Prevent Paramiko deadlock when test sends more than 2MB to stdout

[jburgess777]

The paramiko library has a known problem where checking the exit status can cause a deadlock if the command has written a lot of output to the stdout (or stderr) channel. We hit this problem when using testinfra to return data from log files during a test.

https://docs.paramiko.org/en/stable/api/channel.html#paramiko.channel.Channel.recv_exit_status

We can work around this, for the stdout case, by reading the data before checking the exit status.

I do not attempt to fix the stderr case. Fixing this would likely require a more complex fix, which reads from both stdout and stderr until they are closed, like https://stackoverflow.com/a/78765054

Here is an example of running the test case on the original code. I enabled pytest-timeouts with a 10 second timeout so you can see it stuck in recv_exit_status

$ tox -- -v -k test_command_deadlock test

========================================================================================= test session starts ===========================================================================================
platform linux -- Python 3.10.12, pytest-8.3.3, pluggy-1.5.0 -- /home/jburgess/git/pytest-testinfra/.tox/py/bin/python
cachedir: .tox/py/.pytest_cache
rootdir: /home/jburgess/git/pytest-testinfra
configfile: setup.cfg
plugins: testinfra-0.1.dev789, cov-5.0.0, xdist-3.6.1, timeouts-1.2.1
setup timeout: 0.0s, execution timeout: 0.0s, teardown timeout: 0.0s
collected 177 items / 171 deselected / 6 selected

tes=t/test_backends.py::test_command_deadlock[ssh://debian_bookworm] PASSED                                                                                                                         [ 16%]
test/test_backends.py::test_command_deadlock[safe-ssh://debian_bookworm] PASSED                                                                                                                    [ 33%]
test/test_backends.py::test_command_deadlock[docker://debian_bookworm] PASSED                                                                                                                      [ 50%]
test/test_backends.py::test_command_deadlock[paramiko://debian_bookworm] FAILED                                                                                                                    [ 66%]
test/test_backends.py::test_command_deadlock[ansible://debian_bookworm] PASSED                                                                                                                     [ 83%]
test/test_backends.py::test_command_deadlock[ansible://debian_bookworm?force_ansible=True] PASSED                                                                                                  [100%]

================================================================================================ FAILURES ================================================================================================
___________________________________________________________________________ test_command_deadlock[paramiko://debian_bookworm] ____________________________________________________________________________

host = <testinfra.host.Host paramiko://debian_bookworm>

    @pytest.mark.testinfra_hosts(*HOSTS)
    @pytest.mark.execution_timeout(10)
    def test_command_deadlock(host):
        # Test for deadlock when exceeding Paramiko transport buffer (2MB)
        # https://docs.paramiko.org/en/latest/api/channel.html#paramiko.channel.Channel.recv_exit_status
        size = 3 * 1024 * 1024
>       output = host.check_output(f"python3 -c 'print(\"a\" * {size})'")

test/test_backends.py:651:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
testinfra/host.py:98: in run
    return self.backend.run(command, *args, **kwargs)
testinfra/backend/paramiko.py:150: in run
    rc, stdout, stderr = self._exec_command(cmd)
testinfra/backend/paramiko.py:141: in _exec_command
    rc = chan.recv_exit_status()
.tox/py/lib/python3.10/site-packages/paramiko/channel.py:400: in recv_exit_status
    self.status_event.wait()
/usr/lib/python3.10/threading.py:607: in wait
    signaled = self._cond.wait(timeout)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

With the fix, the test passes

========================================================================================== test session starts ===========================================================================================
platform linux -- Python 3.10.12, pytest-8.3.3, pluggy-1.5.0 -- /home/jburgess/git/pytest-testinfra/.tox/py/bin/python
cachedir: .tox/py/.pytest_cache
rootdir: /home/jburgess/git/pytest-testinfra
configfile: setup.cfg
plugins: testinfra-0.1.dev789, cov-5.0.0, xdist-3.6.1, timeouts-1.2.1
setup timeout: 0.0s, execution timeout: 0.0s, teardown timeout: 0.0s
collected 177 items / 171 deselected / 6 selected

test/test_backends.py::test_command_deadlock[ssh://debian_bookworm] PASSED                                                                                                                         [ 16%]
test/test_backends.py::test_command_deadlock[safe-ssh://debian_bookworm] PASSED                                                                                                                    [ 33%]
test/test_backends.py::test_command_deadlock[docker://debian_bookworm] PASSED                                                                                                                      [ 50%]
test/test_backends.py::test_command_deadlock[paramiko://debian_bookworm] PASSED                                                                                                                    [ 66%]
test/test_backends.py::test_command_deadlock[ansible://debian_bookworm] PASSED                                                                                                                     [ 83%]
test/test_backends.py::test_command_deadlock[ansible://debian_bookworm?force_ansible=True] PASSED                                                                                                  [100%]

=================================================================================== 6 passed, 171 deselected in 4.24s ====================================================================================
py: OK ✔ in 5.43 seconds

I didn't see any regressions in the rest of the test suite.