ENV.deparallelize: add block form for temporary change

Closes Homebrew/homebrew#39026.

Library/Homebrew/extend/ENV/std.rb

@@ -78,8 +78,22 @@ module Stdenv
     paths.select { |d| File.directory? d }.join(File::PATH_SEPARATOR)
   end
 
+  # Removes the MAKEFLAGS environment variable, causing make to use a single job.
+  # This is useful for makefiles with race conditions.
+  # When passed a block, MAKEFLAGS is removed only for the duration of the block and is restored after its completion.
+  # Returns the value of MAKEFLAGS.
   def deparallelize
+    old = self['MAKEFLAGS']
     remove 'MAKEFLAGS', /-j\d+/
+    if block_given?
+      begin
+        yield
+      ensure
+        self['MAKEFLAGS'] = old
+      end
+    end
+
+    old
   end
   alias_method :j1, :deparallelize
 

Library/Homebrew/extend/ENV/super.rb

@@ -241,8 +241,21 @@ module Superenv
 
   public
 
+  # Removes the MAKEFLAGS environment variable, causing make to use a single job.
+  # This is useful for makefiles with race conditions.
+  # When passed a block, MAKEFLAGS is removed only for the duration of the block and is restored after its completion.
+  # Returns the value of MAKEFLAGS.
   def deparallelize
-    delete('MAKEFLAGS')
+    old = delete('MAKEFLAGS')
+    if block_given?
+      begin
+        yield
+      ensure
+        self['MAKEFLAGS'] = old
+      end
+    end
+
+    old
   end
   alias_method :j1, :deparallelize
 

Library/Homebrew/test/test_ENV.rb

@@ -114,6 +114,14 @@ module SharedEnvTests
       assert_equal compiler, @env.compiler
     end
   end
+
+  def test_deparallelize_block_form_restores_makeflags
+    @env['MAKEFLAGS'] = '-j4'
+    @env.deparallelize do
+      assert_nil @env['MAKEFLAGS']
+    end
+    assert_equal '-j4', @env['MAKEFLAGS']
+  end
 end
 
 class StdenvTests < Homebrew::TestCase