capsite/toybox: an OCapN counter vat; no import GC in workerd peers

vats/counter.js keeps its count in the vat's storage; the test checks
that it counts and that two counter vats count apart.

Running it after the greeter test hung: a FinalizationRegistry callback
in the vatAdmin, collecting an import, sent op:gc over the client's
WebSocket, and workerd forbids I/O outside a request. The vatAdmin's and
the vats' OCapN clients now run with enableImportCollection: false, so
imports go when their session does.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

toybox/supervisor.js

@@ -201,7 +201,14 @@ export class VatAdmin extends DurableObject {
 				return harden({ id: id.toString(), root });
 			},
 		});
-		this.ocapn = makeClient({ debugLabel: VAT_ADMIN, swissnumTable: new Map([[VAT_ADMIN, vatAdmin]]) });
+		// workerd runs FinalizationRegistry callbacks outside any request, where
+		// sending the resulting GC message is forbidden I/O. So imports are
+		// never collected here; they go when their session does.
+		this.ocapn = makeClient({
+			debugLabel: VAT_ADMIN,
+			swissnumTable: new Map([[VAT_ADMIN, vatAdmin]]),
+			enableImportCollection: false,
+		});
 		const netlayer = await this.ocapn.registerNetlayer(
 			makeRpcNetLayer({ designator: VAT_ADMIN, openGateway: (designator) => this.gateway(designator) }),
 		);

toybox/test/ocapn.test.js

@@ -66,3 +66,14 @@ test("createVat passes a remote object in; the vat's root comes back", async ()
 	assert.equal(await E(root).hello(), "hello from the vat");
 	assert.equal(await E(root).greetVia("vat"), "hi vat, from carol");
 });
+
+test("a counter vat counts; two counter vats count apart", async () => {
+	const src = await vatSrc("counter.js");
+	const { root: a } = await E(client.vatAdmin).createVat(src);
+	const { root: b } = await E(client.vatAdmin).createVat(src);
+	assert.equal(await E(a).increment(), 1);
+	assert.equal(await E(a).increment(), 2);
+	assert.equal(await E(b).increment(), 1);
+	assert.equal(await E(a).read(), 2);
+	assert.equal(await E(b).read(), 1);
+});

toybox/vat-runtime.js

@@ -37,7 +37,14 @@ export class App extends DurableObject {
 				return root;
 			},
 		});
-		const ocapn = makeClient({ debugLabel: vatId.slice(0, 8), swissnumTable: new Map([[VAT_SWISSNUM, bootstrap]]) });
+		// workerd runs FinalizationRegistry callbacks outside any request, where
+		// sending the resulting GC message is forbidden I/O. So imports are
+		// never collected here; they go when their session does.
+		const ocapn = makeClient({
+			debugLabel: vatId.slice(0, 8),
+			swissnumTable: new Map([[VAT_SWISSNUM, bootstrap]]),
+			enableImportCollection: false,
+		});
 		const netlayer = await ocapn.registerNetlayer(
 			makeRpcNetLayer({ designator: vatId, openGateway: (designator) => router.gateway(designator) }),
 		);

toybox/vats/counter.js

@@ -0,0 +1,10 @@
+/** A counter whose count lives in the vat's storage. */
+export const buildRootObject = ({ Far, storage }) =>
+	Far("counterRoot", {
+		increment: () => {
+			const n = (storage.get("count") ?? 0) + 1;
+			storage.put("count", n);
+			return n;
+		},
+		read: () => storage.get("count") ?? 0,
+	});