Printing an empty string to a PTY in Python with `-u` causes the program to hang
thecodingwizard opened this issue · comments
Description
If I make the following Python file:
print("")
and I run this Python file from within a PTY with the -u flag (ie. python -u main.py from within a PTY), the program will hang indefinitely.
I think you can run the Python file from within a pty with import pty; pty.spawn(["python", "-u", "main.py"]). I used docker run --rm -it --runtime=runsc python3.11-bullseye /bin/bash and manually created + ran the file to test.
I suspect it's because this line of code is being run:
I believe Python will make a write() syscall that writes an empty string to stdout, causing the issue.
I think this also means that print("something", file=sys.stderr, end="") won't work, which means that if an exception is raised in a Python script running within a PTY, it will hang forever.
raise Exception("hello")
^ This will never finish when run inside a PTY with the -u flag; I think it's because it makes a print() call with end="".
Steps to reproduce
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runsc version
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docker version (if using docker)
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uname
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kubectl (if using Kubernetes)
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repo state (if built from source)
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runsc debug logs (if available)
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Thanks for another PR. Similar to the other one, could you add a test to test/syscalls/linux/pty.cc?
I don't like to be too nitpicky about tests for very small changes, but the PTY code especially is finnicky and difficult. Behavior is complex and I just want to ensure we're doing the same thing as Linux.