Possible race condition in saveObjectToFile()

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saveObjectToFile() writes the object to a temporary file first, and then uses asynchronous fs.rename() to move the temporary file to the target file:

json-file-store/Store.es6.js

Lines 78 to 83 in 6aada66

	fs.writeFile(tmpFileName, json, 'utf8', (err) => {
	if (err) return cb(err);
	fs.rename(tmpFileName, file, cb);
	});
	});

If 2 fs.rename()s are called at almost the same time, there will be a race condition, and the execution result is not determined. One of them will lose its operations.

db.save("id", {number: 1}, function (err)
{
    if (err)
    {
        console.error(err);
    }
});

db.save("id", {number: 2}, function (err)
{
    if (err)
    {
        console.error(err);
    }
});

// {"number":1} or {"number":2} in id.json

This problem affects both save() and delete().