Pre-stage cask downloads

- Extract cask containers during queued fetches so installs spend less
  serial time in the staging phase.
- Keep receipt writing and artifact installation in the normal install
  path so installed state does not change early.
- Skip queued extraction when unpack dependencies are still missing so
  dependency installation ordering stays intact.

Author: MikeMcQuaid

Library/Homebrew/cask/download.rb

@@ -5,7 +5,9 @@
 require "fileutils"
 require "unpack_strategy"
 require "cask/cache"
+require "cask/caskroom"
 require "cask/quarantine"
+require "cask/utils"
 
 module Cask
   # A download corresponding to a {Cask}.
@@ -140,6 +142,58 @@ def process_rename_operations(target_dir:)
       end
     end
 
+    sig { returns(Pathname) }
+    def staged_path_from_download_queue
+      HOMEBREW_TEMP/"caskroom"/cask.staged_path.relative_path_from(Caskroom.path)
+    end
+
+    sig { returns(Pathname) }
+    def staged_path_from_download_queue_marker
+      Pathname("#{staged_path_from_download_queue}.staged")
+    end
+
+    sig { params(command: T.class_of(SystemCommand)).void }
+    def purge_staged_from_download_queue(command: SystemCommand)
+      staged_marker = staged_path_from_download_queue_marker
+      FileUtils.rm(staged_marker) if staged_marker.symlink? || staged_marker.exist?
+
+      staged_path = staged_path_from_download_queue
+      return unless staged_path.exist?
+
+      Utils.gain_permissions_remove(staged_path, command:)
+      staged_path.parent.rmdir_if_possible
+    end
+
+    sig { override.params(download: Pathname, pour: T::Boolean).returns(T::Boolean) }
+    def stage_from_download_queue?(download, pour:)
+      return false unless pour
+      return false if cask.staged_path.exist? || staged_path_from_download_queue_marker.exist?
+
+      cask.download ||= download
+      primary_container.dependencies.all? do |dependency|
+        case dependency
+        when Formula
+          dependency.any_version_installed? && dependency.optlinked?
+        when Cask
+          dependency.installed?
+        end
+      end
+    end
+
+    sig { override.params(download: Pathname, pour: T::Boolean).void }
+    def stage_from_download_queue(download, pour:)
+      return unless stage_from_download_queue?(download, pour:)
+
+      purge_staged_from_download_queue
+      cask.download ||= download
+      extract_primary_container(to: staged_path_from_download_queue, verbose: false)
+      process_rename_operations(target_dir: staged_path_from_download_queue)
+      FileUtils.ln_s(staged_path_from_download_queue, staged_path_from_download_queue_marker)
+    rescue
+      purge_staged_from_download_queue
+      raise
+    end
+
     sig { override.returns(T::Boolean) }
     def downloaded_and_valid?
       return false unless super

Library/Homebrew/cask/installer.rb

@@ -132,10 +132,21 @@ def stage
 
       Caskroom.ensure_caskroom_exists
 
-      extract_primary_container
-      process_rename_operations
+      queued_staged_path = downloader.staged_path_from_download_queue
+      queued_staged_marker = downloader.staged_path_from_download_queue_marker
+      if @defer_fetch && queued_staged_marker.exist? && !@cask.staged_path.exist?
+        @cask.staged_path.dirname.mkpath
+        FileUtils.mv(queued_staged_path, @cask.staged_path)
+        FileUtils.rm(queued_staged_marker)
+        queued_staged_path.parent.rmdir_if_possible
+      else
+        downloader.purge_staged_from_download_queue(command: @command) if @defer_fetch
+        extract_primary_container
+        process_rename_operations
+      end
       save_caskfile
     rescue => e
+      downloader.purge_staged_from_download_queue(command: @command) if @defer_fetch
       purge_versioned_files
       raise e
     end

Library/Homebrew/test/cask/installer_spec.rb

@@ -737,6 +737,48 @@ class #{Formulary.class_s(dep_name)} < Formula
 
       installer.enqueue_downloads
     end
+
+    it "stages the main cask download outside Caskroom before install" do
+      cask = Cask::CaskLoader.load(cask_path("local-caffeine"))
+      download_queue = Homebrew::DownloadQueue.new(pour: true)
+      installer = described_class.new(cask, download_queue:, defer_fetch: true)
+
+      begin
+        installer.enqueue_downloads
+        download_queue.fetch
+      ensure
+        download_queue.shutdown
+      end
+
+      expect(cask.staged_path).not_to exist
+      expect(cask).not_to be_installed
+
+      expect(installer).not_to receive(:extract_primary_container)
+
+      installer.stage
+
+      expect(cask.staged_path/"Caffeine.app").to be_a_directory
+      expect(cask).to be_installed
+    end
+
+    it "does not stage queued downloads with missing unpack dependencies" do
+      cask = Cask::CaskLoader.load(cask_path("container-bzip2"))
+      download_queue = Homebrew::DownloadQueue.new(pour: true)
+      installer = described_class.new(cask, download_queue:, defer_fetch: true)
+
+      allow_any_instance_of(Formula).to receive(:any_version_installed?).and_return(false)
+      expect(installer).not_to receive(:extract_primary_container)
+
+      begin
+        installer.enqueue_downloads
+        download_queue.fetch
+      ensure
+        download_queue.shutdown
+      end
+
+      expect(cask.staged_path).not_to exist
+      expect(cask).not_to be_installed
+    end
   end
 
   describe "rename operations" do