internal/core/debug: add cycle detector

Sometimes nodes are shared (using structure sharing)
that have a structural cycle without reporting on
that structural cycle. This caused printing to hang.

This change detects such cases and modifies to output
to inform about such cycles.

Issue #2850

Signed-off-by: Marcel van Lohuizen <[email protected]>
Change-Id: Iba74c91729507a4782c7f5902ef2ac6ed2770716
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1201896
Reviewed-by: Matthew Sackman <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>

Author: mpvl

internal/core/adt/unify.go

@@ -438,6 +438,8 @@ func (n *nodeContext) updateScalar() {
 }
 
 func (n *nodeContext) completeAllArcs(needs condition, mode runMode) bool {
+	// TODO: this can go when lookup vs evaluation distinction
+	// has been improved.
 	if n.node.status == evaluatingArcs {
 		// NOTE: this was an "incomplete" error pre v0.6. If this is a problem
 		// we could make this a CycleError. Technically, this may be correct,

internal/core/debug/compact.go

@@ -89,6 +89,7 @@ func (w *compactPrinter) node(n adt.Node) {
 		case *adt.Vertex:
 			if v, ok := w.printShared(x); !ok {
 				w.node(v)
+				w.popVertex()
 			}
 
 		case adt.Value:

internal/core/debug/debug.go

@@ -70,6 +70,9 @@ type printer struct {
 	indent string
 	cfg    *Config
 
+	// keep track of vertices to avoid cycles.
+	stack []*adt.Vertex
+
 	// modes:
 	// - show vertex
 	// - show original conjuncts
@@ -144,9 +147,32 @@ func (w *printer) printShared(v *adt.Vertex) (x *adt.Vertex, ok bool) {
 		w.shared(s)
 		return v, true
 	}
+	if !w.pushVertex(v) {
+		if s != nil {
+			w.shared(s)
+			w.string(" =>")
+		}
+		w.shared(v)
+		return v, true
+	}
 	return v, false
 }
 
+func (w *printer) pushVertex(v *adt.Vertex) bool {
+	for _, x := range w.stack {
+		if x == v {
+			w.string("<TODO: unmarked structural cycle>")
+			return false
+		}
+	}
+	w.stack = append(w.stack, v)
+	return true
+}
+
+func (w *printer) popVertex() {
+	w.stack = w.stack[:len(w.stack)-1]
+}
+
 func (w *printer) shortError(errs errors.Error) {
 	for {
 		msg, args := errs.Msg()
@@ -201,6 +227,7 @@ func (w *printer) node(n adt.Node) {
 		if ok {
 			return
 		}
+		defer w.popVertex()
 
 		var kind adt.Kind
 		if x.BaseValue != nil {