From 5d2e776ecf68129d27e904a7796f50f26c8edea9 Mon Sep 17 00:00:00 2001
From: Tony Garnock-Jones <tonyg@leastfixedpoint.com>
Date: Fri, 29 Jan 2021 15:37:27 +0100
Subject: [PATCH] Repair error caused by mutation of a set mid-iteration

---
 packages/core/src/runtime/dataflow.ts | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/packages/core/src/runtime/dataflow.ts b/packages/core/src/runtime/dataflow.ts
index 0ca22df..fef04d7 100644
--- a/packages/core/src/runtime/dataflow.ts
+++ b/packages/core/src/runtime/dataflow.ts
@@ -75,6 +75,12 @@ export class Graph<SubjectId, ObjectId> {
         subjectObjects.forEach((oid: ObjectId) => MapSet.del(this.edgesForward, oid, subjectId));
     }
 
+    observersOf(objectId: ObjectId): Array<SubjectId> {
+        const subjects = this.edgesForward.get(objectId);
+        if (subjects === void 0) return [];
+        return Array.from(subjects);
+    }
+
     repairDamage(repairNode: (subjectId: SubjectId) => void) {
         let repairedThisRound = new FlexSet(this.objectIdCanonicalizer);
         while (true) {
@@ -92,8 +98,7 @@ export class Graph<SubjectId, ObjectId> {
             if (workSet.size === 0) break;
 
             workSet.forEach(objectId => {
-                const subjects = this.edgesForward.get(objectId) ?? [] as Array<SubjectId>;
-                subjects.forEach((subjectId: SubjectId) => {
+                this.observersOf(objectId).forEach((subjectId: SubjectId) => {
                     this.forgetSubject(subjectId);
                     this.withSubject(subjectId, () => repairNode(subjectId));
                 });