make test-gc-and-finalize slightly more devious

packages/SwingSet/test/test-gc-and-finalize.js

@@ -15,14 +15,30 @@ test(`have gc() on Node.js`, async t => {
   // `gc` C callback on the global object.
 });
 
-function setup() {
-  const victim = { doomed: 'oh no' };
+function setup1() {
+  // The worst case I can think of is a four-pole counter-rotating cycle,
+  // held in place by N and S, with E as our victim/sensor. Dropping N or S
+  // does not drop a refcount to zero.
+  const N = {};
+  const S = {};
+  const E = { N, S };
+  const W = { N, S };
+  N.E = E;
+  N.W = W;
+  S.E = E;
+  S.W = W;
   const finalized = ['finalizer not called'];
   const fr = new FinalizationRegistry(_tag => {
     finalized[0] = 'finalizer was called';
   });
-  const wr = new WeakRef(victim);
-  fr.register(victim, 'tag');
+  const wr = new WeakRef(E);
+  fr.register(E, 'tag');
+  return { N, S, finalized, fr, wr };
+}
+
+function setup2() {
+  const { N: _ignoreN, S: _ignoreS, finalized, fr, wr } = setup1();
+  // I want N and S to be dropped after E and W.
   return { finalized, fr, wr };
 }
 
@@ -32,7 +48,7 @@ async function provokeGC() {
 
   // we must retain the FinalizationRegistry to let the callback fire
   // eslint-disable-next-line no-unused-vars
-  const { finalized, fr, wr } = setup();
+  const { finalized, fr, wr } = setup2();
 
   // the transition from UNREACHABLE to COLLECTED can happen at any moment,
   // but is far more likely to happen if we force it
@@ -82,7 +98,8 @@ test(`can provoke gc on xsnap`, async t => {
   const vat = xsnap(opts);
   const code = `
 ${gcAndFinalize}
-${setup}
+${setup1}
+${setup2}
 ${provokeGC}
 provokeGC().then(data => issueCommand(ArrayBuffer.fromString(JSON.stringify(data))));
 `;