net: avoid transiting durations through floats

This slightly simplified the code. I stumbled upon this when support was
being added to Fuchsia (and this pattern was initially cargo-culted).

Change-Id: Ica090a118a0056c5c1b51697691bc7308f0d424a
Reviewed-on: https://go-review.googlesource.com/c/go/+/177878Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/tcpsock.go

@@ -337,3 +337,8 @@ func ListenTCP(network string, laddr *TCPAddr) (*TCPListener, error) {
 	}
 	return ln, nil
 }
+
+// roundDurationUp rounds d to the next multiple of to.
+func roundDurationUp(d time.Duration, to time.Duration) time.Duration {
+	return (d + to - 1) / to
+}

src/net/tcpsockopt_darwin.go

@@ -15,8 +15,7 @@ const sysTCP_KEEPINTVL = 0x101
 
 func setKeepAlivePeriod(fd *netFD, d time.Duration) error {
 	// The kernel expects seconds so round to next highest second.
-	d += (time.Second - time.Nanosecond)
-	secs := int(d.Seconds())
+	secs := int(roundDurationUp(d, time.Second))
 	if err := fd.pfd.SetsockoptInt(syscall.IPPROTO_TCP, sysTCP_KEEPINTVL, secs); err != nil {
 		return wrapSyscallError("setsockopt", err)
 	}

src/net/tcpsockopt_dragonfly.go

@@ -13,8 +13,7 @@ import (
 func setKeepAlivePeriod(fd *netFD, d time.Duration) error {
 	// The kernel expects milliseconds so round to next highest
 	// millisecond.
-	d += (time.Millisecond - time.Nanosecond)
-	msecs := int(d / time.Millisecond)
+	msecs := int(roundDurationUp(d, time.Millisecond))
 	if err := fd.pfd.SetsockoptInt(syscall.IPPROTO_TCP, syscall.TCP_KEEPINTVL, msecs); err != nil {
 		return wrapSyscallError("setsockopt", err)
 	}

src/net/tcpsockopt_solaris.go

@@ -13,8 +13,7 @@ import (
 func setKeepAlivePeriod(fd *netFD, d time.Duration) error {
 	// The kernel expects milliseconds so round to next highest
 	// millisecond.
-	d += (time.Millisecond - time.Nanosecond)
-	msecs := int(d / time.Millisecond)
+	msecs := int(roundDurationUp(d, time.Millisecond))
 
 	// Normally we'd do
 	//	syscall.SetsockoptInt(fd.sysfd, syscall.IPPROTO_TCP, syscall.TCP_KEEPINTVL, secs)

src/net/tcpsockopt_unix.go

@@ -14,8 +14,7 @@ import (
 
 func setKeepAlivePeriod(fd *netFD, d time.Duration) error {
 	// The kernel expects seconds so round to next highest second.
-	d += (time.Second - time.Nanosecond)
-	secs := int(d.Seconds())
+	secs := int(roundDurationUp(d, time.Second))
 	if err := fd.pfd.SetsockoptInt(syscall.IPPROTO_TCP, syscall.TCP_KEEPINTVL, secs); err != nil {
 		return wrapSyscallError("setsockopt", err)
 	}

src/net/tcpsockopt_windows.go

@@ -15,8 +15,7 @@ import (
 func setKeepAlivePeriod(fd *netFD, d time.Duration) error {
 	// The kernel expects milliseconds so round to next highest
 	// millisecond.
-	d += (time.Millisecond - time.Nanosecond)
-	msecs := uint32(d / time.Millisecond)
+	msecs := uint32(roundDurationUp(d, time.Millisecond))
 	ka := syscall.TCPKeepalive{
 		OnOff:    1,
 		Time:     msecs,