nsfarm/cli: fix function pexpect_flush

It turns out that nonblocking doesn't mean nonblocking in all cases. In
this case when there was no input the read was blocking.
The timeout set to zero solves this.

nsfarm/board/_board.py

@@ -82,7 +82,7 @@ class Board(abc.ABC):
         # Wait for bootup
         self._pexpect.expect_exact(["Router Turris successfully started.", ], timeout=120)
         # Note Shell sends new line which opens terminal for it
-        shell = cli.Shell(self._pexpect, flush=False)  # TODO why this flush timeouts?
+        shell = cli.Shell(self._pexpect)
         shell.run("sysctl -w kernel.printk='0 4 1 7'")  # disable kernel print to not confuse console flow
         return shell
 

nsfarm/cli.py

@@ -18,6 +18,7 @@ import select
 import socket
 import threading
 import typing
+import pexpect
 
 _FLUSH_BUFFLEN = 2048
 
@@ -25,9 +26,14 @@ _FLUSH_BUFFLEN = 2048
 def pexpect_flush(pexpect_handle):
     """Flush all input on pexpect. This effectively reads everything.
     """
-    # TODO fix: this timeouts if there is nothing to flush
-    while len(pexpect_handle.read_nonblocking(_FLUSH_BUFFLEN)) == _FLUSH_BUFFLEN:
-        pass
+    # The read_nonblocking blocks when there is not at least one byte available for read (yeah you are reading it right
+    # the nonblocking read is blocking). The solution here is to use timeout with zero value. This raises timeout
+    # exception immediately once there is no input available.
+    while True:
+        try:
+            pexpect_handle.read_nonblocking(io.DEFAULT_BUFFER_SIZE, 0)
+        except pexpect.TIMEOUT:
+            return
 
 
 def run_exit_code_zero(exit_code):