Support processing multiple HTTP listeners

Problem:
The Gateway needs to process a Gateway resource. Currently, that is
already supported by the 'state' package. However, that support is
limited. For example, the package doesn't allow processing  multiple
listeners. More over, the approach used in the 'state' package is hard
to extend to support multiple listeners. 
This prompts development of a better solution.

As a first deliverable, we want to support processing multiple HTTP
listeners assuming they all bind to port 80. 
We also assume that the Gateway only supports a single Gateway resource
with known namespace and name.

Solution:
- Introduce a new package as a replacement of the 'state' package.
- Introduce Configuration, which is an internal representation of the
Gateway configuration. 
- Introduce Statuses, which holds status-related information about
processed resources.
- Introduce ChangeProcessor, which processes changes to Gateway API
resources and returns new Configuration and Statuses.

Note: the new package is not plugged in yet - it will be done in
subsequent commits.

Author: pleshakov

internal/newstate/change_processor.go

@@ -0,0 +1,94 @@
+package newstate
+
+import (
+	"fmt"
+	"sync"
+
+	"k8s.io/apimachinery/pkg/types"
+	"sigs.k8s.io/controller-runtime/pkg/client"
+	"sigs.k8s.io/gateway-api/apis/v1alpha2"
+)
+
+// ChangeProcessor processes the changes to resources producing the internal representation of the Gateway configuration.
+// ChangeProcessor only supports one Gateway resource.
+type ChangeProcessor struct {
+	store    *store
+	changed  bool
+	gwNsName types.NamespacedName
+
+	lock sync.Mutex
+}
+
+// NewChangeProcessor creates a new ChangeProcessor for the Gateway resource with the configured namespace name.
+func NewChangeProcessor(gwNsName types.NamespacedName) *ChangeProcessor {
+	return &ChangeProcessor{
+		store:    newStore(),
+		gwNsName: gwNsName,
+	}
+}
+
+// CaptureUpsertChange captures an upsert change to a resource.
+// It panics if the resource is of unsupported type or if the passed Gateway is different from the one this ChangeProcessor
+// was created for.
+func (c *ChangeProcessor) CaptureUpsertChange(obj client.Object) {
+	c.lock.Lock()
+	defer c.lock.Unlock()
+
+	c.changed = true
+
+	switch o := obj.(type) {
+	case *v1alpha2.Gateway:
+		if o.Namespace != c.gwNsName.Namespace || o.Name != c.gwNsName.Name {
+			panic(fmt.Errorf("gateway resource must be %s/%s, got %s/%s", c.gwNsName.Namespace, c.gwNsName.Name, o.Namespace, o.Name))
+		}
+		c.store.gw = o
+	case *v1alpha2.HTTPRoute:
+		c.store.httpRoutes[getNamespacedName(obj)] = o
+	default:
+		panic(fmt.Errorf("ChangeProcessor doesn't support %T", obj))
+	}
+}
+
+// CaptureDeleteChange captures a delete change to a resource.
+// The method panics if the resource is of unsupported type or if the passed Gateway is different from the one this ChangeProcessor
+// was created for.
+func (c *ChangeProcessor) CaptureDeleteChange(resourceType client.Object, nsname types.NamespacedName) {
+	c.lock.Lock()
+	defer c.lock.Unlock()
+
+	c.changed = true
+
+	switch o := resourceType.(type) {
+	case *v1alpha2.Gateway:
+		if nsname != c.gwNsName {
+			panic(fmt.Errorf("gateway resource must be %s/%s, got %s/%s", c.gwNsName.Namespace, c.gwNsName.Name, o.Namespace, o.Name))
+		}
+		c.store.gw = nil
+	case *v1alpha2.HTTPRoute:
+		delete(c.store.httpRoutes, nsname)
+	default:
+		panic(fmt.Errorf("ChangeProcessor doesn't support %T", resourceType))
+	}
+}
+
+// Process processes any captured changes and produces an internal representation of the Gateway configuration and
+// the status information about the processed resources.
+// If no changes were captured, the changed return argument will be false and both the configuration and statuses
+// will be empty.
+func (c *ChangeProcessor) Process() (changed bool, conf Configuration, statuses Statuses) {
+	c.lock.Lock()
+	defer c.lock.Unlock()
+
+	if !c.changed {
+		return false, conf, statuses
+	}
+
+	c.changed = false
+
+	graph := buildGraph(c.store, c.gwNsName)
+
+	conf = buildConfiguration(graph)
+	statuses = buildStatuses(graph)
+
+	return true, conf, statuses
+}