Add support for FreeBSD in portallocator

Signed-off-by: Tomoya Tabuchi <t@tomoyat1.com>

libnetwork/portallocator/portallocator_freebsd.go

@@ -0,0 +1,42 @@
+package portallocator
+
+import (
+	"bytes"
+	"fmt"
+	"os/exec"
+)
+
+func getDynamicPortRange() (start int, end int, err error) {
+	portRangeKernelSysctl := []string{"net.inet.ip.portrange.hifirst", "net.ip.portrange.hilast"}
+	portRangeFallback := fmt.Sprintf("using fallback port range %d-%d", DefaultPortRangeStart, DefaultPortRangeEnd)
+	portRangeLowCmd := exec.Command("/sbin/sysctl", portRangeKernelSysctl[0])
+	var portRangeLowOut bytes.Buffer
+	portRangeLowCmd.Stdout = &portRangeLowOut
+	cmdErr := portRangeLowCmd.Run()
+	if cmdErr != nil {
+		return 0, 0, fmt.Errorf("port allocator - sysctl net.inet.ip.portrange.hifirst failed - %s: %v", portRangeFallback, err)
+	}
+	n, err := fmt.Sscanf(portRangeLowOut.String(), "%d", &start)
+	if n != 1 || err != nil {
+		if err == nil {
+			err = fmt.Errorf("unexpected count of parsed numbers (%d)", n)
+		}
+		return 0, 0, fmt.Errorf("port allocator - failed to parse system ephemeral port range start from %s - %s: %v", portRangeLowOut.String(), portRangeFallback, err)
+	}
+
+	portRangeHighCmd := exec.Command("/sbin/sysctl", portRangeKernelSysctl[1])
+	var portRangeHighOut bytes.Buffer
+	portRangeHighCmd.Stdout = &portRangeHighOut
+	cmdErr = portRangeHighCmd.Run()
+	if cmdErr != nil {
+		return 0, 0, fmt.Errorf("port allocator - sysctl net.inet.ip.portrange.hilast failed - %s: %v", portRangeFallback, err)
+	}
+	n, err = fmt.Sscanf(portRangeHighOut.String(), "%d", &end)
+	if n != 1 || err != nil {
+		if err == nil {
+			err = fmt.Errorf("unexpected count of parsed numbers (%d)", n)
+		}
+		return 0, 0, fmt.Errorf("port allocator - failed to parse system ephemeral port range end from %s - %s: %v", portRangeHighOut.String(), portRangeFallback, err)
+	}
+	return start, end, nil
+}