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/os/system_func.go

@@ -49,10 +49,8 @@ func init() {
 // after the other, the downstream resources might not run for every line unless
 // the "Meta:realize" metaparam is set to true.
 type SystemFunc struct {
-	init *interfaces.Init
-
-	closeChan chan struct{}
-	cancel    context.CancelFunc
+	init   *interfaces.Init
+	cancel context.CancelFunc
 }
 
 // String returns a simple name for this function. This is needed so this struct
@@ -89,12 +87,15 @@ func (obj *SystemFunc) Info() *interfaces.Info {
 // Init runs some startup code for this function.
 func (obj *SystemFunc) Init(init *interfaces.Init) error {
 	obj.init = init
-	obj.closeChan = make(chan struct{})
 	return nil
 }
 
 // Stream returns the changing values that this func has over time.
-func (obj *SystemFunc) Stream() error {
+func (obj *SystemFunc) Stream(ctx context.Context) error {
+	// XXX: this implementation is a bit awkward especially with the port to
+	// the Stream(context.Context) signature change. This is a straight port
+	// but we could refactor this eventually.
+
 	// Close the output chan to signal that no more values are coming.
 	defer close(obj.init.Output)
 
@@ -115,7 +116,7 @@ func (obj *SystemFunc) Stream() error {
 
 	// Kill the current process, if any. A new cancel function is created
 	// each time a new process is started.
-	var ctx context.Context
+	var innerCtx context.Context
 	defer func() {
 		if obj.cancel == nil {
 			return
@@ -132,7 +133,7 @@ func (obj *SystemFunc) Stream() error {
 				select {
 				case <-processedChan:
 					return nil
-				case <-obj.closeChan:
+				case <-ctx.Done():
 					return nil
 				}
 			}
@@ -147,8 +148,8 @@ func (obj *SystemFunc) Stream() error {
 			// Run the command, connecting it to ctx so we can kill
 			// it if needed, and to two Readers so we can read its
 			// stdout and stderr.
-			ctx, obj.cancel = context.WithCancel(context.Background())
-			cmd := exec.CommandContext(ctx, "sh", "-c", shellCommand)
+			innerCtx, obj.cancel = context.WithCancel(context.Background())
+			cmd := exec.CommandContext(innerCtx, "sh", "-c", shellCommand)
 			stdoutReader, err := cmd.StdoutPipe()
 			if err != nil {
 				return err
@@ -205,14 +206,8 @@ func (obj *SystemFunc) Stream() error {
 				wg.Wait()
 				close(processedChan)
 			}()
-		case <-obj.closeChan:
+		case <-ctx.Done():
 			return nil
 		}
 	}
 }
-
-// Close runs some shutdown code for this function and turns off the stream.
-func (obj *SystemFunc) Close() error {
-	close(obj.closeChan)
-	return nil
-}