lang: interfaces, funcs: Port Func API to new Stream signature

This removes the `Close() error` and replaces it with a more modern
Stream API that takes a context. This removes boilerplate and makes
integration with concurrent code easier. The only downside is that there
isn't an explicit cleanup step, but only one function was even using
that and it was possible to switch it to a defer in Stream.

This also renames the functions from polyfunc to just func which we
determine by API not naming.

lang/funcs/core/sys/load_fact.go

@@ -18,6 +18,7 @@
 package coresys
 
 import (
+	"context"
 	"time"
 
 	"github.com/purpleidea/mgmt/lang/funcs/facts"
@@ -39,8 +40,7 @@ func init() {
 
 // LoadFact is a fact which returns the current system load.
 type LoadFact struct {
-	init      *facts.Init
-	closeChan chan struct{}
+	init *facts.Init
 }
 
 // String returns a simple name for this fact. This is needed so this struct can
@@ -65,12 +65,11 @@ func (obj *LoadFact) Info() *facts.Info {
 // Init runs some startup code for this fact.
 func (obj *LoadFact) Init(init *facts.Init) error {
 	obj.init = init
-	obj.closeChan = make(chan struct{})
 	return nil
 }
 
 // Stream returns the changing values that this fact has over time.
-func (obj *LoadFact) Stream() error {
+func (obj *LoadFact) Stream(ctx context.Context) error {
 	defer close(obj.init.Output) // always signal when we're done
 
 	// it seems the different values only update once every 5
@@ -88,7 +87,7 @@ func (obj *LoadFact) Stream() error {
 			startChan = nil // disable
 		case <-ticker.C: // received the timer event
 			// pass
-		case <-obj.closeChan:
+		case <-ctx.Done():
 			return nil
 		}
 
@@ -107,14 +106,8 @@ func (obj *LoadFact) Stream() error {
 		select {
 		case obj.init.Output <- st:
 			// send
-		case <-obj.closeChan:
+		case <-ctx.Done():
 			return nil
 		}
 	}
 }
-
-// Close runs some shutdown code for this fact and turns off the stream.
-func (obj *LoadFact) Close() error {
-	close(obj.closeChan)
-	return nil
-}