lang: funcs: structs: Core function struct modifications

These changes are needed for the lambdas implementation.

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

lang/funcs/structs/channel_based_sink.go

@@ -0,0 +1,129 @@
+// Mgmt
+// Copyright (C) 2013-2023+ 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 structs
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/purpleidea/mgmt/lang/interfaces"
+	"github.com/purpleidea/mgmt/lang/types"
+)
+
+const (
+	// ChannelBasedSinkFuncArgName is the name for the edge which connects
+	// the input value to ChannelBasedSinkFunc.
+	ChannelBasedSinkFuncArgName = "channelBasedSinkFuncArg"
+)
+
+// ChannelBasedSinkFunc is a Func which receives values from upstream nodes and
+// emits them to a golang channel.
+type ChannelBasedSinkFunc struct {
+	Name     string
+	EdgeName string
+	Target   interfaces.Func // for drawing dashed edges in the Graphviz visualization
+
+	Chan chan types.Value
+	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 *ChannelBasedSinkFunc) String() string {
+	return obj.Name
+}
+
+// ArgGen returns the Nth arg name for this function.
+func (obj *ChannelBasedSinkFunc) ArgGen(index int) (string, error) {
+	if index != 0 {
+		return "", fmt.Errorf("the ChannelBasedSinkFunc only has one argument")
+	}
+	return obj.EdgeName, nil
+}
+
+// Validate makes sure we've built our struct properly. It is usually unused for
+// normal functions that users can use directly.
+func (obj *ChannelBasedSinkFunc) Validate() error {
+	if obj.Chan == nil {
+		return fmt.Errorf("the Chan was not set")
+	}
+	return nil
+}
+
+// Info returns some static info about itself.
+func (obj *ChannelBasedSinkFunc) Info() *interfaces.Info {
+	return &interfaces.Info{
+		Pure: false,
+		Memo: false,
+		Sig:  types.NewType(fmt.Sprintf("func(%s %s) %s", obj.EdgeName, obj.Type, obj.Type)),
+		Err:  obj.Validate(),
+	}
+}
+
+// Init runs some startup code for this function.
+func (obj *ChannelBasedSinkFunc) Init(init *interfaces.Init) error {
+	obj.init = init
+	return nil
+}
+
+// Stream returns the changing values that this func has over time.
+func (obj *ChannelBasedSinkFunc) Stream(ctx context.Context) error {
+	defer close(obj.init.Output) // the sender closes
+	defer close(obj.Chan)        // the sender closes
+
+	for {
+		select {
+		case input, ok := <-obj.init.Input:
+			if !ok {
+				return nil // can't output any more
+			}
+
+			value, exists := input.Struct()[obj.EdgeName]
+			if !exists {
+				return fmt.Errorf("programming error, can't find edge")
+			}
+
+			if obj.last != nil && value.Cmp(obj.last) == nil {
+				continue // value didn't change, skip it
+			}
+			obj.last = value // store so we can send after this select
+
+		case <-ctx.Done():
+			return nil
+		}
+
+		select {
+		case obj.Chan <- obj.last: // send
+		case <-ctx.Done():
+			return nil
+		}
+
+		// Also send the value downstream. If we don't, then when we
+		// close the Output channel, the function engine is going to
+		// complain that we closed that channel without sending it any
+		// value.
+		select {
+		case obj.init.Output <- obj.last: // send
+		case <-ctx.Done():
+			return nil
+		}
+	}
+}