net: simplify readProtocols via sync.OnceFunc

In this case, using sync.OnceFunc is a better choice.

Change-Id: I52d27b9741265c90300a04a03537020e1aaaaaa7
GitHub-Last-Rev: a281daea255f1508a9042e8c8c7eb7ca1cef2430
GitHub-Pull-Request: golang/go#73434
Reviewed-on: https://go-review.googlesource.com/c/go/+/666635


Reviewed-by: David Chase <drchase@google.com>
Auto-Submit: Jorropo <jorropo.pgm@gmail.com>
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Jorropo <jorropo.pgm@gmail.com>

src/net/lookup_unix.go

@@ -12,11 +12,9 @@ import (
 	"sync"
 )
 
-var onceReadProtocols sync.Once
-
-// readProtocols loads contents of /etc/protocols into protocols map
+// readProtocolsOnce loads contents of /etc/protocols into protocols map
 // for quick access.
-func readProtocols() {
+var readProtocolsOnce = sync.OnceFunc(func() {
 	file, err := open("/etc/protocols")
 	if err != nil {
 		return
@@ -43,12 +41,12 @@ func readProtocols() {
 			}
 		}
 	}
-}
+})
 
 // lookupProtocol looks up IP protocol name in /etc/protocols and
 // returns correspondent protocol number.
 func lookupProtocol(_ context.Context, name string) (int, error) {
-	onceReadProtocols.Do(readProtocols)
+	readProtocolsOnce()
 	return lookupProtocolMap(name)
 }