Feature server implementation (#3899)

* Feature server implementation

Signed-off-by: Spike Curtis <spike@coder.com>

* Fix imports

Signed-off-by: Spike Curtis <spike@coder.com>

Signed-off-by: Spike Curtis <spike@coder.com>

coderd/features.go

@@ -2,7 +2,11 @@ package coderd
 
 import (
 	"net/http"
+	"reflect"
 
+	"golang.org/x/xerrors"
+
+	"github.com/coder/coder/coderd/audit"
 	"github.com/coder/coder/coderd/httpapi"
 	"github.com/coder/coder/codersdk"
 )
@@ -11,11 +15,10 @@ import (
 type FeaturesService interface {
 	EntitlementsAPI(w http.ResponseWriter, r *http.Request)
 
-	// TODO
-	// Get returns the implementations for feature interfaces. Parameter `s `must be a pointer to a
+	// Get returns the implementations for feature interfaces. Parameter `s` must be a pointer to a
 	// struct type containing feature interfaces as fields.  The FeatureService sets all fields to
 	// the correct implementations depending on whether the features are turned on.
-	// Get(s any) error
+	Get(s any) error
 }
 
 type featuresService struct{}
@@ -34,3 +37,57 @@ func (featuresService) EntitlementsAPI(rw http.ResponseWriter, _ *http.Request)
 		HasLicense: false,
 	})
 }
+
+// Get returns the implementations for feature interfaces. Parameter `s` must be a pointer to a
+// struct type containing feature interfaces as fields.  The AGPL featureService always returns the
+// "disabled" version of the feature interface because it doesn't include any enterprise features
+// by definition.
+func (featuresService) Get(ps any) error {
+	if reflect.TypeOf(ps).Kind() != reflect.Pointer {
+		return xerrors.New("input must be pointer to struct")
+	}
+	vs := reflect.ValueOf(ps).Elem()
+	if vs.Kind() != reflect.Struct {
+		return xerrors.New("input must be pointer to struct")
+	}
+	for i := 0; i < vs.NumField(); i++ {
+		vf := vs.Field(i)
+		tf := vf.Type()
+		if tf.Kind() != reflect.Interface {
+			return xerrors.Errorf("fields of input struct must be interfaces: %s", tf.String())
+		}
+		err := setImplementation(vf, tf)
+		if err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// setImplementation finds the correct implementation for the field's type, and sets it on the
+// struct.  It returns an error if unsuccessful
+func setImplementation(vf reflect.Value, tf reflect.Type) error {
+	// when we get more than a few features it might make sense to have a data structure for finding
+	// the correct implementation that's faster than just a linear search, but for now just spin
+	// through the implementations we have.
+	vd := reflect.ValueOf(DisabledImplementations)
+	for j := 0; j < vd.NumField(); j++ {
+		vdf := vd.Field(j)
+		if vdf.Type() == tf {
+			vf.Set(vdf)
+			return nil
+		}
+	}
+	return xerrors.Errorf("unable to find implementation of interface %s", tf.String())
+}
+
+// FeatureInterfaces contains a field for each interface controlled by an enterprise feature.
+type FeatureInterfaces struct {
+	Auditor audit.Auditor
+}
+
+// DisabledImplementations includes all the implementations of turned-off features.  There are no
+// turned-on implementations in AGPL code.
+var DisabledImplementations = FeatureInterfaces{
+	Auditor: audit.NewNop(),
+}