lang: ast, funcs: Remove the secret channel from call

This removes the secret channel from the call function. Having it made
it more complicated to write new function engines, and it's not clear
why it was even needed in the first place. It seems that even the
current generation of function engines work just fine without it.

Co-authored-by: Samuel Gélineau <gelisam@gmail.com>

lang/funcs/structs/output.go

@@ -0,0 +1,146 @@
+// Mgmt
+// Copyright (C) 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 <https://www.gnu.org/licenses/>.
+//
+// Additional permission under GNU GPL version 3 section 7
+//
+// If you modify this program, or any covered work, by linking or combining it
+// with embedded mcl code and modules (and that the embedded mcl code and
+// modules which link with this program, contain a copy of their source code in
+// the authoritative form) containing parts covered by the terms of any other
+// license, the licensors of this program grant you additional permission to
+// convey the resulting work. Furthermore, the licensors of this program grant
+// the original author, James Shubin, additional permission to update this
+// additional permission if he deems it necessary to achieve the goals of this
+// additional permission.
+
+package structs
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/purpleidea/mgmt/lang/interfaces"
+	"github.com/purpleidea/mgmt/lang/types"
+)
+
+const (
+	// OutputFuncArgName is the name for the edge which connects
+	// the input value to OutputFunc.
+	OutputFuncArgName = "out"
+
+	OutputFuncDummyArgName = "dummy"
+)
+
+// OutputFunc is a Func which receives values from upstream nodes and emits them
+// downstream. It accepts (and ignores) a "dummy" arg as well.
+type OutputFunc struct {
+	interfaces.Textarea // XXX: Do we want this here for this func as well ?
+
+	Name     string
+	EdgeName string
+	Type     *types.Type
+
+	init *interfaces.Init
+	last types.Value // last value received to use for diff
+}
+
+// String returns a simple name for this function. This is needed so this struct
+// can satisfy the pgraph.Vertex interface.
+func (obj *OutputFunc) String() string {
+	return obj.Name
+}
+
+// ArgGen returns the Nth arg name for this function.
+func (obj *OutputFunc) ArgGen(index int) (string, error) {
+	seq := []string{obj.EdgeName, OutputFuncDummyArgName}
+	if l := len(seq); index >= l {
+		return "", fmt.Errorf("index %d exceeds arg length of %d", index, l)
+	}
+	return seq[index], nil
+}
+
+// Validate makes sure we've built our struct properly. It is usually unused for
+// normal functions that users can use directly.
+func (obj *OutputFunc) Validate() error {
+	return nil
+}
+
+// Info returns some static info about itself.
+func (obj *OutputFunc) Info() *interfaces.Info {
+	// XXX: contains "dummy" return type
+	s := fmt.Sprintf("func(%s %s, %s float) %s", obj.EdgeName, obj.Type, OutputFuncDummyArgName, obj.Type)
+	return &interfaces.Info{
+		Pure: false,
+		Memo: false,
+		Sig:  types.NewType(s),
+		Err:  obj.Validate(),
+	}
+}
+
+// Init runs some startup code for this function.
+func (obj *OutputFunc) Init(init *interfaces.Init) error {
+	obj.init = init
+	return nil
+}
+
+// Stream returns the changing values that this func has over time.
+func (obj *OutputFunc) Stream(ctx context.Context) error {
+	defer close(obj.init.Output) // the sender closes
+
+	for {
+		select {
+		case input, ok := <-obj.init.Input:
+			if !ok {
+				return nil // can't output any more
+			}
+
+			args, err := interfaces.StructToCallableArgs(input) // []types.Value, error)
+			if err != nil {
+				return err
+			}
+
+			result, err := obj.Call(ctx, args) // get the value...
+			if err != nil {
+				return err
+			}
+
+			if obj.last != nil && result.Cmp(obj.last) == nil {
+				continue // value didn't change, skip it
+			}
+			obj.last = result // store so we can send after this select
+
+		case <-ctx.Done():
+			return nil
+		}
+
+		select {
+		case obj.init.Output <- obj.last: // send
+		case <-ctx.Done():
+			return nil
+		}
+	}
+}
+
+// Call this function with the input args and return the value if it is possible
+// to do so at this time.
+func (obj *OutputFunc) Call(ctx context.Context, args []types.Value) (types.Value, error) {
+	if len(args) != 2 {
+		return nil, fmt.Errorf("programming error, can't find edge")
+	}
+	// Send the useful input arg, not the dummy arg.
+	return args[0], nil
+}