context: reduce init-time allocations

Small cleanup to remove a couple of needless global variables.
Instead of relying on two instances of emptyCtx having different
addresses, we use different types.

For #26775

Change-Id: I0bc4813e94226f7b3f52bf4b1b3c3a3bbbebcc9e
Reviewed-on: https://go-review.googlesource.com/c/go/+/455455
Reviewed-by: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Sameer Ajmani <sameer@golang.org>

diff --git a/src/context/context.go b/src/context/context.go
index d6ed7443e9091552ee64d9abedf75af5d37b182d..c60179e70fb0a0d6ea1df9b3a61436a961713e99 100644 (file)
--- a/src/context/context.go
+++ b/src/context/context.go
@@ -172,47 +172,44 @@ func (deadlineExceededError) Error() string   { return "context deadline exceede
 func (deadlineExceededError) Timeout() bool   { return true }
 func (deadlineExceededError) Temporary() bool { return true }
 
-// An emptyCtx is never canceled, has no values, and has no deadline. It is not
-// struct{}, since vars of this type must have distinct addresses.
-type emptyCtx int
+// An emptyCtx is never canceled, has no values, and has no deadline.
+// It is the common base of backgroundCtx and todoCtx.
+type emptyCtx struct{}
 
-func (*emptyCtx) Deadline() (deadline time.Time, ok bool) {
+func (emptyCtx) Deadline() (deadline time.Time, ok bool) {
        return
 }
 
-func (*emptyCtx) Done() <-chan struct{} {
+func (emptyCtx) Done() <-chan struct{} {
        return nil
 }
 
-func (*emptyCtx) Err() error {
+func (emptyCtx) Err() error {
        return nil
 }
 
-func (*emptyCtx) Value(key any) any {
+func (emptyCtx) Value(key any) any {
        return nil
 }
 
-func (e *emptyCtx) String() string {
-       switch e {
-       case background:
-               return "context.Background"
-       case todo:
-               return "context.TODO"
-       }
-       return "unknown empty Context"
+type backgroundCtx struct{ emptyCtx }
+
+func (backgroundCtx) String() string {
+       return "context.Background"
 }
 
-var (
-       background = new(emptyCtx)
-       todo       = new(emptyCtx)
-)
+type todoCtx struct{ emptyCtx }
+
+func (todoCtx) String() string {
+       return "context.TODO"
+}
 
 // Background returns a non-nil, empty Context. It is never canceled, has no
 // values, and has no deadline. It is typically used by the main function,
 // initialization, and tests, and as the top-level Context for incoming
 // requests.
 func Background() Context {
-       return background
+       return backgroundCtx{}
 }
 
 // TODO returns a non-nil, empty Context. Code should use context.TODO when
@@ -220,7 +217,7 @@ func Background() Context {
 // surrounding function has not yet been extended to accept a Context
 // parameter).
 func TODO() Context {
-       return todo
+       return todoCtx{}
 }
 
 // A CancelFunc tells an operation to abandon its work.
@@ -653,7 +650,7 @@ func value(c Context, key any) any {
                                return &ctx.cancelCtx
                        }
                        c = ctx.Context
-               case *emptyCtx:
+               case backgroundCtx, todoCtx:
                        return nil
                default:
                        return c.Value(key)