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/sys/hostname_fact.go

@@ -18,6 +18,8 @@
 package coresys
 
 import (
+	"context"
+
 	"github.com/purpleidea/mgmt/lang/funcs/facts"
 	"github.com/purpleidea/mgmt/lang/types"
 )
@@ -35,8 +37,7 @@ func init() {
 // HostnameFact is a function that returns the hostname.
 // TODO: support hostnames that change in the future.
 type HostnameFact 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
@@ -61,26 +62,19 @@ func (obj *HostnameFact) Info() *facts.Info {
 // Init runs some startup code for this fact.
 func (obj *HostnameFact) Init(init *facts.Init) error {
 	obj.init = init
-	obj.closeChan = make(chan struct{})
 	return nil
 }
 
 // Stream returns the single value that this fact has, and then closes.
-func (obj *HostnameFact) Stream() error {
+func (obj *HostnameFact) Stream(ctx context.Context) error {
 	select {
 	case obj.init.Output <- &types.StrValue{
 		V: obj.init.Hostname,
 	}:
 		// 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 fact and turns off the stream.
-func (obj *HostnameFact) Close() error {
-	close(obj.closeChan)
-	return nil
-}