Handling Subprocess Pipe Buffer Limits and Avoiding Deadlocks in Python

When a Python program invokes external commands via subprocess.Popen with stdout=PIPE or stderr=PIPE , the operating system pipe buffer can become full if the child process produces a large amount of output, causing the parent process to block on wait() and resulting in a deadlock.

Linux's default pipe buffer size is 4 MiB (the PIPE_BUF limit); if the child writes more than this, the pipe fills and the parent waiting for the process to finish cannot read the data, so the child also blocks, freezing both processes.

The Python documentation warns that data is buffered in memory and that using Popen.wait() with large or unbounded output can cause deadlocks, essentially advising against reading large streams via PIPE .

To avoid the issue, do not set stdout or stderr to PIPE for large outputs. Instead, use communicate() which reads the streams incrementally, or redirect output to real files or a StringIO buffer while reading in chunks.

<code>from subprocess import Popen, PIPE
p = Popen(cmd, stdout=PIPE, stderr=PIPE)
(out, err) = p.communicate()
</code>

Another pattern uses StringIO to collect output while polling the process:

<code>from subprocess import Popen, PIPE
from io import StringIO

def run(cmd):
    out = StringIO()
    p = Popen(cmd, stdout=PIPE, stderr=PIPE, stdin=PIPE, shell=True)
    while p.poll() is None:
        data = p.stdout.read()
        out.write(data)
    if p.returncode != 0:
        print(f'Run `{cmd}` failed with exit code {p.returncode}\n{p.stderr.read()}')
        sys.exit(p.returncode)
    return out.getvalue()
</code>

By using these approaches, the parent process reads the child’s output before the pipe buffer fills, preventing the deadlock and allowing reliable execution of external commands from Python.