From 6d43b73af18d23a4cab88dccf8d93a8eb423d2ed Mon Sep 17 00:00:00 2001
From: Joe Wreschnig <joe.wreschnig@gmail.com>
Date: Sat, 13 Jun 2020 17:41:23 +0200
Subject: [PATCH] Reuse the same channel for all precompleted contexts

This avoids making a new channel and immediately closing it. `benchcmp`
says:

    benchmark                      old ns/op     new ns/op     delta
    BenchmarkPrecompleted-4        213           138           -35.21%

    benchmark                      old allocs    new allocs    delta
    BenchmarkPrecompleted-4        3             2             -33.33%

    benchmark                      old bytes     new bytes     delta
    BenchmarkPrecompleted-4        176           80            -54.55%
---
 context.go | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/context.go b/context.go
index 1bd24d3..3681417 100644
--- a/context.go
+++ b/context.go
@@ -28,14 +28,14 @@ type Context struct {
 func UntilSignal(parent context.Context, sig ...os.Signal) *Context {
 	ctx := new(Context)
 	ctx.parent = parent
-	ctx.done = make(chan struct{})
 
 	if err := parent.Err(); err != nil {
-		close(ctx.done)
+		ctx.done = alreadyclosed
 		ctx.err = err
 		return ctx
 	}
 
+	ctx.done = make(chan struct{})
 	ctx.c = make(chan os.Signal, 1)
 	signal.Notify(ctx.c, sig...)
 	go ctx.wait(sig...)
@@ -103,3 +103,11 @@ func (s *Context) Err() error {
 	s.m.Unlock()
 	return err
 }
+
+// Reuse the same channel for all contexts which begin life already
+// completed.
+var alreadyclosed = make(chan struct{})
+
+func init() {
+	close(alreadyclosed)
+}
-- 
2.20.1