internal/core/adt: restrict notification

This restricts the cases where notifications are added,
limiting the overhead of the mechanism.

Issue #661

Change-Id: I4d8929dfb6136ef994e39eb50c1ede39c2f13f60
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/8402
Reviewed-by: CUE cueckoo <cueckoo@gmail.com>
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>

cue/testdata/cycle/049_self-reference_cycles_conflicts_with_strings.txtar

@@ -42,10 +42,6 @@ Result:
       //     ./in.cue:2:5
       //     ./in.cue:5:7
     }
-    y: (_|_){
-      // [eval] a.x: conflicting values "hey!?" and "hey":
-      //     ./in.cue:2:5
-      //     ./in.cue:5:7
-    }
+    y: (string){ "hey!" }
   }
 }

internal/core/adt/eval.go

@@ -144,7 +144,14 @@ func (c *OpContext) evaluate(v *Vertex, state VertexStatus) Value {
 	}
 
 	if v.status < Finalized && v.state != nil {
-		v.state.addNotify(c.vertex)
+		// TODO: errors are slightly better if we always add addNotify, but
+		// in this case it is less likely to cause a performance penalty.
+		// See https://github.com/cuelang/cue/issues/661. It may be possible to
+		// relax this again once we have proper tests to prevent regressions of
+		// that issue.
+		if !v.state.done() || v.state.errs != nil {
+			v.state.addNotify(c.vertex)
+		}
 	}
 
 	return v