Add Cask::Installer#prelude to check before download queueing

Fixes #20374

When using HOMEBREW_DOWNLOAD_CONCURRENCY, cask binaries were being
downloaded before checking if the cask could actually be installed
(e.g., disabled casks or conflict checks). This resulted in unnecessary
downloads for casks that would ultimately fail to install.

This change adds a `prelude` method to Cask::Installer that performs
early validation checks (deprecation/disable status and conflicts)
similar to Formula#prelude_fetch. The prelude method is called before
enqueueing downloads in all download queue scenarios (install, reinstall,
and upgrade commands), ensuring that validation failures occur before
the "Fetching downloads for:" message is displayed.

Key changes:
- Add Cask::Installer#prelude method with @ran_prelude tracking
- Call prelude before enqueueing downloads in install/reinstall/upgrade
- Refactor to avoid creating installer objects multiple times
- Maintain backward compatibility for non-download-queue scenarios

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Library/Homebrew/cask/installer.rb

@@ -46,6 +46,7 @@ module Cask
       @verify_download_integrity = verify_download_integrity
       @quiet = quiet
       @download_queue = download_queue
+      @ran_prelude = T.let(false, T::Boolean)
     end
 
     sig { returns(T::Boolean) }
@@ -140,8 +141,7 @@ module Cask
       old_config = @cask.config
       predecessor = @cask if reinstall? && @cask.installed?
 
-      check_deprecate_disable
-      check_conflicts
+      prelude
 
       print caveats
       fetch
@@ -791,6 +791,16 @@ on_request: true)
       )
     end
 
+    sig { void }
+    def prelude
+      return if @ran_prelude
+
+      check_deprecate_disable
+      check_conflicts
+
+      @ran_prelude = true
+    end
+
     sig { void }
     def enqueue_downloads
       download_queue = @download_queue

Library/Homebrew/cask/reinstall.rb

@@ -30,6 +30,8 @@ module Cask
       end
 
       if download_queue
+        cask_installers.each(&:prelude)
+
         oh1 "Fetching downloads for: #{casks.map { |cask| Formatter.identifier(cask.full_name) }.to_sentence}",
             truncate: false
         cask_installers.each(&:enqueue_downloads)

Library/Homebrew/cask/upgrade.rb

@@ -115,19 +115,22 @@ module Cask
         download_queue = Homebrew::DownloadQueue.new(pour: true)
 
         fetchable_casks = upgradable_casks.map(&:last)
-        fetchable_casks_sentence = fetchable_casks.map { |cask| Formatter.identifier(cask.full_name) }.to_sentence
-        oh1 "Fetching downloads for: #{fetchable_casks_sentence}", truncate: false
-
-        fetchable_casks.each do |cask|
+        fetchable_cask_installers = fetchable_casks.map do |cask|
           # This is significantly easier given the weird difference in Sorbet signatures here.
           # rubocop:disable Style/DoubleNegation
           Installer.new(cask, binaries: !!binaries, verbose: !!verbose, force: !!force,
                                                skip_cask_deps: !!skip_cask_deps, require_sha: !!require_sha,
                                                upgrade: true, quarantine:, download_queue:)
-                   .enqueue_downloads
           # rubocop:enable Style/DoubleNegation
         end
 
+        fetchable_cask_installers.each(&:prelude)
+
+        fetchable_casks_sentence = fetchable_casks.map { |cask| Formatter.identifier(cask.full_name) }.to_sentence
+        oh1 "Fetching downloads for: #{fetchable_casks_sentence}", truncate: false
+
+        fetchable_cask_installers.each(&:enqueue_downloads)
+
         download_queue.fetch
       end
 

Library/Homebrew/cmd/install.rb

@@ -248,17 +248,21 @@ module Homebrew
           fetch_casks = Homebrew::EnvConfig.no_install_upgrade? ? new_casks : casks
 
           if download_queue
-            fetch_casks_sentence = fetch_casks.map { |cask| Formatter.identifier(cask.full_name) }.to_sentence
-            oh1 "Fetching downloads for: #{fetch_casks_sentence}", truncate: false
-
-            fetch_casks.each do |cask|
+            fetch_cask_installers = fetch_casks.map do |cask|
               Cask::Installer.new(cask, binaries: args.binaries?, verbose: args.verbose?,
                                                    force: args.force?, skip_cask_deps: args.skip_cask_deps?,
                                                    require_sha: args.require_sha?, reinstall: true,
                                                    quarantine: args.quarantine?, zap: args.zap?, download_queue:)
-                             .enqueue_downloads
             end
 
+            # Run prelude checks for all casks before enqueueing downloads
+            fetch_cask_installers.each(&:prelude)
+
+            fetch_casks_sentence = fetch_casks.map { |cask| Formatter.identifier(cask.full_name) }.to_sentence
+            oh1 "Fetching downloads for: #{fetch_casks_sentence}", truncate: false
+
+            fetch_cask_installers.each(&:enqueue_downloads)
+
             download_queue.fetch
           end