length
+
+100000.times do
+  iter
+end