lang: funcs: simple: Don't block on simple, pure, static functions

I forgot to handle the special case of a function using this API that
received no inputs. It was waiting for the first input to come in, and
as a result was never producing any output.

Remember that functions like this should *almost* be thought of as
constants of the system. You would expect their output to never change
during the lifetime of a particular program invocation.

examples/lang/answer.mcl

@@ -0,0 +1,4 @@
+# it was a lovely surprise to me, when i realized that mgmt had the answer!
+print "answer" {
+	msg => printf("the answer to life, the universe, and everything is: %d", answer()),
+}

lang/funcs/facts/core/answer_fact.go

@@ -1,76 +0,0 @@
-// Mgmt
-// Copyright (C) 2013-2018+ James Shubin and the project contributors
-// Written by James Shubin <james@shubin.ca> and the project contributors
-//
-// This program is free software: you can redistribute it and/or modify
-// it under the terms of the GNU General Public License as published by
-// the Free Software Foundation, either version 3 of the License, or
-// (at your option) any later version.
-//
-// This program is distributed in the hope that it will be useful,
-// but WITHOUT ANY WARRANTY; without even the implied warranty of
-// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-// GNU General Public License for more details.
-//
-// You should have received a copy of the GNU General Public License
-// along with this program.  If not, see <http://www.gnu.org/licenses/>.
-
-package core // TODO: should this be in its own individual package?
-
-import (
-	"github.com/purpleidea/mgmt/lang/funcs/facts"
-	"github.com/purpleidea/mgmt/lang/types"
-)
-
-func init() {
-	facts.Register("answer", func() facts.Fact { return &AnswerFact{} }) // must register the fact and name
-}
-
-// Answer is the Answer to Life, the Universe and Everything.
-const Answer = 42
-
-// AnswerFact returns the Answer to Life, the Universe and Everything.
-type AnswerFact struct {
-	init      *facts.Init
-	closeChan chan struct{}
-}
-
-// Validate makes sure we've built our struct properly. It is usually unused for
-// normal facts that users can use directly.
-//func (obj *AnswerFact) Validate() error {
-//	return nil
-//}
-
-// Info returns some static info about itself.
-func (obj *AnswerFact) Info() *facts.Info {
-	return &facts.Info{
-		Output: types.NewType("int"),
-	}
-}
-
-// Init runs some startup code for this fact.
-func (obj *AnswerFact) Init(init *facts.Init) error {
-	obj.init = init
-	obj.closeChan = make(chan struct{})
-	return nil
-}
-
-// Stream returns the changing values that this fact has over time. In this
-// case, it returns a single value and then exits, since this is a static fact.
-func (obj *AnswerFact) Stream() error {
-	select {
-	case obj.init.Output <- &types.IntValue{
-		V: Answer,
-	}:
-	case <-obj.closeChan:
-		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 *AnswerFact) Close() error {
-	close(obj.closeChan)
-	return nil
-}

lang/funcs/simple/answer_func.go

@@ -0,0 +1,34 @@
+// Mgmt
+// Copyright (C) 2013-2018+ James Shubin and the project contributors
+// Written by James Shubin <james@shubin.ca> and the project contributors
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package simple // TODO: should this be in its own individual package?
+
+import (
+	"github.com/purpleidea/mgmt/lang/types"
+)
+
+// Answer is the Answer to Life, the Universe and Everything.
+const Answer = 42
+
+func init() {
+	Register("answer", &types.FuncValue{
+		T: types.NewType("func() int"),
+		V: func([]types.Value) (types.Value, error) {
+			return &types.IntValue{V: Answer}, nil
+		},
+	})
+}

lang/funcs/simple/simple.go

@@ -15,7 +15,7 @@
 // You should have received a copy of the GNU General Public License
 // along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
-package simple // TODO: should this be in its own individual package?
+package simple
 
 import (
 	"fmt"
@@ -30,7 +30,7 @@ import (
 // RegisteredFuncs maps a function name to the corresponding static, pure func.
 var RegisteredFuncs = make(map[string]*types.FuncValue) // must initialize
 
-// Register registers a simple, static, pure function. It is easier to use that
+// Register registers a simple, static, pure function. It is easier to use than
 // the raw function API, but also limits you to simple, static, pure functions.
 func Register(name string, fn *types.FuncValue) {
 	if _, exists := RegisteredFuncs[name]; exists {
@@ -88,16 +88,21 @@ func (obj *simpleFunc) Stream() error {
 		select {
 		case input, ok := <-obj.init.Input:
 			if !ok {
-				return nil // can't output any more
+				if len(obj.Fn.Type().Ord) > 0 {
+					return nil // can't output any more
+				}
+				// no inputs were expected, pass through once
 			}
-			//if err := input.Type().Cmp(obj.Info().Sig.Input); err != nil {
-			//	return errwrap.Wrapf(err, "wrong function input")
-			//}
+			if ok {
+				//if err := input.Type().Cmp(obj.Info().Sig.Input); err != nil {
+				//	return errwrap.Wrapf(err, "wrong function input")
+				//}
 
-			if obj.last != nil && input.Cmp(obj.last) == nil {
-				continue // value didn't change, skip it
+				if obj.last != nil && input.Cmp(obj.last) == nil {
+					continue // value didn't change, skip it
+				}
+				obj.last = input // store for next
 			}
-			obj.last = input // store for next
 
 			values := []types.Value{}
 			for _, name := range obj.Fn.Type().Ord {
@@ -121,7 +126,9 @@ func (obj *simpleFunc) Stream() error {
 
 		select {
 		case obj.init.Output <- obj.result: // send
-			// pass
+			if len(obj.Fn.Type().Ord) == 0 {
+				return nil // no more values, we're a pure func
+			}
 		case <-obj.closeChan:
 			return nil
 		}