From d15a0cd9ebbd4ebd51a52f7547a18a66c52ef411 Mon Sep 17 00:00:00 2001
From: Mike McQuaid <mike@mikemcquaid.com>
Date: Tue, 29 Jul 2025 15:15:37 +0100
Subject: [PATCH] env_config: add `auto` option for `download_concurrency`

This will allow the user to set `HOMEBREW_DOWNLOAD_CONCURRENCY` to
`auto` to have Homebrew use the number of cores on the machine * 2.
---
 Library/Homebrew/env_config.rb  | 13 ++++++++++---
 Library/Homebrew/env_config.rbi |  8 ++++++++
 2 files changed, 18 insertions(+), 3 deletions(-)
 create mode 100644 Library/Homebrew/env_config.rbi

diff --git a/Library/Homebrew/env_config.rb b/Library/Homebrew/env_config.rb
index 6a2a558232..1d363b5d57 100644
--- a/Library/Homebrew/env_config.rb
+++ b/Library/Homebrew/env_config.rb
@@ -634,9 +634,16 @@ module Homebrew
     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
+      concurrency = ENV.fetch("HOMEBREW_DOWNLOAD_CONCURRENCY", 1)
+      concurrency = if concurrency == "auto"
+        require "os"
+        require "hardware"
+        Hardware::CPU.cores * 2
+      else
+        concurrency.to_i
+      end
+
+      [concurrency, 1].max
     end
   end
 end
diff --git a/Library/Homebrew/env_config.rbi b/Library/Homebrew/env_config.rbi
new file mode 100644
index 0000000000..d6e994727e
--- /dev/null
+++ b/Library/Homebrew/env_config.rbi
@@ -0,0 +1,8 @@
+# typed: strict
+# frozen_string_literal: true
+
+module Homebrew
+  module EnvConfig
+    include Kernel
+  end
+end