Ability to get a net listener in a given port range

packer/rpc/build.go

@@ -29,6 +29,7 @@ func (b *Build) Prepare() {
 
 func (b *Build) Run(ui packer.Ui) {
 	// Create and start the server for the UI
+	// TODO: Error handling
 	server := NewServer()
 	server.RegisterUi(ui)
 	server.Start()

packer/rpc/port.go

+package rpc
+
+import (
+	"fmt"
+	"net"
+)
+
+var portRangeMin int = 0
+var portRangeMax int = 0
+
+// This sets the port range that the RPC stuff will use when creating
+// new temporary servers. Some RPC calls require the creation of temporary
+// RPC servers. These allow you to pick a range these bind to.
+func PortRange(min, max int) {
+	portRangeMin = min
+	portRangeMax = max
+}
+
+// This finds an open port in the given range and returns a listener
+// bound to that port.
+func netListenerInRange(min, max int) net.Listener {
+	for port := min; port <= max; port++ {
+		l, err := net.Listen("tcp", fmt.Sprintf(":%d", port))
+		if err == nil {
+			return l
+		}
+	}
+
+	return nil
+}

packer/rpc/port_test.go

+package rpc
+
+import (
+	"cgl.tideland.biz/asserts"
+	"net"
+	"strings"
+	"testing"
+)
+
+func addrPort(address net.Addr) string {
+	parts := strings.Split(address.String(), ":")
+	return parts[len(parts) - 1]
+}
+
+func Test_netListenerInRange(t *testing.T) {
+	assert := asserts.NewTestingAsserts(t, true)
+
+	// Verify it selects an open port
+	L1000, err := net.Listen("tcp", ":10000")
+	defer L1000.Close()
+	assert.Nil(err, "should be able to bind to port 10000")
+
+	L := netListenerInRange(10000, 10005)
+	assert.NotNil(L, "should have a listener")
+	assert.Equal(addrPort(L.Addr()), "10001", "should bind to open port")
+
+	// Returns nil if there are no open ports
+	L = netListenerInRange(10000, 10000)
+	assert.Nil(L, "should not get a listener")
+}