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/composite.go

@@ -18,6 +18,7 @@
 package structs
 
 import (
+	"context"
 	"fmt"
 
 	"github.com/purpleidea/mgmt/lang/interfaces"
@@ -41,8 +42,6 @@ type CompositeFunc struct {
 	init   *interfaces.Init
 	last   types.Value // last value received to use for diff
 	result types.Value // last calculated output
-
-	closeChan chan struct{}
 }
 
 // String returns a simple name for this function. This is needed so this struct
@@ -117,14 +116,13 @@ func (obj *CompositeFunc) Info() *interfaces.Info {
 // Init runs some startup code for this composite function.
 func (obj *CompositeFunc) Init(init *interfaces.Init) error {
 	obj.init = init
-	obj.closeChan = make(chan struct{})
 	return nil
 }
 
 // Stream takes an input struct in the format as described in the Func and Graph
 // methods of the Expr, and returns the actual expected value as a stream based
 // on the changing inputs to that value.
-func (obj *CompositeFunc) Stream() error {
+func (obj *CompositeFunc) Stream(ctx context.Context) error {
 	defer close(obj.init.Output) // the sender closes
 	for {
 		select {
@@ -139,7 +137,7 @@ func (obj *CompositeFunc) Stream() error {
 					select {
 					case obj.init.Output <- result: // send
 						// pass
-					case <-obj.closeChan:
+					case <-ctx.Done():
 						return nil
 					}
 				}
@@ -207,21 +205,15 @@ func (obj *CompositeFunc) Stream() error {
 			}
 			obj.result = result // store new result
 
-		case <-obj.closeChan:
+		case <-ctx.Done():
 			return nil
 		}
 
 		select {
 		case obj.init.Output <- obj.result: // send
 			// pass
-		case <-obj.closeChan:
+		case <-ctx.Done():
 			return nil
 		}
 	}
 }
-
-// Close runs some shutdown code for this function and turns off the stream.
-func (obj *CompositeFunc) Close() error {
-	close(obj.closeChan)
-	return nil
-}