Add with_env helper to set temporary ENV vars

Library/Homebrew/test/utils_spec.rb

@@ -270,4 +270,30 @@ describe "globally-scoped helper methods" do
       }.to raise_error(MethodDeprecatedError, %r{method.*replacement.*homebrew/homebrew-core.*homebrew/core}m)
     end
   end
+
+  describe "#with_env" do
+    it "sets environment variables within the block" do
+      expect(ENV["PATH"]).not_to eq("/bin")
+      with_env "PATH" => "/bin" do
+        expect(ENV["PATH"]).to eq("/bin")
+      end
+    end
+
+    it "restores ENV after the block" do
+      with_env "PATH" => "/bin" do
+        expect(ENV["PATH"]).to eq("/bin")
+      end
+      expect(ENV["PATH"]).not_to eq("/bin")
+    end
+
+    it "restores ENV if an exception is raised" do
+      expect {
+        with_env "PATH" => "/bin" do
+          raise StandardError, "boom"
+        end
+      }.to raise_error(StandardError)
+
+      expect(ENV["PATH"]).not_to eq("/bin")
+    end
+  end
 end

Library/Homebrew/utils.rb

@@ -533,3 +533,27 @@ def migrate_legacy_keg_symlinks_if_necessary
   end
   FileUtils.rm_rf legacy_pinned_kegs
 end
+
+# Calls the given block with the passed environment variables
+# added to ENV, then restores ENV afterwards.
+# Example:
+# with_env "PATH" => "/bin" do
+#   system "echo $PATH"
+# end
+#
+# Note that this method is *not* thread-safe - other threads
+# which happen to be scheduled during the block will also
+# see these environment variables.
+def with_env(hash)
+  old_values = {}
+  begin
+    hash.each do |key, value|
+      old_values[key] = ENV.delete(key)
+      ENV[key] = value
+    end
+
+    yield if block_given?
+  ensure
+    ENV.update(old_values)
+  end
+end