[215] add `util.repeateUntilCancelled`

* calls an async function repeatedly on an interval
* returns a function that, if invoked, cancels the next call of the
  async function

app/util.js

@@ -38,6 +38,19 @@ const repeatUntil = (fn, interval, predicate) =>
         .then(() => wait(interval))
         .then(() => repeatUntil(fn, interval, predicate))
 
+const repeatUntilCancelled = (fn, interval) => {
+  let shouldContinue = true
+  const cancel = () => (shouldContinue = false)
+  const repeat = (fn, interval) =>
+    !shouldContinue
+      ? Promise.resolve()
+      : Promise.resolve(fn())
+          .then(() => wait(interval))
+          .then(() => repeat(fn, interval))
+  repeat(fn, interval)
+  return cancel
+}
+
 const repeatUntilTimeout = (fn, interval, timeout) => {
   const now = nowInMillis()
   repeatUntil(fn, interval, () => nowInMillis() > now + timeout)
@@ -145,6 +158,7 @@ module.exports = {
   redact,
   repeatEvery,
   repeatUntilTimeout,
+  repeatUntilCancelled,
   sha256Hash,
   sequence,
   statuses,

test/unit/util.spec.js

 import { expect } from 'chai'
 import { describe, it, beforeEach, afterEach } from 'mocha'
+import sinon from 'sinon'
 import util from '../../app/util'
 
 describe('utility module', () => {
@@ -91,4 +92,17 @@ describe('utility module', () => {
       })
     })
   })
+
+  describe('#repeatUntilCancelled', () => {
+    it('calls a function repeatedly until it is cancelled', async () => {
+      const fn = sinon.stub().returns(Promise.resolve())
+      const interval = 20
+      const cancel = util.repeatUntilCancelled(fn, interval)
+      await util.wait(2 * interval)
+      expect(fn.callCount).to.eql(2)
+      cancel()
+      await util.wait(2 * interval)
+      expect(fn.callCount).to.eql(2)
+    })
+  })
 })