lang: ast: structs, funcs: structs: Exprif without a channel

This adds an improved "expr if" which only adds the active branch to the
graph and removes the "secret" channel.

Author: purpleidea

lang/ast/structs.go

@@ -12716,59 +12716,75 @@ func (obj *ExprIf) Check(typ *types.Type) ([]*interfaces.UnificationInvariant, e
 	return interfaces.GenericCheck(obj, typ)
 }
 
-// Func returns a function which returns the correct branch based on the ever
-// changing conditional boolean input.
-func (obj *ExprIf) Func() (interfaces.Func, error) {
+// Graph returns the reactive function graph which is expressed by this node. It
+// includes the condition produced by this node, and the appropriate edges of
+// that. The then or else side of the graph is added at runtime based on the
+// value of the condition.
+// TODO: If we know the condition is static, generate only that side statically.
+func (obj *ExprIf) Graph(env *interfaces.Env) (*pgraph.Graph, interfaces.Func, error) {
+	graph, err := pgraph.NewGraph("if")
+	if err != nil {
+		return nil, nil, err
+	}
+
 	typ, err := obj.Type()
 	if err != nil {
-		return nil, err
+		return nil, nil, err
 	}
 
-	return &structs.IfFunc{
-		Textarea: obj.Textarea,
+	// XXX: can we speculate if it's static?
 
-		Type: typ, // this is the output type of the expression
-	}, nil
-}
+	g, f, err := obj.Condition.Graph(env)
+	if err != nil {
+		return nil, nil, err
+	}
+	graph.AddGraph(g)
 
-// Graph returns the reactive function graph which is expressed by this node. It
-// includes any vertices produced by this node, and the appropriate edges to any
-// vertices that are produced by its children. Nodes which fulfill the Expr
-// interface directly produce vertices (and possible children) where as nodes
-// that fulfill the Stmt interface do not produces vertices, where as their
-// children might. This particular if expression doesn't do anything clever here
-// other than adding in both branches of the graph. Since we're functional, this
-// shouldn't have any ill effects.
-// XXX: is this completely true if we're running technically impure, but safe
-// built-in functions on both branches? Can we turn off half of this?
-func (obj *ExprIf) Graph(env *interfaces.Env) (*pgraph.Graph, interfaces.Func, error) {
-	graph, err := pgraph.NewGraph("if")
+	thenGraph, thenFunc, err := obj.ThenBranch.Graph(env)
 	if err != nil {
 		return nil, nil, err
 	}
-	function, err := obj.Func()
+	elseGraph, elseFunc, err := obj.ElseBranch.Graph(env)
 	if err != nil {
 		return nil, nil, err
 	}
 
-	exprs := map[string]interfaces.Expr{
-		"c": obj.Condition,
-		"a": obj.ThenBranch,
-		"b": obj.ElseBranch,
+	edgeName := structs.ExprIfFuncArgNameCondition
+	edgeNameDummy := structs.OutputFuncDummyArgName
+
+	exprIfSubgraphOutput := &structs.OutputFunc{ // the new graph shape thing!
+		Textarea: obj.Textarea,
+		Name:     "exprIfSubgraphOutput",
+		Type:     obj.typ,
+		EdgeName: structs.OutputFuncArgName,
 	}
-	for _, argName := range []string{"c", "a", "b"} { // deterministic order
-		x := exprs[argName]
-		g, f, err := x.Graph(env)
-		if err != nil {
-			return nil, nil, err
-		}
-		graph.AddGraph(g)
+	graph.AddVertex(exprIfSubgraphOutput)
 
-		edge := &interfaces.FuncEdge{Args: []string{argName}}
-		graph.AddEdge(f, function, edge) // branch -> if
+	function := &structs.ExprIfFunc{
+		Textarea: obj.Textarea,
+
+		Type: typ, // this is the output type of the expression
+
+		EdgeName: edgeName,
+
+		ThenGraph: thenGraph,
+		ElseGraph: elseGraph,
+
+		ThenFunc: thenFunc,
+		ElseFunc: elseFunc,
+
+		OutputVertex: exprIfSubgraphOutput,
 	}
+	graph.AddVertex(function)
 
-	return graph, function, nil
+	edge := &interfaces.FuncEdge{Args: []string{edgeName}}
+	graph.AddEdge(f, function, edge) // condition -> exprif
+
+	graph.AddEdge(function, exprIfSubgraphOutput, &interfaces.FuncEdge{
+		Args: []string{edgeNameDummy},
+	})
+
+	return graph, exprIfSubgraphOutput, nil
 }
 
 // SetValue here is a no-op, because algorithmically when this is called from

lang/funcs/structs/exprif.go

@@ -0,0 +1,231 @@
+// 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"
+	"sync"
+
+	"github.com/purpleidea/mgmt/lang/interfaces"
+	"github.com/purpleidea/mgmt/lang/types"
+	"github.com/purpleidea/mgmt/pgraph"
+	"github.com/purpleidea/mgmt/util/errwrap"
+)
+
+const (
+	// ExprIfFuncName is the unique name identifier for this function.
+	ExprIfFuncName = "exprif"
+
+	// ExprIfFuncArgNameCondition is the name for the edge which connects
+	// the input condition to ExprIfFunc.
+	ExprIfFuncArgNameCondition = "condition"
+)
+
+// ExprIfFunc is a function that passes through the value of the correct branch
+// based on the conditional value it gets.
+type ExprIfFunc struct {
+	interfaces.Textarea
+
+	Type *types.Type // this is the type of the if expression output we hold
+
+	EdgeName string // name of the edge used
+
+	ThenGraph *pgraph.Graph
+	ElseGraph *pgraph.Graph
+
+	ThenFunc interfaces.Func
+	ElseFunc interfaces.Func
+
+	OutputVertex interfaces.Func
+
+	init *interfaces.Init
+	last *bool // 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 *ExprIfFunc) String() string {
+	return ExprIfFuncName
+}
+
+// Validate tells us if the input struct takes a valid form.
+func (obj *ExprIfFunc) Validate() error {
+	if obj.Type == nil {
+		return fmt.Errorf("must specify a type")
+	}
+
+	if obj.EdgeName == "" {
+		return fmt.Errorf("must specify an edge name")
+	}
+
+	if obj.ThenFunc == nil {
+		return fmt.Errorf("must specify a then func")
+	}
+	if obj.ElseFunc == nil {
+		return fmt.Errorf("must specify an else func")
+	}
+
+	t1 := obj.ThenFunc.Info().Sig.Out
+	t2 := obj.ElseFunc.Info().Sig.Out
+	if err := t1.Cmp(t2); err != nil {
+		return errwrap.Wrapf(err, "types of exprif branches must match")
+	}
+
+	if obj.OutputVertex == nil {
+		return fmt.Errorf("the output vertex is missing")
+	}
+
+	return nil
+}
+
+// Info returns some static info about itself.
+func (obj *ExprIfFunc) Info() *interfaces.Info {
+	var typ *types.Type
+	if obj.Type != nil { // don't panic if called speculatively
+		typ = &types.Type{
+			Kind: types.KindFunc, // function type
+			Map: map[string]*types.Type{
+				ExprIfFuncArgNameCondition: types.TypeBool, // conditional must be a boolean
+			},
+			Ord: []string{ExprIfFuncArgNameCondition}, // conditional
+			Out: obj.Type,                             // result type must match
+		}
+	}
+
+	return &interfaces.Info{
+		Pure: true,
+		Memo: false, // TODO: ???
+		Sig:  typ,
+		Err:  obj.Validate(),
+	}
+}
+
+// Init runs some startup code for this if expression function.
+func (obj *ExprIfFunc) Init(init *interfaces.Init) error {
+	obj.init = init
+	return nil
+}
+
+// Stream takes an input struct in the format as described in the Func and Graph
+// methods of the Expr, and returns the actual expected value as a stream based
+// on the changing inputs to that value.
+func (obj *ExprIfFunc) Stream(ctx context.Context) error {
+	// 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
+
+		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()
+	}()
+
+	for {
+		select {
+		case input, ok := <-obj.init.Input:
+			if !ok {
+				obj.init.Input = nil // block looping back here
+				if !done {
+					return fmt.Errorf("input closed without ever sending anything")
+				}
+				return nil
+			}
+
+			value, exists := input.Struct()[obj.EdgeName]
+			if !exists {
+				return fmt.Errorf("programming error, can't find edge")
+			}
+
+			b := value.Bool()
+
+			if obj.last != nil && *obj.last == b {
+				continue // result didn't change
+			}
+			obj.last = &b // store new result
+
+			if err := obj.replaceSubGraph(b); err != nil {
+				return errwrap.Wrapf(err, "could not replace subgraph")
+			}
+
+			send(ctx, true) // send dummy and then close
+
+			continue
+
+		case <-ctx.Done():
+			return nil
+		}
+	}
+}
+
+func (obj *ExprIfFunc) replaceSubGraph(b bool) error {
+	// delete the old subgraph
+	if err := obj.init.Txn.Reverse(); err != nil {
+		return errwrap.Wrapf(err, "could not Reverse")
+	}
+
+	var f interfaces.Func
+	if b {
+		obj.init.Txn.AddGraph(obj.ThenGraph)
+		f = obj.ThenFunc
+	} else {
+		obj.init.Txn.AddGraph(obj.ElseGraph)
+		f = obj.ElseFunc
+	}
+
+	// create the new subgraph
+	edgeName := OutputFuncArgName
+	edge := &interfaces.FuncEdge{Args: []string{edgeName}}
+	obj.init.Txn.AddVertex(f)
+	obj.init.Txn.AddEdge(f, obj.OutputVertex, edge)
+
+	return obj.init.Txn.Commit()
+}