cue: limit validation depth

This is a temporary workaround to not having an
occurs check. See Issue #29.

Change-Id: I2d6f3bfe3493322cf507e0088d415afaf97bda4f
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/2365
Reviewed-by: Marcel van Lohuizen <[email protected]>

Author: mpvl

cue/context.go

@@ -42,6 +42,10 @@ type context struct {
 	// tracing
 	trace bool
 	level int
+
+	// TODO: replace with proper structural cycle detection/ occurs check.
+	// See Issue #29.
+	maxDepth int
 }
 
 func (c *context) incEvalDepth() {

cue/validate.go

@@ -23,11 +23,16 @@ func validate(ctx *context, v value) *bottom {
 	switch x := eval.(type) {
 	case *structLit:
 		x = x.expandFields(ctx)
+		if ctx.maxDepth++; ctx.maxDepth > 20 {
+			return nil
+		}
 		for i := range x.arcs {
 			if err := validate(ctx, x.at(ctx, i)); err != nil {
+				ctx.maxDepth--
 				return err
 			}
 		}
+		ctx.maxDepth--
 	case *list:
 		// TODO: also validate types for open lists?
 		return validate(ctx, x.elem)