timers: bail from intervals if _repeat is bad

PR-URL: #10365
Ref: #9685

Reviewed-By: Myles Borins <myles.borins@gmail.com>

Author: Fishrock123

lib/timers.js

@@ -272,6 +272,10 @@ exports.setInterval = function(callback, repeat) {
   return timer;
 
   function wrapper() {
+    // If _repeat was overriden we are effectively canceled. Bail.
+    if (typeof timer._repeat !== 'function')
+      return;
+
     timer._repeat();
 
     // Do not re-arm unenroll'd or closed timers.

test/parallel/test-timers-unenroll-unref-interval.js

@@ -1,6 +1,7 @@
 'use strict';
 
 const common = require('../common');
+const assert = require('assert');
 const timers = require('timers');
 
 {
@@ -33,6 +34,19 @@ const timers = require('timers');
   }), 1).unref();
 }
 
+{
+  const interval = setInterval(common.mustCall(() => {
+    // This case is only necessary / valid prior to
+    // c8c2544cd9c339cdde881fc9a7f0851971b94d72
+    // which is not on the v4.x branch.
+    assert.strictEqual(typeof interval._repeat, 'function');
+
+    process.nextTick(common.mustCall(() => {
+      interval._repeat = null;
+    }));
+  }), 1).unref();
+}
+
 // Use timers' intrinsic behavior to keep this open
 // exactly long enough for the problem to manifest.
 //