Refactor DownloadQueue handling

- Use undocumented (for now) `HOMEBREW_DOWNLOAD_CONCURRENCY` instead
  of `--concurrency` flag and avoid passing around `concurrency`
- Create and use `Formula#enqueue_resources_and_patches` helper method
- Rename some method calls to be more obvious
- Use `Downloadable` type to simplify type checks
- General refactoring

Library/Homebrew/cmd/fetch.rb

@@ -26,7 +26,6 @@ module Homebrew
                             "(Pass `all` to download for all architectures.)"
         flag   "--bottle-tag=",
                description: "Download a bottle for given tag."
-        flag "--concurrency=", description: "Number of concurrent downloads.", hidden: true
         switch "--HEAD",
                description: "Fetch HEAD version instead of stable version."
         switch "-f", "--force",
@@ -150,12 +149,7 @@ module Homebrew
                   download_queue.enqueue(resource)
                 end
 
-                formula.resources.each do |r|
-                  download_queue.enqueue(r)
-                  r.patches.each { |patch| download_queue.enqueue(patch.resource) if patch.external? }
-                end
-
-                formula.patchlist.each { |patch| download_queue.enqueue(patch.resource) if patch.external? }
+                formula.enqueue_resources_and_patches(download_queue:)
               end
             end
           else
@@ -181,18 +175,13 @@ module Homebrew
           end
         end
 
-        download_queue.start
+        download_queue.fetch
       ensure
         download_queue.shutdown
       end
 
       private
 
-      sig { returns(Integer) }
-      def concurrency
-        @concurrency ||= T.let(args.concurrency&.to_i || 1, T.nilable(Integer))
-      end
-
       sig { returns(Integer) }
       def retries
         @retries ||= T.let(args.retry? ? FETCH_MAX_TRIES : 0, T.nilable(Integer))
@@ -201,7 +190,7 @@ module Homebrew
       sig { returns(DownloadQueue) }
       def download_queue
         @download_queue ||= T.let(begin
-          DownloadQueue.new(concurrency:, retries:, force: args.force?)
+          DownloadQueue.new(retries:, force: args.force?)
         end, T.nilable(DownloadQueue))
       end
     end

Library/Homebrew/download_queue.rb

@@ -8,16 +8,16 @@ require "retryable_download"
 
 module Homebrew
   class DownloadQueue
-    sig { params(concurrency: Integer, retries: Integer, force: T::Boolean).void }
-    def initialize(concurrency:, retries:, force:)
-      @concurrency = concurrency
-      @quiet = T.let(concurrency > 1, T::Boolean)
+    sig { params(retries: Integer, force: T::Boolean).void }
+    def initialize(retries: 0, force: false)
+      @concurrency = T.let(EnvConfig.download_concurrency, Integer)
+      @quiet = T.let(@concurrency > 1, T::Boolean)
       @tries = T.let(retries + 1, Integer)
       @force = force
       @pool = T.let(Concurrent::FixedThreadPool.new(concurrency), Concurrent::FixedThreadPool)
     end
 
-    sig { params(downloadable: T.any(Resource, Bottle, Cask::Download)).void }
+    sig { params(downloadable: Downloadable).void }
     def enqueue(downloadable)
       downloads[downloadable] ||= Concurrent::Promises.future_on(
         pool, RetryableDownload.new(downloadable, tries:), force, quiet
@@ -28,7 +28,7 @@ module Homebrew
     end
 
     sig { void }
-    def start
+    def fetch
       if concurrency == 1
         downloads.each do |downloadable, promise|
           promise.wait!
@@ -133,6 +133,8 @@ module Homebrew
           $stdout.flush
         end
       end
+
+      downloads.clear
     end
 
     sig { void }
@@ -166,10 +168,9 @@ module Homebrew
     sig { returns(T::Boolean) }
     attr_reader :quiet
 
-    sig { returns(T::Hash[T.any(Resource, Bottle, Cask::Download), Concurrent::Promises::Future]) }
+    sig { returns(T::Hash[Downloadable, Concurrent::Promises::Future]) }
     def downloads
-      @downloads ||= T.let({}, T.nilable(T::Hash[T.any(Resource, Bottle, Cask::Download),
-                                                 Concurrent::Promises::Future]))
+      @downloads ||= T.let({}, T.nilable(T::Hash[Downloadable, Concurrent::Promises::Future]))
     end
 
     class Spinner

Library/Homebrew/env_config.rb

@@ -630,5 +630,13 @@ module Homebrew
     def devcmdrun?
       Homebrew::Settings.read("devcmdrun") == "true"
     end
+
+    sig { returns(Integer) }
+    def download_concurrency
+      # TODO: document this variable when ready to publicly announce it.
+      concurrency = ENV.fetch("HOMEBREW_DOWNLOAD_CONCURRENCY", 1).to_i
+      concurrency = 1 if concurrency <= 1
+      concurrency
+    end
   end
 end

Library/Homebrew/formula.rb

@@ -3202,6 +3202,15 @@ class Formula
     end
   end
 
+  sig { params(download_queue: Homebrew::DownloadQueue).void }
+  def enqueue_resources_and_patches(download_queue:)
+    resources.each do |resource|
+      download_queue.enqueue(resource)
+      resource.patches.select(&:external?).each { |patch| download_queue.enqueue(patch.resource) }
+    end
+    patchlist.select(&:external?).each { |patch| download_queue.enqueue(patch.resource) }
+  end
+
   sig { void }
   def fetch_patches
     patchlist.select(&:external?).each(&:fetch)

Library/Homebrew/retryable_download.rb

@@ -5,10 +5,6 @@ module Homebrew
   class RetryableDownload
     include Downloadable
 
-    sig { returns(Downloadable) }
-    attr_reader :downloadable
-    private :downloadable
-
     sig { override.returns(T.any(NilClass, String, URL)) }
     def url = downloadable.url
 
@@ -92,5 +88,10 @@ module Homebrew
 
     sig { override.returns(String) }
     def download_name = downloadable.download_name
+
+    private
+
+    sig { returns(Downloadable) }
+    attr_reader :downloadable
   end
 end

Library/Homebrew/test/cmd/fetch_spec.rb

@@ -19,7 +19,7 @@ RSpec.describe Homebrew::Cmd::FetchCmd do
     setup_test_formula "testball1"
     setup_test_formula "testball2"
 
-    expect { brew "fetch", "testball1", "testball2", "--concurrency=2" }.to be_a_success
+    expect { brew "fetch", "testball1", "testball2", "HOMEBREW_DOWNLOAD_CONCURRENCY" => "2" }.to be_a_success
 
     expect(HOMEBREW_CACHE/"testball1--0.1.tbz").to be_a_symlink
     expect(HOMEBREW_CACHE/"testball1--0.1.tbz").to exist
@@ -33,7 +33,7 @@ RSpec.describe Homebrew::Cmd::FetchCmd do
     setup_test_formula "testball1"
     setup_test_formula "testball3"
 
-    expect { brew "fetch", "testball1", "testball3", "--concurrency=2" }.to be_a_failure
+    expect { brew "fetch", "testball1", "testball3", "HOMEBREW_DOWNLOAD_CONCURRENCY" => "2" }.to be_a_failure
       .and output(/Error:.*process has already locked/).to_stderr
   end
 end