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/template_func.go

@@ -19,6 +19,7 @@ package core // TODO: should this be in its own individual package?
 
 import (
 	"bytes"
+	"context"
 	"fmt"
 	"reflect"
 	"strings"
@@ -73,8 +74,6 @@ type TemplateFunc struct {
 	last types.Value // last value received to use for diff
 
 	result *string // last calculated output
-
-	closeChan chan struct{}
 }
 
 // String returns a simple name for this function. This is needed so this struct
@@ -367,7 +366,6 @@ func (obj *TemplateFunc) Info() *interfaces.Info {
 // Init runs some startup code for this function.
 func (obj *TemplateFunc) Init(init *interfaces.Init) error {
 	obj.init = init
-	obj.closeChan = make(chan struct{})
 	return nil
 }
 
@@ -491,7 +489,7 @@ Loop:
 }
 
 // Stream returns the changing values that this func has over time.
-func (obj *TemplateFunc) Stream() error {
+func (obj *TemplateFunc) Stream(ctx context.Context) error {
 	defer close(obj.init.Output) // the sender closes
 	for {
 		select {
@@ -526,7 +524,7 @@ func (obj *TemplateFunc) Stream() error {
 			}
 			obj.result = &result // store new result
 
-		case <-obj.closeChan:
+		case <-ctx.Done():
 			return nil
 		}
 
@@ -534,18 +532,12 @@ func (obj *TemplateFunc) Stream() error {
 		case obj.init.Output <- &types.StrValue{
 			V: *obj.result,
 		}:
-		case <-obj.closeChan:
+		case <-ctx.Done():
 			return nil
 		}
 	}
 }
 
-// Close runs some shutdown code for this function and turns off the stream.
-func (obj *TemplateFunc) Close() error {
-	close(obj.closeChan)
-	return nil
-}
-
 // safename renames the functions so they're valid inside the template. This is
 // a limitation of the template library, and it might be worth moving to a new
 // one.