Initial implementation of ForwardCRD plugin (#4512)

* Add forwardcrd plugin README.md

Co-authored-by: Aidan Obley <aobley@vmware.com>

Signed-off-by: Christian Ang <angc@vmware.com>

* Create forwardcrd plugin

- Place forwardcrd before forward plugin in plugin list. This will avoid
forward from preventing the forwardcrd plugin from handling any queries
in the case of having a default upstream forwarder in a server block (as
is the case in the default kubernetes Corefile).

Co-authored-by: Aidan Obley <aobley@vmware.com>

Signed-off-by: Christian Ang <angc@vmware.com>

* Add Forward CRD

Signed-off-by: Christian Ang <angc@vmware.com>

* Add NewWithConfig to forward plugin

- allows external packages to instanciate forward plugins

Co-authored-by: Aidan Obley <aobley@vmware.com>

Signed-off-by: Christian Ang <angc@vmware.com>

* ForwardCRD plugin handles requests for Forward CRs

- add a Kubernetes controller that can read Forward CRs
- instances of the forward plugin are created based on Forward CRs from
the Kubernetes controller
- DNS requests are handled by calling matching Forward plugin instances
based on zone name
- Defaults to the kube-system namespace to align with Corefile RBAC

Signed-off-by: Christian Ang <angc@vmware.com>

Use klog v2 in forwardcrd plugin

* Refactor forward setup to use NewWithConfig

Co-authored-by: Christian Ang <angc@vmware.com>

Signed-off-by: Edwin Xie <exie@vmware.com>

* Use ParseInt instead of Atoi

- to ensure that the bitsize is 32 for later casting to uint32

Signed-off-by: Christian Ang <angc@vmware.com>

* Add @christianang to CODEOWNERS for forwardcrd

Signed-off-by: Christian Ang <angc@vmware.com>

Co-authored-by: Edwin Xie <exie@vmware.com>

plugin/forwardcrd/plugin_map.go

@@ -0,0 +1,83 @@
+package forwardcrd
+
+import (
+	"sync"
+
+	"github.com/coredns/coredns/plugin"
+)
+
+// PluginInstanceMap represents a map of zones to coredns plugin instances that
+// is thread-safe. It enables the forwardcrd plugin to save the state of
+// which plugin instances should be delegated to for a given zone.
+type PluginInstanceMap struct {
+	mutex          *sync.RWMutex
+	zonesToPlugins map[string]lifecyclePluginHandler
+	keyToZones     map[string]string
+}
+
+// NewPluginInstanceMap returns a new instance of PluginInstanceMap.
+func NewPluginInstanceMap() *PluginInstanceMap {
+	return &PluginInstanceMap{
+		mutex:          &sync.RWMutex{},
+		zonesToPlugins: make(map[string]lifecyclePluginHandler),
+		keyToZones:     make(map[string]string),
+	}
+}
+
+// Upsert adds or updates the map with a zone to plugin handler mapping. If the
+// same key is provided it will overwrite the old zone for that key with the
+// new zone. Returns the plugin instance and true if the upsert was an update
+// operation and not a create operation.
+func (p *PluginInstanceMap) Upsert(key, zone string, handler lifecyclePluginHandler) (lifecyclePluginHandler, bool) {
+	var isUpdate bool
+	var oldPlugin lifecyclePluginHandler
+	p.mutex.Lock()
+	normalizedZone := plugin.Host(zone).NormalizeExact()[0] // there can only be one here, won't work with non-octet reverse
+	oldZone, ok := p.keyToZones[key]
+	if ok {
+		oldPlugin = p.zonesToPlugins[oldZone]
+		isUpdate = true
+		delete(p.zonesToPlugins, oldZone)
+	}
+
+	p.keyToZones[key] = normalizedZone
+	p.zonesToPlugins[normalizedZone] = handler
+	p.mutex.Unlock()
+	return oldPlugin, isUpdate
+}
+
+// Get gets the plugin handler provided a zone name. It will return true if the
+// plugin handler exists and false if it does not exist.
+func (p *PluginInstanceMap) Get(zone string) (lifecyclePluginHandler, bool) {
+	p.mutex.RLock()
+	normalizedZone := plugin.Host(zone).NormalizeExact()[0] // there can only be one here, won't work with non-octet reverse
+	handler, ok := p.zonesToPlugins[normalizedZone]
+	p.mutex.RUnlock()
+	return handler, ok
+}
+
+// List lists all the plugin instances in the map.
+func (p *PluginInstanceMap) List() []lifecyclePluginHandler {
+	p.mutex.RLock()
+	plugins := make([]lifecyclePluginHandler, len(p.zonesToPlugins))
+	var i int
+	for _, v := range p.zonesToPlugins {
+		plugins[i] = v
+		i++
+	}
+	p.mutex.RUnlock()
+	return plugins
+}
+
+// Delete deletes the zone and plugin handler from the map. Returns the plugin
+// instance that was deleted, useful for shutting down. Returns nil if no
+// plugin was found.
+func (p *PluginInstanceMap) Delete(key string) lifecyclePluginHandler {
+	p.mutex.Lock()
+	zone := p.keyToZones[key]
+	plugin := p.zonesToPlugins[zone]
+	delete(p.zonesToPlugins, zone)
+	delete(p.keyToZones, key)
+	p.mutex.Unlock()
+	return plugin
+}