Asynchronous file logging writes out-of-order entries

The implementation of asynchronous (sync = false) file logging doesn’t guarantee the logs are written out in order.

The (abbreviated) code looks like this:

writeLine(text) {
   ...
  this.asyncWriteQueue.push(text);
  this.nextAsyncWrite();
}

nextAsyncWrite() {
    var text = this.asyncWriteQueue.shift();
   fs.writeFile(..., err => {
         this.nextAsyncWrite();
   });
}

When writeLine() is called multiple times, it results in multiple in-flight fs.writeFile() calls - the order in which these calls complete is not guaranteed (that is, the second fs.writeFile() might complete before the first one, depending on I/O scheduling).

This is demonstrated by the following:

const logger = require('electron-log');
logger.transports.file.sync = false;

for (let i=0; i < 100; i++) {
  logger.info(`${i}`);
}

It should log the numbers in order in the file, but doesn’t.

(Also note that the code is quite inefficient - it calls fs.writeFile() for every single log line pushed to asyncWriteQueue - it should probably just write all the buffered entries in a single write each time nextAsyncWrite() is called.)

[megahertz]

@ramaralo This task has low priority for me, so it won’t be changed quickly

[xmedeko]

Note: pino hooks handlers to beforeExit, exit, uncaughtException, SIGHUP, SIGINT, SIGQUIT, SIGTERM to flush async logger, see https://github.com/pinojs/pino/blob/master/docs/asynchronous.md#caveats (But I do not find it necessary.)