fix: permit waking async interval with unreliable clocks

src/utils.ts

@@ -1036,6 +1036,9 @@ export interface InterruptableAsyncIntervalOptions {
   minInterval: number;
   /** Whether the method should be called immediately when the interval is started  */
   immediate: boolean;
+
+  /* @internal only used for testing unreliable timer environments */
+  clock: () => number;
 }
 
 /** @internal */
@@ -1066,12 +1069,13 @@ export function makeInterruptableAsyncInterval(
   const interval = options.interval || 1000;
   const minInterval = options.minInterval || 500;
   const immediate = typeof options.immediate === 'boolean' ? options.immediate : false;
+  const clock = typeof options.clock === 'function' ? options.clock : now;
 
   function wake() {
-    const currentTime = now();
+    const currentTime = clock();
     const timeSinceLastWake = currentTime - lastWakeTime;
     const timeSinceLastCall = currentTime - lastCallTime;
-    const timeUntilNextCall = Math.max(interval - timeSinceLastCall, 0);
+    const timeUntilNextCall = interval - timeSinceLastCall;
     lastWakeTime = currentTime;
 
     // For the streaming protocol: there is nothing obviously stopping this
@@ -1090,6 +1094,14 @@ export function makeInterruptableAsyncInterval(
     if (timeUntilNextCall > minInterval) {
       reschedule(minInterval);
     }
+
+    // This is possible in virtualized environments like AWS Lambda where our
+    // clock is unreliable. In these cases the timer is "running" but never
+    // actually completes, so we want to execute immediately and then attempt
+    // to reschedule.
+    if (timeUntilNextCall < 0) {
+      executeAndReschedule();
+    }
   }
 
   function stop() {
@@ -1114,7 +1126,7 @@ export function makeInterruptableAsyncInterval(
 
   function executeAndReschedule() {
     lastWakeTime = 0;
-    lastCallTime = now();
+    lastCallTime = clock();
 
     fn(err => {
       if (err) throw err;
@@ -1125,7 +1137,7 @@ export function makeInterruptableAsyncInterval(
   if (immediate) {
     executeAndReschedule();
   } else {
-    lastCallTime = now();
+    lastCallTime = clock();
     reschedule(undefined);
   }
 

test/unit/utils.test.js

@@ -114,5 +114,51 @@ describe('utils', function () {
 
       this.clock.tick(250);
     });
+
+    it('should immediately schedule if the clock is unreliable', function (done) {
+      let clockCalled = 0;
+      let lastTime = now();
+      const marks = [];
+      const executor = makeInterruptableAsyncInterval(
+        callback => {
+          marks.push(now() - lastTime);
+          lastTime = now();
+          callback();
+        },
+        {
+          interval: 50,
+          minInterval: 10,
+          immediate: true,
+          clock() {
+            clockCalled += 1;
+
+            // needs to happen on the third call because `wake` checks
+            // the `currentTime` at the beginning of the function
+            if (clockCalled === 3) {
+              return now() - 100000;
+            }
+
+            return now();
+          }
+        }
+      );
+
+      // force mark at 20ms, and then the unreliable system clock
+      // will report a very stale `lastCallTime` on this mark.
+      setTimeout(() => executor.wake(), 10);
+
+      // try to wake again in another `minInterval + immediate`, now
+      // using a very old `lastCallTime`. This should result in an
+      // immediate scheduling: 0ms (immediate), 20ms (wake with minIterval)
+      // and then 10ms for another immediate.
+      setTimeout(() => executor.wake(), 30);
+
+      setTimeout(() => {
+        executor.stop();
+        expect(marks).to.eql([0, 20, 10, 50, 50, 50, 50]);
+        done();
+      }, 250);
+      this.clock.tick(250);
+    });
   });
 });