Disallow assignments that can lead to race conditions due to usage of await or yield (require-atomic-updates)

When writing asynchronous code, it is possible to create subtle race condition bugs. Consider the following example:

let totalLength = 0;

async function addLengthOfSinglePage(pageNum) {
  totalLength += await getPageLength(pageNum);
}

Promise.all([addLengthOfSinglePage(1), addLengthOfSinglePage(2)]).then(() => {
  console.log('The combined length of both pages is', totalLength);
});

This code looks like it will sum the results of calling getPageLength(1) and getPageLength(2), but in reality the final value of totalLength will only be the length of one of the two pages. The bug is in the statement totalLength += await getPageLength(pageNum);. This statement first reads an initial value of totalLength, then calls getPageLength(pageNum) and waits for that Promise to fulfill. Finally, it sets the value of totalLength to the sum of await getPageLength(pageNum) and the initial value of totalLength. If the totalLength variable is updated in a separate function call during the time that the getPageLength(pageNum) Promise is pending, that update will be lost because the new value is overwritten without being read.

One way to fix this issue would be to ensure that totalLength is read at the same time as it’s updated, like this:

async function addLengthOfSinglePage(pageNum) {
  const lengthOfThisPage = await getPageLength(pageNum);

  totalLength += lengthOfThisPage;
}

Another solution would be to avoid using a mutable variable reference at all:

Promise.all([getPageLength(1), getPageLength(2)]).then(pageLengths => {
  const totalLength = pageLengths.reduce((accumulator, length) => accumulator + length, 0);

  console.log('The combined length of both pages is', totalLength);
});

Rule Details

This rule aims to report assignments to variables or properties where all of the following are true:

• A variable or property is reassigned to a new value which is based on its old value.
• A yield or await expression interrupts the assignment after the old value is read, and before the new value is set.
• The rule cannot easily verify that the assignment is safe (e.g. if an assigned variable is local and would not be readable from anywhere else while the function is paused).

Examples of incorrect code for this rule:

/* eslint require-atomic-updates: error */

let result;
async function foo() {
  result += await somethingElse;

  result = result + await somethingElse;

  result = result + doSomething(await somethingElse);
}

function* bar() {
  result += yield;

  result = result + (yield somethingElse);

  result = result + doSomething(yield somethingElse);
}

Examples of correct code for this rule:

/* eslint require-atomic-updates: error */

let result;
async function foo() {
  result = await somethingElse + result;

  let tmp = await somethingElse;
  result += tmp;

  let localVariable = 0;
  localVariable += await somethingElse;
}

function* bar() {
  result += yield;

  result = (yield somethingElse) + result;

  result = doSomething(yield somethingElse, result);
}

When Not To Use It

If you don’t use async or generator functions, you don’t need to enable this rule.

Version

This rule was introduced in ESLint 5.3.0.

Resources

• Rule source
• Documentation source