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/test/fastcount_fact.go

@@ -31,6 +31,7 @@ package coretest
 
 import (
 	"context"
+	"sync"
 
 	"github.com/purpleidea/mgmt/lang/funcs/facts"
 	"github.com/purpleidea/mgmt/lang/types"
@@ -50,6 +51,9 @@ func init() {
 // FastCountFact is a fact that counts up as fast as possible from zero forever.
 type FastCountFact struct {
 	init *facts.Init
+
+	mutex *sync.Mutex
+	count int
 }
 
 // String returns a simple name for this fact. This is needed so this struct can
@@ -74,6 +78,7 @@ func (obj *FastCountFact) Info() *facts.Info {
 // Init runs some startup code for this fact.
 func (obj *FastCountFact) Init(init *facts.Init) error {
 	obj.init = init
+	obj.mutex = &sync.Mutex{}
 	return nil
 }
 
@@ -81,16 +86,32 @@ func (obj *FastCountFact) Init(init *facts.Init) error {
 func (obj *FastCountFact) Stream(ctx context.Context) error {
 	defer close(obj.init.Output) // always signal when we're done
 
-	count := int64(0)
-
 	// streams must generate an initial event on startup
 	for {
+		result, err := obj.Call(ctx)
+		if err != nil {
+			return err
+		}
+
+		obj.mutex.Lock()
+		obj.count++
+		obj.mutex.Unlock()
+
 		select {
-		case obj.init.Output <- &types.IntValue{V: count}:
-			count++
+		case obj.init.Output <- result:
 
 		case <-ctx.Done():
 			return nil
 		}
 	}
 }
+
+// Call this fact and return the value if it is possible to do so at this time.
+func (obj *FastCountFact) Call(ctx context.Context) (types.Value, error) {
+	obj.mutex.Lock() // TODO: could be a read lock
+	count := obj.count
+	obj.mutex.Unlock()
+	return &types.IntValue{
+		V: int64(count),
+	}, nil
+}