为什么它是以下代码中的竞争条件？

Question

I have the following code ES Lint marks as a race condition:

const fillPersonPositions = async (person) => {
    person.positions = await getPositions(person.id);
};

Full error:

Possible race condition: `person.positions` might be reassigned based on an outdated value of `person.positions`.eslint(require-atomic-updates)

If I change "person.id" to something unrelated to person object like "null", the error vanishes. What is the problem here and how to avoid it? (I mean fix code if it's a real race condition problem or tell the linter that it's not a problem if not).

Answer 1

This code could be problematic if you called fillPersonPositions more than once, and the id of the person changed in the meantime. Eg:

person.id = 1;
const p1 = fillPersonPositions(person);
person.id = 3;
const p2 = fillPersonPositions(person);
await Promise.all([p1, p2]);

Here, there's no guarantee that the first fillPersonPositions resolves first, so at the end, you might end up with person having an ID of 3, but a positions property corresponding to the ID of 1.

Of course, the above is very contrived, but ESLint does not know that.

One possible fix would be to avoid mutation, and instead return (and use) a new object:

const fillPersonPositions = async (person) => {
    const positions = await getPositions(person.id);
    return { ...person, positions };
};

(It's often a good idea to avoid mutation when possible anyway, for the sake of more understandable code - for similar reasons, const is a better choice than let when you have the option)

Answer 2

The code is marked because person is not atomic because it theoretically could be changed elsewehre during your asynchronous opertation.

You can turn of this rule by changing the following entry within your .eslintrc.json file:

"require-atomic-updates": "off"