internal/poll: check return value instead of errno for copy_file_range(2)

There is one special case of (0, nil) indicating EOF where the updates
of zero to remain and written are redundant.

Change-Id: I017471657a9424fab88c72d14d3eb66d14a7e5c7
Reviewed-on: https://go-review.googlesource.com/c/go/+/609297
Run-TryBot: Andy Pan <panjf2000@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>

src/internal/poll/copy_file_range_freebsd.go

@@ -22,7 +22,7 @@ func handleCopyFileRangeErr(err error, copied, written int64) (bool, error) {
 	switch err {
 	case syscall.ENOSYS:
 		// The copy_file_range(2) function first appeared in FreeBSD 13.0.
-		// Go supports FreeBSD>= 12, so the system call
+		// Go supports FreeBSD >= 12, so the system call
 		// may not be present. We've detected the FreeBSD version with
 		// unix.SupportCopyFileRange() at the beginning of this function,
 		// but we still want to check for ENOSYS here to prevent some rare

src/internal/poll/copy_file_range_unix.go

@@ -24,7 +24,7 @@ func CopyFileRange(dst, src *FD, remain int64) (written int64, handled bool, err
 			max = maxCopyFileRangeRound
 		}
 		n, e := copyFileRange(dst, src, int(max))
-		if e == nil {
+		if n > 0 {
 			remain -= n
 			written += n
 		}