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/ast/structs.go

@@ -11000,20 +11000,36 @@ func (obj *ExprCall) Graph(env *interfaces.Env) (*pgraph.Graph, interfaces.Func,
 
 	// Add a vertex for the call itself.
 	edgeName := structs.CallFuncArgNameFunction
+	edgeNameDummy := structs.OutputFuncDummyArgName
+
+	callSubgraphOutput := &structs.OutputFunc{ // the new graph shape thing!
+		Textarea: obj.Textarea,
+		Name:     "callSubgraphOutput",
+		Type:     obj.typ,
+		EdgeName: structs.OutputFuncArgName,
+	}
+	graph.AddVertex(callSubgraphOutput)
+
 	callFunc := &structs.CallFunc{
 		Textarea: obj.Textarea,
 
-		Type:        obj.typ,
-		FuncType:    ftyp,
-		EdgeName:    edgeName,
-		ArgVertices: argFuncs,
+		Type:         obj.typ,
+		FuncType:     ftyp,
+		EdgeName:     edgeName,
+		ArgVertices:  argFuncs,
+		OutputVertex: callSubgraphOutput,
 	}
 	graph.AddVertex(callFunc)
+
 	graph.AddEdge(funcValueFunc, callFunc, &interfaces.FuncEdge{
 		Args: []string{edgeName},
 	})
 
-	return graph, callFunc, nil
+	graph.AddEdge(callFunc, callSubgraphOutput, &interfaces.FuncEdge{
+		Args: []string{edgeNameDummy},
+	})
+
+	return graph, callSubgraphOutput, nil
 }
 
 // funcValueFunc is a helper function to make the code more readable. This was

lang/funcs/structs/call.go

@@ -32,6 +32,7 @@ package structs
 import (
 	"context"
 	"fmt"
+	"sync"
 
 	"github.com/purpleidea/mgmt/lang/interfaces"
 	"github.com/purpleidea/mgmt/lang/types"
@@ -61,14 +62,11 @@ type CallFunc struct {
 
 	ArgVertices []interfaces.Func
 
+	OutputVertex interfaces.Func
+
 	init *interfaces.Init
 
 	lastFuncValue *full.FuncValue // remember the last function value
-
-	// outputChan is an initially-nil channel from which we receive output
-	// lists from the subgraph. This channel is reset when the subgraph is
-	// recreated.
-	outputChan chan types.Value
 }
 
 // String returns a simple name for this function. This is needed so this struct
@@ -98,6 +96,10 @@ func (obj *CallFunc) Validate() error {
 		return fmt.Errorf("number of arg Funcs must match number of func args in the type")
 	}
 
+	if obj.OutputVertex == nil {
+		return fmt.Errorf("the output vertex is missing")
+	}
+
 	return nil
 }
 
@@ -127,29 +129,46 @@ func (obj *CallFunc) Init(init *interfaces.Init) error {
 // methods of the Expr, and returns the actual expected value as a stream based
 // on the changing inputs to that value.
 func (obj *CallFunc) Stream(ctx context.Context) error {
-	defer close(obj.init.Output) // the sender closes
+	// XXX: is there a sync.Once sort of solution that would be more elegant here?
+	mutex := &sync.Mutex{}
+	done := false
+	send := func(ctx context.Context, b bool) error {
+		mutex.Lock()
+		defer mutex.Unlock()
+		if done {
+			return nil
+		}
+		done = true
+		defer close(obj.init.Output) // the sender closes
 
-	obj.outputChan = nil
+		if !b {
+			return nil
+		}
+
+		// send dummy value to the output
+		select {
+		case obj.init.Output <- types.NewFloat(): // XXX: dummy value
+		case <-ctx.Done():
+			return ctx.Err()
+		}
+
+		return nil
+	}
+	defer send(ctx, false) // just close
 
 	defer func() {
 		obj.init.Txn.Reverse()
 	}()
 
-	canReceiveMoreFuncValues := true
-	canReceiveMoreOutputValues := true
 	for {
-
-		if !canReceiveMoreFuncValues && !canReceiveMoreOutputValues {
-			// break
-			return nil
-		}
-
 		select {
 		case input, ok := <-obj.init.Input:
 			if !ok {
 				obj.init.Input = nil // block looping back here
-				canReceiveMoreFuncValues = false
-				continue
+				if !done {
+					return fmt.Errorf("input closed without ever sending anything")
+				}
+				return nil
 			}
 
 			value, exists := input.Struct()[obj.EdgeName]
@@ -179,24 +198,11 @@ func (obj *CallFunc) Stream(ctx context.Context) error {
 			if err := obj.replaceSubGraph(newFuncValue); err != nil {
 				return errwrap.Wrapf(err, "could not replace subgraph")
 			}
-			canReceiveMoreOutputValues = true
+
+			send(ctx, true) // send dummy and then close
+
 			continue
 
-		case outputValue, ok := <-obj.outputChan:
-			// send the new output value downstream
-			if !ok {
-				obj.outputChan = nil
-				canReceiveMoreOutputValues = false
-				continue
-			}
-
-			// send to the output
-			select {
-			case obj.init.Output <- outputValue:
-			case <-ctx.Done():
-				return nil
-			}
-
 		case <-ctx.Done():
 			return nil
 		}
@@ -214,13 +220,13 @@ func (obj *CallFunc) replaceSubGraph(newFuncValue *full.FuncValue) error {
 	//
 	//   outputFunc -> "callSubgraphOutput"
 	// }
+	// XXX
 
 	// delete the old subgraph
 	if err := obj.init.Txn.Reverse(); err != nil {
 		return errwrap.Wrapf(err, "could not Reverse")
 	}
 
-	// create the new subgraph
 	// This passed in Txn has AddVertex, AddEdge, and possibly AddGraph
 	// methods called on it. Nothing else. It will _not_ call Commit or
 	// Reverse. It adds to the graph, and our Commit and Reverse operations
@@ -230,17 +236,11 @@ func (obj *CallFunc) replaceSubGraph(newFuncValue *full.FuncValue) error {
 		return errwrap.Wrapf(err, "could not call newFuncValue.Call()")
 	}
 
-	obj.outputChan = make(chan types.Value)
-	edgeName := ChannelBasedSinkFuncArgName
-	subgraphOutput := &ChannelBasedSinkFunc{
-		Name:     "callSubgraphOutput",
-		Target:   obj,
-		EdgeName: edgeName,
-		Chan:     obj.outputChan,
-		Type:     obj.Type,
-	}
+	// create the new subgraph
+	edgeName := OutputFuncArgName
 	edge := &interfaces.FuncEdge{Args: []string{edgeName}}
-	obj.init.Txn.AddVertex(subgraphOutput)
-	obj.init.Txn.AddEdge(outputFunc, subgraphOutput, edge)
+	obj.init.Txn.AddVertex(outputFunc)
+	obj.init.Txn.AddEdge(outputFunc, obj.OutputVertex, edge)
+
 	return obj.init.Txn.Commit()
 }

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
+}

lang/interpret_test/TestAstFunc1/changing-func.txtar

@@ -25,6 +25,8 @@ Edge: FuncValue -> if # a
 Edge: FuncValue -> if # a
 Edge: FuncValue -> if # b
 Edge: FuncValue -> if # b
+Edge: call -> callSubgraphOutput # dummy
+Edge: call -> callSubgraphOutput # dummy
 Edge: const: bool(false) -> if # c
 Edge: const: bool(true) -> if # c
 Edge: if -> call # fn
@@ -35,6 +37,8 @@ Vertex: FuncValue
 Vertex: FuncValue
 Vertex: call
 Vertex: call
+Vertex: callSubgraphOutput
+Vertex: callSubgraphOutput
 Vertex: const: bool(false)
 Vertex: const: bool(true)
 Vertex: if

lang/interpret_test/TestAstFunc1/returned-func.txtar

@@ -12,5 +12,7 @@ $out = $fn()
 test "${out}" {}
 -- OUTPUT --
 Edge: FuncValue -> call # fn
+Edge: call -> callSubgraphOutput # dummy
 Vertex: FuncValue
 Vertex: call
+Vertex: callSubgraphOutput

lang/interpret_test/TestAstFunc1/returned-lambda.txtar

@@ -11,5 +11,7 @@ $out = $fn()
 test "${out}" {}
 -- OUTPUT --
 Edge: FuncValue -> call # fn
+Edge: call -> callSubgraphOutput # dummy
 Vertex: FuncValue
 Vertex: call
+Vertex: callSubgraphOutput

lang/interpret_test/TestAstFunc1/shape0.txtar

@@ -13,11 +13,13 @@ $z = apply($add1, 1)
 test [fmt.printf("%d", $z),] {}
 -- OUTPUT --
 Edge: FuncValue -> call # fn
-Edge: call -> printf: func(format str, a int) str # a
+Edge: call -> callSubgraphOutput # dummy
+Edge: callSubgraphOutput -> printf: func(format str, a int) str # a
 Edge: const: str("%d") -> printf: func(format str, a int) str # format
 Edge: printf: func(format str, a int) str -> composite: []str # 0
 Vertex: FuncValue
 Vertex: call
+Vertex: callSubgraphOutput
 Vertex: composite: []str
 Vertex: const: int(1)
 Vertex: const: str("%d")

lang/interpret_test/TestAstFunc1/shape8.txtar

@@ -21,7 +21,8 @@ test [$lambda("hello"),] {}
 Edge: FuncValue -> if # a
 Edge: FuncValue -> if # b
 Edge: _operator -> if # c
-Edge: call -> composite: []str # 0
+Edge: call -> callSubgraphOutput # dummy
+Edge: callSubgraphOutput -> composite: []str # 0
 Edge: const: int(0) -> _operator # b
 Edge: const: int(10) -> _operator # a
 Edge: const: str(">") -> _operator # op
@@ -30,6 +31,7 @@ Vertex: FuncValue
 Vertex: FuncValue
 Vertex: _operator
 Vertex: call
+Vertex: callSubgraphOutput
 Vertex: composite: []str
 Vertex: const: int(0)
 Vertex: const: int(10)