gh-156378: Stop logging.config.listen() spinning on a truncated request

[ayaangazali]

ConfigStreamHandler.handle() reads a 4-byte length and then loops until that many bytes have arrived:

chunk = self.connection.recv(slen)
while len(chunk) < slen:
    chunk = chunk + conn.recv(slen - len(chunk))

At end of stream recv() returns b'', so chunk stops growing while the loop condition stays true. A client that announces a length and then closes without sending the body leaves the handler calling recv() forever. verify cannot help, since it only runs after this loop.

This treats an empty read as end of stream and drops the partial request, which matches what the handler does with anything else it cannot act on.

Verifying

Announce one byte, then hang up, and watch process CPU while otherwise idle:

t = logging.config.listen(0)
t.start(); t.ready.wait(5)
sock = socket.create_connection(('localhost', t.port), timeout=2.0)
sock.sendall(struct.pack('>L', 1))
sock.close()
cpu0 = time.process_time(); time.sleep(0.8); cpu1 = time.process_time()
print(cpu1 - cpu0)

Before: 0.805 seconds of CPU over 0.8 seconds of wall clock, so a core is pinned. After: 0.001.

test_logging passes in full, 292 tests.

On the test

The added test waits for the handler thread to exit rather than measuring CPU, because a CPU threshold would be flaky on a loaded machine. It only fails if the thread never exits, so a slow box just makes it slower, not red. Verified both directions: with Lib/logging/config.py reverted it fails with AssertionError: the request handler did not exit after EOF, and it passes with the fix.

I did not try to make the handler report the truncated request. It looked like a separate behaviour question from stopping the spin, and the surrounding code already discards input it cannot use without comment.

sorry if I've missed something here. I reproduced it from the issue, traced the loop myself, and used Claude Code to sanity check the fix and the test design before settling on waiting for thread exit instead of timing CPU. happy to change any of it. freshman in college, still learning my way around this codebase :)

• Issue: logging.config.listen() spins forever after a truncated length-prefixed request #156378

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