connection: Fix race in ParamikoBackgroundCommand API

When using an ssh background command, the data is read from paramiko as
it comes and stored in a buffering pipe by a thread. Currently, the
ParamikoBackgroundCommand API will report the command as having
completed as soon as paramiko reports it. This however does not
necessarily mean that the pipe-filling thread is finished copying the
streams, and the client will end up assuming there is no more data to
read even though it's not the case.

Fix that by ensuring that when poll() returns non-None, the output
streams are ready to be drained.

devlib/connection.py

@@ -266,10 +266,20 @@ class ParamikoBackgroundCommand(BackgroundCommand):
         return self._pid
 
     def wait(self):
-        return self.chan.recv_exit_status()
+        status = self.chan.recv_exit_status()
+        # Ensure that the redirection thread is finished copying the content
+        # from paramiko to the pipe.
+        self.redirect_thread.join()
+        return status
 
     def poll(self):
-        if self.chan.exit_status_ready():
+        # Wait for the redirection thread to finish, otherwise we would
+        # indicate the caller that the command is finished and that the streams
+        # are safe to drain, but actually the redirection thread is not
+        # finished yet, which would end up in lost data.
+        if self.redirect_thread.is_alive():
+            return None
+        elif self.chan.exit_status_ready():
             return self.wait()
         else:
             return None