fix: permit waking async interval with unreliable clock

The logic for waking the AsyncInterruptibleInterval sooner than its
interval is dependent on an ability to reliably mark the last call
made to the wrapped function. In environments like AWS Lambda where
instances can be frozen and later thawed, it's possible for the
last call to be in a distant past even though the internal timer
has not completed yet. This change ensures that we immediately
reschedule in these situations.

NODE-2829

Author: mbroadst

lib/utils.js

@@ -713,12 +713,13 @@ function makeInterruptableAsyncInterval(fn, options) {
   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
@@ -737,6 +738,14 @@ function makeInterruptableAsyncInterval(fn, options) {
     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() {
@@ -758,7 +767,7 @@ function makeInterruptableAsyncInterval(fn, options) {
 
   function executeAndReschedule() {
     lastWakeTime = 0;
-    lastCallTime = now();
+    lastCallTime = clock();
 
     fn(err => {
       if (err) throw err;
@@ -769,7 +778,7 @@ function makeInterruptableAsyncInterval(fn, options) {
   if (immediate) {
     executeAndReschedule();
   } else {
-    lastCallTime = now();
+    lastCallTime = clock();
     reschedule();
   }
 

test/unit/utils.test.js

@@ -116,5 +116,51 @@ 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);
+    });
   });
 });