PERF: Terminate `rake` process during `ember build` (#15920)

The `assets:precompile` rake task loads the full Ruby app, which can consume around 500mb of RAM by itself. Using `exec` to run `ember build` allows us to free up the Ruby memory and make more space for `ember build`

lib/tasks/assets.rake

@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+EMBER_CLI = ENV["EMBER_CLI_PROD_ASSETS"] != "0"
+
 task 'assets:precompile:before' do
 
   require 'uglifier'
@@ -9,6 +11,14 @@ task 'assets:precompile:before' do
     raise "rake assets:precompile should only be run in RAILS_ENV=production, you are risking unminified assets"
   end
 
+  if EMBER_CLI && !(ENV["EMBER_CLI_COMPILE_DONE"] == "1")
+    # Using exec to free up Rails app memory during ember build
+    exec <<~SCRIPT
+      NODE_OPTIONS='--max-old-space-size=2048' yarn --cwd app/assets/javascripts/discourse run ember build -prod && \
+      EMBER_CLI_COMPILE_DONE=1 bin/rake assets:precompile
+    SCRIPT
+  end
+
   # Ensure we ALWAYS do a clean build
   # We use many .erbs that get out of date quickly, especially with plugins
   STDERR.puts "Purging temp files"
@@ -35,7 +45,7 @@ task 'assets:precompile:before' do
   require 'sprockets'
   require 'digest/sha1'
 
-  if ENV['EMBER_CLI_PROD_ASSETS'] != "0"
+  if EMBER_CLI
     # Remove the assets that Ember CLI will handle for us
     Rails.configuration.assets.precompile.reject! do |asset|
       asset.is_a?(String) && is_ember_cli_asset?(asset)
@@ -82,7 +92,7 @@ task 'assets:flush_sw' => 'environment' do
 end
 
 def is_ember_cli_asset?(name)
-  return false if ENV['EMBER_CLI_PROD_ASSETS'] == '0'
+  return false if !EMBER_CLI
   %w(application.js admin.js ember_jquery.js pretty-text-bundle.js start-discourse.js vendor.js).include?(name)
 end
 
@@ -244,13 +254,6 @@ def copy_ember_cli_assets
   assets = {}
   files = {}
 
-  log_task_duration('ember build -prod') {
-    unless system("NODE_OPTIONS='--max-old-space-size=2048' yarn --cwd #{ember_dir} run ember build -prod")
-      STDERR.puts "Error running ember build"
-      exit 1
-    end
-  }
-
   # Copy assets and generate manifest data
   log_task_duration('Copy assets and generate manifest data') {
     Dir["#{ember_cli_assets}**/*"].each do |f|
@@ -317,7 +320,7 @@ end
 
 task 'assets:precompile' => 'assets:precompile:before' do
 
-  copy_ember_cli_assets if ENV['EMBER_CLI_PROD_ASSETS'] != '0'
+  copy_ember_cli_assets if EMBER_CLI
 
   refresh_days = GlobalSetting.refresh_maxmind_db_during_precompile_days