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/structs/const.go

@@ -18,6 +18,7 @@
 package structs
 
 import (
+	"context"
 	"fmt"
 
 	"github.com/purpleidea/mgmt/lang/interfaces"
@@ -33,8 +34,7 @@ const (
 type ConstFunc struct {
 	Value types.Value
 
-	init      *interfaces.Init
-	closeChan chan struct{}
+	init *interfaces.Init
 }
 
 // String returns a simple name for this function. This is needed so this struct
@@ -70,24 +70,17 @@ func (obj *ConstFunc) Info() *interfaces.Info {
 // Init runs some startup code for this const.
 func (obj *ConstFunc) Init(init *interfaces.Init) error {
 	obj.init = init
-	obj.closeChan = make(chan struct{})
 	return nil
 }
 
 // Stream returns the single value that this const has, and then closes.
-func (obj *ConstFunc) Stream() error {
+func (obj *ConstFunc) Stream(ctx context.Context) error {
 	select {
 	case obj.init.Output <- obj.Value:
 		// pass
-	case <-obj.closeChan:
+	case <-ctx.Done():
 		return nil
 	}
 	close(obj.init.Output) // signal that we're done sending
 	return nil
 }
-
-// Close runs some shutdown code for this const and turns off the stream.
-func (obj *ConstFunc) Close() error {
-	close(obj.closeChan)
-	return nil
-}