DEV: Publish and enable use of pre-built JS assets (#33973)

Building the Discourse ember app is resource-intensive process. This
commit introduces a framework for us to build these assets centrally,
and make them available for people to download.

On every commit to `main`, a new GitHub actions workflow will build
development & production versions of the core assets, and publish them
as a github release under the `discourse/discourse-assets` repository. A
separate repository is being used to avoid polluting the main
`discourse/discourse` repository with one-git-tag-per-release.

The `assemble_ember_build.rb` script is updated to fetch the relevant
asset bundle. Requests are made to `get.discourse.org`, which then
redirects to GitHub releases. This redirection service is being used so
that we have the option to switch away from GitHub releases in future
without breaking existing Discourse installations.

For now, this behavior can be enabled by setting
`DISCOURSE_DOWNLOAD_PRE_BUILT_ASSETS=1`. In the near future, we hope to
make this the default, with opt-out via
`DISCOURSE_DOWNLOAD_PRE_BUILT_ASSETS=0`.

script/assemble_ember_build.rb

@@ -6,6 +6,14 @@ require "fileutils"
 require "tempfile"
 require "open3"
 require "json"
+require "time"
+
+require_relative "../lib/version"
+
+DOWNLOAD_PRE_BUILT_ASSETS = ENV["DISCOURSE_DOWNLOAD_PRE_BUILT_ASSETS"] == "1"
+DOWNLOAD_TEMP_FILE = "#{__dir__}/../tmp/assets.tar.gz"
+
+PRE_BUILD_ROOT = "https://get.discourse.org/discourse-assets"
 
 BUILD_INFO_FILE = "dist/BUILD_INFO.json"
 
@@ -17,6 +25,10 @@ def capture(*args)
   output
 end
 
+def log(message)
+  STDERR.puts "[assemble_ember_build] #{message}"
+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
@@ -41,7 +53,7 @@ def low_memory_environment?
 end
 
 def resolved_ember_env
-  ENV["EMBER_ENV"] || "production"
+  ENV["EMBER_ENV"] || "development"
 end
 
 def build_info
@@ -50,7 +62,7 @@ end
 
 def existing_core_build_usable?
   if !File.exist?(BUILD_INFO_FILE)
-    STDERR.puts "No existing build info file found."
+    log "No existing build info file found."
     return false
   end
 
@@ -60,7 +72,7 @@ def existing_core_build_usable?
   if existing == expected
     true
   else
-    STDERR.puts <<~MSG
+    log <<~MSG
       Existing build is not reusable.
       - Existing: #{existing.inspect}
       - Current: #{expected.inspect}
@@ -69,6 +81,43 @@ def existing_core_build_usable?
   end
 end
 
+def download_prebuild_assets!
+  return false if !DOWNLOAD_PRE_BUILT_ASSETS
+
+  git_is_clean = capture("git", "status", "--porcelain").strip.empty?
+  if !git_is_clean
+    log "Git working directory is not clean. Cannot download prebuilt assets."
+    return false
+  end
+
+  core_commit_hash = capture("git", "rev-parse", "HEAD").strip
+  version_string = "#{Discourse::VERSION::STRING}-#{core_commit_hash.slice(0, 8)}"
+
+  url = "#{PRE_BUILD_ROOT}/#{version_string}/#{resolved_ember_env}.tar.gz"
+  puts "Fetching and extracting #{url}..."
+
+  begin
+    system("curl", "--fail-with-body", "-L", url, "-o", DOWNLOAD_TEMP_FILE, exception: true)
+  rescue RuntimeError => e
+    log "Failed to download prebuilt assets: #{e.message}"
+    return false
+  end
+
+  FileUtils.rm_rf("dist")
+  FileUtils.mkdir_p("dist")
+  begin
+    system("tar", "--strip-components=1", "-xzf", DOWNLOAD_TEMP_FILE, "-C", "dist", exception: true)
+  rescue RuntimeError => e
+    log "Failed to extract prebuilt assets: #{e.message}"
+    return false
+  end
+
+  puts "Prebuilt assets downloaded and extracted successfully."
+  true
+ensure
+  FileUtils.rm_f(DOWNLOAD_TEMP_FILE) if File.exist?(DOWNLOAD_TEMP_FILE)
+end
+
 build_cmd = %w[pnpm ember build]
 build_env = { "CI" => "1" }
 
@@ -78,7 +127,7 @@ if Etc.nprocessors > 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"
+  log "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"
   build_env["JOBS"] = "1"
@@ -86,8 +135,8 @@ end
 
 build_cmd << "-prod" if resolved_ember_env == "production"
 
-if existing_core_build_usable?
-  STDERR.puts "Reusing existing core ember build. Only building plugins..."
+if existing_core_build_usable? || (download_prebuild_assets! && existing_core_build_usable?)
+  log "Reusing existing core ember build. Only building plugins..."
   build_env["SKIP_CORE_BUILD"] = "1"
   build_cmd << "-o" << "dist/_plugin_only_build"
   begin
@@ -97,9 +146,9 @@ if existing_core_build_usable?
   ensure
     FileUtils.rm_rf("dist/_plugin_only_build")
   end
-  STDERR.puts "Plugin build successfully integrated into dist"
+  log "Plugin build successfully integrated into dist"
 else
-  STDERR.puts "Running full core build..."
+  log "Running full core build..."
   system(build_env, *build_cmd, exception: true)
   File.write(BUILD_INFO_FILE, JSON.pretty_generate(build_info))
 end

script/publish_built_assets.rb

@@ -0,0 +1,58 @@
+#!/usr/bin/env ruby
+# frozen_string_literal: true
+# rubocop:disable Discourse/NoChdir
+
+require "fileutils"
+require_relative "../lib/version"
+require "open3"
+
+tmp_dir = "#{__dir__}/../tmp/prebuilt_asset_bundles"
+FileUtils.mkdir_p(tmp_dir)
+
+core_commit_hash = `git rev-parse HEAD`.strip
+version_string = "v#{Discourse::VERSION::STRING}-#{core_commit_hash.slice(0, 8)}"
+release_repo = "discourse/discourse-assets"
+
+puts "Checking if release #{version_string} already exists in #{release_repo}..."
+
+out, status =
+  Open3.capture2e("gh", "--repo", release_repo, "release", "view", version_string, "--json", "name")
+puts out
+
+if status.success?
+  puts "Release #{version_string} already exists in #{release_repo}. Exiting."
+  exit 0
+end
+
+common_env = { "DISCOURSE_DOWNLOAD_PRE_BUILT_ASSETS" => "0", "LOAD_PLUGINS" => "0" }
+
+Dir.chdir("#{__dir__}/../app/assets/javascripts/discourse")
+FileUtils.rm_rf("dist")
+
+system({ **common_env, "EMBER_ENV" => "production" }, "#{__dir__}/assemble_ember_build.rb")
+FileUtils.rm_rf("dist/assets/plugins")
+system("tar", "-czf", "#{tmp_dir}/production.tar.gz", "dist", exception: true)
+
+FileUtils.rm_rf("dist")
+system({ **common_env, "EMBER_ENV" => "development" }, "#{__dir__}/assemble_ember_build.rb")
+FileUtils.rm_rf("dist/assets/plugins")
+system("tar", "-czf", "#{tmp_dir}/development.tar.gz", "dist", exception: true)
+
+puts "Creating release #{version_string} in #{release_repo}..."
+system(
+  "gh",
+  "--repo",
+  release_repo,
+  "release",
+  "create",
+  version_string,
+  "#{tmp_dir}/production.tar.gz",
+  "#{tmp_dir}/development.tar.gz",
+  "--title",
+  version_string,
+  "--notes",
+  version_string,
+  exception: true,
+)
+
+FileUtils.rm_rf(tmp_dir)