per sergeyberezin@ comment in https://chromium-review.googlesource.com/c/chromium/src/+/1330204/34/ios/build/bots/scripts/xcodebuild_runner.py#459

The bot expects a command to print something once in a while, otherwise it'll kill it in 20 min of inactivity, assuming that the process has hung. So we can't just quietly accumulate logs and then dump them all in the end (though that's a good thing to do for a sane output).
We'd need to come up with something to solve this puzzle. Maybe we can print heartbeat log lines from a special thread?