`tornado.process.Subprocess()` hangs forever in `write()` for large data about tornado HOT 1 CLOSED

This is a classic deadlock: A pipe only has a buffer of a few KB. So when the child process tries to write 140K, it's only able to write a little bit to the buffer, and then it has to wait for that buffer to be consumed to write more. The child process exits after it has written all of its data, but it can't write all its data until the parent process reads it. So in this particular case, you need to call proc.stdout.read_until_close() before wait_for_exit.

In general, if you mix multiple operations on a subprocess (wait_for_exit and reading from a stream, or reading from multiple streams), you need to think carefully about the order in which things might happen and this will likely require a more complicated structure than simple sequence of awaits (maybe a tornado.gen.WaitIterator or asyncio equivalents). But in the common cases reading from a single stream until close and then waiting for exit will work.

from tornado.