pgraph, lang: funcs: dage: Add an awkward graphviz interface

This refactors the code to make it not depend on the specific engine.

lang/funcs/dage/dage.go

@@ -1451,10 +1451,10 @@ func (obj *Engine) Stats() string {
 	return obj.stats.String()
 }
 
-// Graphviz writes out the diagram of a graph to be used for visualization and
-// debugging. You must not modify the graph (eg: during Lock) when calling this
-// method.
-func (obj *Engine) Graphviz(dir string) error {
+// ExecGraphviz writes out the diagram of a graph to be used for visualization
+// and debugging. You must not modify the graph (eg: during Lock) when calling
+// this method.
+func (obj *Engine) ExecGraphviz(dir string) error {
 	// XXX: would this deadlock if we added this?
 	//obj.graphMutex.Lock()         // XXX: should this be a RLock?
 	//defer obj.graphMutex.Unlock() // XXX: should this be an RUnlock?

lang/funcs/dage/txn.go

@@ -498,12 +498,12 @@ func (obj *graphTxn) commit() error {
 
 	// XXX: running this on each commit has a huge performance hit.
 	// XXX: we could write out the .dot files and run graphviz afterwards
-	if engine, ok := obj.GraphAPI.(*Engine); ok && GraphvizDebug {
+	if g, ok := obj.GraphAPI.(pgraph.Graphvizable); ok && GraphvizDebug {
 		//d := time.Now().Unix()
-		//if err := engine.graph.ExecGraphviz(fmt.Sprintf("/tmp/txn-graphviz-%d.dot", d)); err != nil {
+		//if err := g.ExecGraphviz(fmt.Sprintf("/tmp/txn-graphviz-%d.dot", d)); err != nil {
 		//	panic("no graphviz")
 		//}
-		if err := engine.Graphviz(""); err != nil {
+		if err := g.ExecGraphviz(""); err != nil {
 			panic(err) // XXX: improve me
 		}
 

pgraph/graphviz.go

@@ -40,6 +40,23 @@ const (
 	ptrLabelsSize = 10
 )
 
+// Graphvizable is a simple interface to handle the common signature for this
+// useful graphviz exec method that is used in debugging. Expect that this
+// signature might change if the authors find a different variant more useful.
+type Graphvizable interface {
+
+	// ExecGraphviz runs graphviz and stores the result at this absolute
+	// path. It (awkwardly) can be used by someone expecting either a
+	// filename, or a directory. The filename scenario should be used if you
+	// are expecting a single .dot output file. The directory scenario
+	// should be used if you are expecting a series of .dot graphs.
+	// Directories must end with a trailing slash. A filename passed will
+	// get this location overwritten if there is something already there. If
+	// the string is empty, it might create a file in a temporary directory
+	// somewhere.
+	ExecGraphviz(filename string) error
+}
+
 // Graphviz adds some visualization features for pgraph.
 type Graphviz struct {
 	// Name is the display name of the graph. If specified it overrides an