Make control handlers registry an object and extend its functionality

It is not a singleton anymore. Instead it is an object with a registry
backend. The default registry backend is provided, which is equivalent
to what used to be before. Custom backend can be provided for testing
purposes.

The registry also supports batch operations to remove and add handlers
as an atomic step.

probe/host/controls.go

@@ -16,11 +16,11 @@ const (
 )
 
 func (r *Reporter) registerControls() {
-	controls.Register(ExecHost, r.execHost)
+	r.handlerRegistry.Register(ExecHost, r.execHost)
 }
 
-func (*Reporter) deregisterControls() {
-	controls.Rm(ExecHost)
+func (r *Reporter) deregisterControls() {
+	r.handlerRegistry.Rm(ExecHost)
 }
 
 func (r *Reporter) execHost(req xfer.Request) xfer.Response {

probe/host/reporter.go

@@ -52,24 +52,26 @@ var (
 
 // Reporter generates Reports containing the host topology.
 type Reporter struct {
-	hostID       string
-	hostName     string
-	probeID      string
-	version      string
-	pipes        controls.PipeClient
-	hostShellCmd []string
+	hostID          string
+	hostName        string
+	probeID         string
+	version         string
+	pipes           controls.PipeClient
+	hostShellCmd    []string
+	handlerRegistry *controls.HandlerRegistry
 }
 
 // NewReporter returns a Reporter which produces a report containing host
 // topology for this host.
-func NewReporter(hostID, hostName, probeID, version string, pipes controls.PipeClient) *Reporter {
+func NewReporter(hostID, hostName, probeID, version string, pipes controls.PipeClient, handlerRegistry *controls.HandlerRegistry) *Reporter {
 	r := &Reporter{
-		hostID:       hostID,
-		hostName:     hostName,
-		probeID:      probeID,
-		pipes:        pipes,
-		version:      version,
-		hostShellCmd: getHostShellCmd(),
+		hostID:          hostID,
+		hostName:        hostName,
+		probeID:         probeID,
+		pipes:           pipes,
+		version:         version,
+		hostShellCmd:    getHostShellCmd(),
+		handlerRegistry: handlerRegistry,
 	}
 	r.registerControls()
 	return r

probe/host/reporter_test.go

@@ -7,6 +7,7 @@ import (
 	"time"
 
 	"github.com/weaveworks/scope/common/mtime"
+	"github.com/weaveworks/scope/probe/controls"
 	"github.com/weaveworks/scope/probe/host"
 	"github.com/weaveworks/scope/report"
 )
@@ -55,7 +56,8 @@ func TestReporter(t *testing.T) {
 	host.GetMemoryUsageBytes = func() (float64, float64) { return 60.0, 100.0 }
 	host.GetLocalNetworks = func() ([]*net.IPNet, error) { return []*net.IPNet{ipnet}, nil }
 
-	rpt, err := host.NewReporter(hostID, hostname, "", "", nil).Report()
+	hr := controls.NewDefaultHandlerRegistry()
+	rpt, err := host.NewReporter(hostID, hostname, "", "", nil, hr).Report()
 	if err != nil {
 		t.Fatal(err)
 	}