lang: gapi, unification: Shutdown unification quickly when asked

This is a bit of a hack until we improve the GAPI a bit, but will let us
shut down type unification a bit faster if we want to interrupt a long
running operation. Hopefully our future algorthmic performance
improvements will obliviate the need for this to be a common issue.

lang/gapi/gapi.go

@@ -503,6 +503,16 @@ func (obj *GAPI) Graph() (*pgraph.Graph, error) {
 
 // Next returns nil errors every time there could be a new graph.
 func (obj *GAPI) Next() chan gapi.Next {
+	// TODO: This ctx stuff is temporary until we improve the Next() API.
+	ctx, cancel := context.WithCancel(context.Background())
+	obj.wg.Add(1)
+	go func() {
+		defer obj.wg.Done()
+		select {
+		case <-obj.closeChan:
+			cancel() // close the ctx to unblock type unification
+		}
+	}()
 	ch := make(chan gapi.Next)
 	obj.wg.Add(1)
 	go func() {
@@ -553,7 +563,7 @@ func (obj *GAPI) Next() chan gapi.Next {
 				// run up to these three but fail on err
 				if e := obj.LangClose(); e != nil { // close any old lang
 					err = e // pass through the err
-				} else if e := obj.LangInit(context.TODO()); e != nil { // init the new one!
+				} else if e := obj.LangInit(ctx); e != nil { // init the new one!
 					err = e // pass through the err
 
 					// Always run LangClose after LangInit

lang/lang.go

@@ -232,6 +232,7 @@ func (obj *Lang) Init(ctx context.Context) error {
 		Debug:  obj.Debug,
 		Logf:   logf,
 	}
+	// NOTE: This is the "real" Unify that runs. (This is not for deploy.)
 	unifyErr := unifier.Unify(ctx)
 	obj.Logf("type unification took: %s", time.Since(timing))
 	if unifyErr != nil {

lang/unification/simplesolver.go

@@ -771,6 +771,13 @@ Loop:
 				}
 				// is there another EqualityWrapFuncInvariant with the same Expr1 pointer?
 				for _, fn := range fnInvariants {
+					// XXX: I think we're busy in this loop a lot.
+					select {
+					case <-ctx.Done():
+						return nil, ctx.Err()
+					default:
+						// pass
+					}
 					// is this fn.Expr1 related by equivalency graph to eq.Expr1 ?
 					if (eq.Expr1 != fn.Expr1) && !inEquiv(fn.Expr1) {
 						if obj.Debug {