internal/poll: remove detection of buggy splice on old Linux versions

The splice syscall is buggy prior to Linux 2.6.29. CL 113999 added a
workaround to detect buggy versions and disable use of splice for these.
As of Go 1.18 the minumum Linux version is 2.6.32. Thus, a non-buggy
implementation of the splice syscall can be assumed.

For #45964
Fixes #54505

Change-Id: Ied3a3334da7a3f7fa1280b7c5b1dfb9030219336
Reviewed-on: https://go-review.googlesource.com/c/go/+/422979
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

src/internal/poll/splice_linux.go

@@ -5,10 +5,8 @@
 package poll
 
 import (
-	"internal/syscall/unix"
 	"runtime"
 	"sync"
-	"sync/atomic"
 	"syscall"
 	"unsafe"
 )
@@ -207,40 +205,13 @@ func putPipe(p *splicePipe) {
 	splicePipePool.Put(p)
 }
 
-var disableSplice unsafe.Pointer
-
 // newPipe sets up a pipe for a splice operation.
-func newPipe() (sp *splicePipe) {
-	p := (*bool)(atomic.LoadPointer(&disableSplice))
-	if p != nil && *p {
-		return nil
-	}
-
+func newPipe() *splicePipe {
 	var fds [2]int
-	// pipe2 was added in 2.6.27 and our minimum requirement is 2.6.23, so it
-	// might not be implemented. Falling back to pipe is possible, but prior to
-	// 2.6.29 splice returns -EAGAIN instead of 0 when the connection is
-	// closed.
-	const flags = syscall.O_CLOEXEC | syscall.O_NONBLOCK
-	if err := syscall.Pipe2(fds[:], flags); err != nil {
+	if err := syscall.Pipe2(fds[:], syscall.O_CLOEXEC|syscall.O_NONBLOCK); err != nil {
 		return nil
 	}
-
-	sp = &splicePipe{splicePipeFields: splicePipeFields{rfd: fds[0], wfd: fds[1]}}
-
-	if p == nil {
-		p = new(bool)
-		defer atomic.StorePointer(&disableSplice, unsafe.Pointer(p))
-
-		// F_GETPIPE_SZ was added in 2.6.35, which does not have the -EAGAIN bug.
-		if _, _, errno := syscall.Syscall(unix.FcntlSyscall, uintptr(fds[0]), syscall.F_GETPIPE_SZ, 0); errno != 0 {
-			*p = true
-			destroyPipe(sp)
-			return nil
-		}
-	}
-
-	return
+	return &splicePipe{splicePipeFields: splicePipeFields{rfd: fds[0], wfd: fds[1]}}
 }
 
 // destroyPipe destroys a pipe.