internal/poll: use sync.OnceValue to determine kernel version ≥ 5.3 in CopyFileRange

Change-Id: I13fdf86c3f46bf3c83cb116e9dd3bc4ab1a949d7
Reviewed-on: https://go-review.googlesource.com/c/go/+/573755
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: qiulaidongfeng <2645477756@qq.com>

src/internal/poll/copy_file_range_linux.go

@@ -10,27 +10,23 @@ import (
 	"syscall"
 )
 
-var (
-	kernelVersion53Once sync.Once
-	kernelVersion53     bool
-)
+var isKernelVersionGE53 = sync.OnceValue(func() bool {
+	major, minor := unix.KernelVersion()
+	// copy_file_range(2) is broken in various ways on kernels older than 5.3,
+	// see https://go.dev/issue/42400 and
+	// https://man7.org/linux/man-pages/man2/copy_file_range.2.html#VERSIONS
+	if major > 5 || (major == 5 && minor >= 3) {
+		return true
+	}
+	return false
+})
 
 const maxCopyFileRangeRound = 1 << 30
 
 // CopyFileRange copies at most remain bytes of data from src to dst, using
 // the copy_file_range system call. dst and src must refer to regular files.
 func CopyFileRange(dst, src *FD, remain int64) (written int64, handled bool, err error) {
-	kernelVersion53Once.Do(func() {
-		major, minor := unix.KernelVersion()
-		// copy_file_range(2) is broken in various ways on kernels older than 5.3,
-		// see issue #42400 and
-		// https://man7.org/linux/man-pages/man2/copy_file_range.2.html#VERSIONS
-		if major > 5 || (major == 5 && minor >= 3) {
-			kernelVersion53 = true
-		}
-	})
-
-	if !kernelVersion53 {
+	if !isKernelVersionGE53() {
 		return 0, false, nil
 	}