lang: core, funcs: Port some functions to CallableFunc API

Some modern features of our function engine and language might require
this new API, so port what we can and figure out the rest later.

lang/core/example/flipflop_fact.go

@@ -31,6 +31,7 @@ package coreexample
 
 import (
 	"context"
+	"sync"
 	"time"
 
 	"github.com/purpleidea/mgmt/lang/funcs/facts"
@@ -52,6 +53,7 @@ func init() {
 // function which you could specify an interval for.
 type FlipFlopFact struct {
 	init  *facts.Init
+	mutex *sync.Mutex
 	value bool
 }
 
@@ -77,6 +79,7 @@ func (obj *FlipFlopFact) Info() *facts.Info {
 // Init runs some startup code for this fact.
 func (obj *FlipFlopFact) Init(init *facts.Init) error {
 	obj.init = init
+	obj.mutex = &sync.Mutex{}
 	return nil
 }
 
@@ -100,14 +103,30 @@ func (obj *FlipFlopFact) Stream(ctx context.Context) error {
 			return nil
 		}
 
+		result, err := obj.Call(ctx)
+		if err != nil {
+			return err
+		}
+
+		obj.mutex.Lock()
+		obj.value = !obj.value // flip it
+		obj.mutex.Unlock()
+
 		select {
-		case obj.init.Output <- &types.BoolValue{ // flip
-			V: obj.value,
-		}:
+		case obj.init.Output <- result:
+
 		case <-ctx.Done():
 			return nil
 		}
-
-		obj.value = !obj.value // flip it
 	}
 }
+
+// Call this fact and return the value if it is possible to do so at this time.
+func (obj *FlipFlopFact) Call(ctx context.Context) (types.Value, error) {
+	obj.mutex.Lock() // TODO: could be a read lock
+	value := obj.value
+	obj.mutex.Unlock()
+	return &types.BoolValue{
+		V: value,
+	}, nil
+}