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/datetime/now_fact.go

@@ -18,6 +18,7 @@
 package coredatetime
 
 import (
+	"context"
 	"time"
 
 	"github.com/purpleidea/mgmt/lang/funcs/facts"
@@ -36,8 +37,7 @@ func init() {
 
 // DateTimeFact is a fact which returns the current date and time.
 type DateTimeFact 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
@@ -62,12 +62,11 @@ func (obj *DateTimeFact) Info() *facts.Info {
 // Init runs some startup code for this fact.
 func (obj *DateTimeFact) 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 *DateTimeFact) Stream() error {
+func (obj *DateTimeFact) Stream(ctx context.Context) error {
 	defer close(obj.init.Output) // always signal when we're done
 	// XXX: this might be an interesting fact to write because:
 	// 1) will the sleeps from the ticker be in sync with the second ticker?
@@ -87,7 +86,7 @@ func (obj *DateTimeFact) Stream() error {
 			startChan = nil // disable
 		case <-ticker.C: // received the timer event
 			// pass
-		case <-obj.closeChan:
+		case <-ctx.Done():
 			return nil
 		}
 
@@ -95,14 +94,8 @@ func (obj *DateTimeFact) Stream() error {
 		case obj.init.Output <- &types.IntValue{ // seconds since 1970...
 			V: time.Now().Unix(), // .UTC() not necessary
 		}:
-		case <-obj.closeChan:
+		case <-ctx.Done():
 			return nil
 		}
 	}
 }
-
-// Close runs some shutdown code for this fact and turns off the stream.
-func (obj *DateTimeFact) Close() error {
-	close(obj.closeChan)
-	return nil
-}