lang: New function engine

This mega patch primarily introduces a new function engine. The main
reasons for this new engine are:

1) Massively improved performance with lock-contended graphs.

Certain large function graphs could have very high lock-contention which
turned out to be much slower than I would have liked. This new algorithm
happens to be basically lock-free, so that's another helpful
improvement.

2) Glitch-free function graphs.

The function graphs could "glitch" (an FRP term) which could be
undesirable in theory. In practice this was never really an issue, and
I've not explicitly guaranteed that the new graphs are provably
glitch-free, but in practice things are a lot more consistent.

3) Simpler graph shape.

The new graphs don't require the private channels. This makes
understanding the graphs a lot easier.

4) Branched graphs only run half.

Previously we would run two pure side of an if statement, and while this
was mostly meant as an early experiment, it stayed in for far too long
and now was the right time to remove this. This also means our graphs
are much smaller and more efficient too.

Note that this changed the function API slightly. Everything has been
ported. It's possible that we introduce a new API in the future, but it
is unexpected to cause removal of the two current APIs.

In addition, we finally split out the "schedule" aspect from
world.schedule(). The "pick me" aspects now happen in a separate
resource, rather than as a yucky side-effect in the function. This also
lets us more precisely choose when we're scheduled, and we can observe
without being chosen too.

As usual many thanks to Sam for helping through some of the algorithmic
graph shape issues!

lang/funcs/structs/exprif.go

@@ -32,7 +32,6 @@ package structs
 import (
 	"context"
 	"fmt"
-	"sync"
 
 	"github.com/purpleidea/mgmt/lang/interfaces"
 	"github.com/purpleidea/mgmt/lang/types"
@@ -134,78 +133,6 @@ func (obj *ExprIfFunc) Init(init *interfaces.Init) error {
 	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 {
@@ -229,3 +156,30 @@ func (obj *ExprIfFunc) replaceSubGraph(b bool) error {
 
 	return obj.init.Txn.Commit()
 }
+
+// Call this func and return the value if it is possible to do so at this time.
+func (obj *ExprIfFunc) Call(ctx context.Context, args []types.Value) (types.Value, error) {
+	if len(args) < 1 {
+		return nil, fmt.Errorf("not enough args")
+	}
+	value := args[0]
+	b := value.Bool()
+
+	if obj.last == nil || *obj.last != b {
+		obj.last = &b // store new result
+
+		if err := obj.replaceSubGraph(b); err != nil {
+			return nil, errwrap.Wrapf(err, "could not replace subgraph")
+		}
+
+		return nil, interfaces.ErrInterrupt
+	}
+
+	// send dummy value to the output
+	return types.NewNil(), nil // dummy value
+}
+
+// Cleanup runs after that function was removed from the graph.
+func (obj *ExprIfFunc) Cleanup(ctx context.Context) error {
+	return obj.init.Txn.Reverse()
+}