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

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:

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.

test/test_backends.py

@@ -640,3 +640,12 @@ def test_get_hosts():
         ("a", 10),
         ("a", 1),
     ]
+
+
+@pytest.mark.testinfra_hosts(*HOSTS)
+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})'")
+    assert len(output) == size

testinfra/backend/paramiko.py

@@ -138,9 +138,9 @@ def _exec_command(self, command: bytes) -> tuple[int, bytes, bytes]:
         if self.get_pty:
             chan.get_pty()
         chan.exec_command(command)
-        rc = chan.recv_exit_status()
         stdout = b"".join(chan.makefile("rb"))
         stderr = b"".join(chan.makefile_stderr("rb"))
+        rc = chan.recv_exit_status()
         return rc, stdout, stderr
 
     def run(self, command: str, *args: str, **kwargs: Any) -> base.CommandResult: