PERF: Reuse existing core JS build where possible (#32311)

Building the Discourse JS app is very resource-intensive. This commit
introduces an `assemble_ember_build` script which will check the
existing content of the `dist/` directory and re-use the core build if
possible. Plugins will always be rebuilt.

For now, this functionality is only useful for multi-stage (i.e.
non-standard) Discourse deployments. But in future, this script may be
extended to pull the contents of the `dist/` directory from a remote
location.

script/assemble_ember_build.rb

@@ -0,0 +1,104 @@
+#!/usr/bin/env ruby
+# frozen_string_literal: true
+# rubocop:disable Discourse/NoChdir
+
+require "fileutils"
+require "tempfile"
+require "open3"
+require "json"
+
+BUILD_INFO_FILE = "dist/BUILD_INFO.json"
+
+Dir.chdir("#{__dir__}/../app/assets/javascripts/discourse")
+
+def capture(*args)
+  output, status = Open3.capture2(*args)
+  raise "Command failed: #{args.inspect}" if status != 0
+  output
+end
+
+# Returns a git tree-hash representing the current state of Discourse core.
+# If the working directory is clean, it will match the tree hash (note: different to the commit hash) of the HEAD commit.
+def core_tree_hash
+  Tempfile.create do |f|
+    f.close
+
+    git_dir = capture("git", "rev-parse", "--git-dir").strip
+    FileUtils.cp "#{git_dir}/index", f.path
+
+    env = { "GIT_INDEX_FILE" => f.path }
+    system(env, "git", "add", "-A", exception: true)
+    return capture(env, "git", "write-tree").strip
+  end
+end
+
+def node_heap_size_limit
+  capture("node", "-e", "console.log(v8.getHeapStatistics().heap_size_limit/1024/1024)").to_f
+end
+
+def low_memory_environment?
+  node_heap_size_limit < 2048
+end
+
+def resolved_ember_env
+  ENV["EMBER_ENV"] || "production"
+end
+
+def build_info
+  { "ember_env" => resolved_ember_env, "core_tree_hash" => core_tree_hash }
+end
+
+def existing_core_build_usable?
+  if !File.exist?(BUILD_INFO_FILE)
+    STDERR.puts "No existing build info file found."
+    return false
+  end
+
+  existing = JSON.parse(File.read(BUILD_INFO_FILE))
+  expected = build_info
+
+  if existing == expected
+    true
+  else
+    STDERR.puts <<~MSG
+      Existing build is not reusable.
+      - Existing: #{existing.inspect}
+      - Current: #{expected.inspect}
+    MSG
+    false
+  end
+end
+
+build_cmd = %w[pnpm ember build]
+build_env = { "CI" => "1" }
+
+if Etc.nprocessors > 2
+  # Anything more than 2 doesn't seem to improve build times
+  build_env["JOBS"] ||= "2"
+end
+
+if low_memory_environment?
+  STDERR.puts "Node.js heap_size_limit is less than 2048MB. Setting --max-old-space-size=2048 and CHEAP_SOURCE_MAPS=1"
+  build_env["NODE_OPTIONS"] = "--max_old_space_size=2048"
+  build_env["CHEAP_SOURCE_MAPS"] = "1"
+end
+
+build_cmd << "-prod" if resolved_ember_env == "production"
+
+if existing_core_build_usable?
+  STDERR.puts "Reusing existing core ember build. Only building plugins..."
+  build_env["SKIP_CORE_BUILD"] = "1"
+  build_cmd << "-o" << "dist/_plugin_only_build"
+  begin
+    system(build_env, *build_cmd, exception: true)
+    FileUtils.rm_rf("dist/assets/plugins")
+    FileUtils.mv("dist/_plugin_only_build/assets/plugins", "dist/assets/plugins")
+  ensure
+    FileUtils.rm_rf("dist/_plugin_only_build")
+  end
+  STDERR.puts "Plugin build successfully integrated into dist"
+else
+  STDERR.puts "Running full core build..."
+  system(build_env, *build_cmd, exception: true)
+  File.write(BUILD_INFO_FILE, JSON.pretty_generate(build_info))
+end