Error in parsing output from probe-cli

[sarathms]

Describe the bug
Sometimes, when probe-desktop reads the output from running ooniprobe run <test> --batch as a ReadableStream, the on('data') event handler receives non-JSON data. In this case, it was this error message:

2021/03/10 12:18:22 failed to sufficiently increase receive buffer size (was: 208 kiB, wanted: 2048 kiB, got: 416 kiB). See https://github.com/lucas-clemente/quic-go/wiki/UDP-Receive-Buffer-Size for details.

It seems to swallow a small amount of data that would otherwise be shown in the UI when the test is running. This doesn't happen when it is run directly from the terminal with/without the --batch flag.

To Reproduce
This sample code can reproduce the bug. Although it happens intermittently.

const { spawn } = require('child_process')
const split2 = require('split2')

const pathToOONI = '/usr/bin/ooniprobe'

const cli = spawn(pathToOONI, ['--batch', 'run', 'middlebox'], { stdio: ['pipe', 'pipe', 'pipe'] })

cli.stderr.pipe(split2()).on('data', line => {
  try {
    console.log(`line: ${line.toString('utf8')}`)
    const msg = JSON.parse(line.toString('utf8'))
    // console.log('msg: ', msg)
  } catch (e) {
    console.error(e)
  }
})

Expected behavior
Errors are expected on stdout in batch mode where we do not attempt to JSON.parse the content.

System information (please complete the following information):

• OS: Linux
• Version: probe-cli 3.7.0

[bassosimone]

This is intermittent because it depends on which resolver is selected. We see this warning only when http3 is used. We are now using http3 to reach some dns-over-https resolvers. The right fix to me seems to use stdout for JSON messages and stderr for everything else that is not related to messages. I can confirm by manually running a test program that the error you are talking about is emitted by a dependency and is indeed emitted onto the standard error. So, I'll just change ooniprobe to emit JSON on the stdout.

[bassosimone]

All done!