pino/file logs do not get populated to docker container via mounted volume until the app closes

[mjarosie]

I'm trying to write logs from an application running on a host machine to a Docker container via mounted volume. Using pino/file transport that targets a file that is in a directory mounted to a Docker container (via docker-compose) does not propagate the file changes to the container.

Given the following options that log to both stdout and a file:

const pino = require("pino");

const transport = pino.transport({
  targets: [
    {
      target: "pino/file",
      options: {
        destination: 1, // stdout
      },
    },
    {
      target: "pino/file",
      options: {
        destination: "./tmp/my-logs/test.log",
        mkdir: true,
        // Neither combination of the two options below makes any difference.
        sync: true,
        fsync: true,
      },
    },
  ],
});

const logger = pino(transport);

And the following compose config:

version: "3.9"

services:
  container:
    image: alpine:latest
    command: >
      sh -c "watch -n 1 cat /tmp/my-logs/test.log"
    volumes:
      - type: bind
        read_only: true
        source: ./tmp/my-logs/
        target: /tmp/my-logs/
        bind:
          create_host_path: true

The container does not receive logs as they're written into the log file. The container only sees the logs lines once the node application gets closed.

Is there any way of "forcing" pino/file transport to somehow "flush" those changes to the file with each new log line?

Initially I'd though it's a Docker issue, but then I have also noticed that the same happens with my VSCode editor - if I have the log file opened and the application runs, new logs do not appear, but once I close the application - all new logs suddenly appear in the editor.

Here's the repo that reproduces the issue: https://github.com/mjarosie/pino-file-docker-volume-issue

[jsumners]

I am unable to reproduce the problem with the script:

const pino = require("pino");

const transport = pino.transport({
  targets: [
    {
      target: "pino/file",
      options: {
        destination: 1, // stdout
      },
    },
    {
      target: "pino/file",
      options: {
        destination: "./test.log",
        mkdir: true,
        // Neither combination of the two options below makes any difference.
        sync: true,
        fsync: true,
      },
    },
  ],
});

const logger = pino(transport);

let run = true
setTimeout(() => { run = false }, 60_000)

let i = 0
while (run === true) {
  logger.info({ i }, 'test %i', i)
  i += 1
}

In one terminal run node index.js. In a second, run tail -f test.log. The log file is tailed successfully.

[mcollina]

What version of Node.js are you using?