fix(vow): handle resolution loops in vows

packages/vow/src/watch.js

@@ -65,8 +65,23 @@ const settle = (resolver, watcher, wcb, value, watcherArgs = []) => {
  * @param {IsRetryableReason} isRetryableReason
  * @param {ReturnType<typeof makeWatchNextStep>} watchNextStep
  */
-const preparePromiseWatcher = (zone, isRetryableReason, watchNextStep) =>
-  zone.exoClass(
+const preparePromiseWatcher = (zone, isRetryableReason, watchNextStep) => {
+  // We use an ephemeral WeakSet for the previously seen vows in a watch operation
+  // While watch is durable, it suffices to detect the cycle in a single incarnation
+  /** @type {WeakMap<PromiseWatcher, WeakSet<any>>} */
+  const watcherSeenPayloads = new WeakMap();
+
+  /** @param {PromiseWatcher} watcher */
+  const getSeenPayloads = watcher => {
+    let seenPayloads = watcherSeenPayloads.get(watcher);
+    if (!seenPayloads) {
+      seenPayloads = new WeakSet();
+      watcherSeenPayloads.set(watcher, seenPayloads);
+    }
+    return seenPayloads;
+  };
+
+  return zone.exoClass(
     'PromiseWatcher',
     PromiseWatcherI,
     /**
@@ -91,12 +106,20 @@ const preparePromiseWatcher = (zone, isRetryableReason, watchNextStep) =>
       /** @type {Required<PromiseWatcher>['onFulfilled']} */
       onFulfilled(value) {
         const { watcher, watcherArgs, resolver } = this.state;
-        if (getVowPayload(value)) {
+        const payload = getVowPayload(value);
+        if (payload) {
+          const seenPayloads = getSeenPayloads(this.self);
+          // TODO: rely on endowed helper to get storable cap from payload
+          if (seenPayloads.has(payload.vowV0)) {
+            return this.self.onRejected(Error('Vow resolution cycle detected'));
+          }
+          seenPayloads.add(payload.vowV0);
           // We've been shortened, so reflect our state accordingly, and go again.
           this.state.vow = value;
           watchNextStep(value, this.self);
           return;
         }
+        watcherSeenPayloads.delete(this.self);
         this.state.priorRetryValue = undefined;
         this.state.watcher = undefined;
         this.state.resolver = undefined;
@@ -115,13 +138,15 @@ const preparePromiseWatcher = (zone, isRetryableReason, watchNextStep) =>
             return;
           }
         }
+        watcherSeenPayloads.delete(this.self);
         this.state.priorRetryValue = undefined;
         this.state.resolver = undefined;
         this.state.watcher = undefined;
         settle(resolver, watcher, 'onRejected', reason, watcherArgs);
       },
     },
   );
+};
 
 /**
  * @param {Zone} zone

packages/vow/src/when.js

@@ -28,11 +28,19 @@ export const makeWhen = (
     let result = await specimenP;
     let payload = getVowPayload(result);
     let priorRetryValue;
+    const seenPayloads = new WeakSet();
     while (payload) {
-      result = await basicE(payload.vowV0)
+      // TODO: rely on endowed helpers for getting storable cap and performing
+      // shorten "next step"
+      const { vowV0 } = payload;
+      if (seenPayloads.has(vowV0)) {
+        throw Error('Vow resolution cycle detected');
+      }
+      result = await basicE(vowV0)
         .shorten()
         .then(
           res => {
+            seenPayloads.add(vowV0);
             priorRetryValue = undefined;
             return res;
           },

packages/vow/test/watch.test.js

@@ -171,6 +171,59 @@ test('watcher args arity - shim', async t => {
   }
 });
 
+test('vow self resolution', async t => {
+  const zone = makeHeapZone();
+  const { watch, when, makeVowKit } = prepareVowTools(zone);
+
+  // A direct self vow resolution
+  const { vow: vow1, resolver: resolver1 } = makeVowKit();
+  resolver1.resolve(vow1);
+
+  // A self vow resolution through promise
+  const { vow: vow2, resolver: resolver2 } = makeVowKit();
+  const vow2P = Promise.resolve(vow2);
+  resolver2.resolve(vow2P);
+
+  // A 2 vow loop
+  const { vow: vow3, resolver: resolver3 } = makeVowKit();
+  const { vow: vow4, resolver: resolver4 } = makeVowKit();
+  resolver3.resolve(vow4);
+  resolver4.resolve(vow3);
+
+  // A head vow pointing to a 2 vow loop (a lasso?)
+  const { vow: vow5, resolver: resolver5 } = makeVowKit();
+  resolver5.resolve(vow4);
+
+  const turnTimeout = async n => {
+    if (n > 0) {
+      return Promise.resolve(n - 1).then(turnTimeout);
+    }
+
+    return 'timeout';
+  };
+
+  /**
+   * @param {number} n
+   * @param {Promise<any>} promise
+   */
+  const raceTurnTimeout = async (n, promise) =>
+    Promise.race([promise, turnTimeout(n)]);
+
+  const expectedError = {
+    message: 'Vow resolution cycle detected',
+  };
+
+  await t.throwsAsync(raceTurnTimeout(20, when(vow1)), expectedError);
+  await t.throwsAsync(raceTurnTimeout(20, when(vow2)), expectedError);
+  await t.throwsAsync(raceTurnTimeout(20, when(vow3)), expectedError);
+  await t.throwsAsync(raceTurnTimeout(20, when(vow5)), expectedError);
+
+  await t.throwsAsync(raceTurnTimeout(20, when(watch(vow1))), expectedError);
+  await t.throwsAsync(raceTurnTimeout(20, when(watch(vow2))), expectedError);
+  await t.throwsAsync(raceTurnTimeout(20, when(watch(vow3))), expectedError);
+  await t.throwsAsync(raceTurnTimeout(20, when(watch(vow5))), expectedError);
+});
+
 test('disconnection of non-vow informs watcher', async t => {
   const zone = makeHeapZone();
   const { watch, when } = prepareVowTools(zone, {