Fix Huawei special_login_handler (#2711)

[fharbe]

This is a proposal to fix the special_login_handler() issue (#2711).

TL;DR: I removed the special_login_handler() completely and moved the handling for the password change prompt into session_preparation(), because I did not see a better way of fixing the issues altogether (read the whole story below).

After spending some time investigating the issue I found out that the pattern matching logic in read_until_pattern expects the pattern to split exactly in 3 parts (using re.split), see base_connection.py#L579 for reference. Seems this change was introduced during some fundamental changes to the channel reading logic in in 793100d.

At first I tried to simply fix the password_change_prompt regex (see huawei.py#L104) by replacing the inner groups (parenthesis) with (?:) like so:

        password_change_prompt = r"((?:Change now|Please choose))|(?:[\]>]\s*$)"

However, when looking more closely at the code, I stumbled upon the if re.search(password_change_prompt, output): condition, which seemed wrong (would always evaluate to True). So I modified the if-condition to actually search for the password_change_prompt only, like so:

password_change_prompt = r"(?:Change now|Please choose).+"
combined_pattern = r"({}|[>\]])".format(password_change_prompt)
output = self.read_until_pattern(combined_pattern, read_timeout=3.0)
if re.search(password_change_prompt, output):
    self.write_channel("N\n")

At first it seemed to work, but it turns out that connecting to the device now only works if a password change prompt is present. When no password change prompt is found, the following exception is raised:

netmiko.exceptions.ReadTimeout: 
Pattern not detected: '[>\\]]' in output.

Huh? Inspecting the debug logs, it turns out that self.write_channel("N\n") (which was always called before changing the if-condition) was responsible for the following debug logs:

DEBUG:netmiko:read_channel: N
DEBUG:netmiko:read_channel: 
        ^
Error: Unrecognized command found at '^' position.
<AC6508>

This means, the self.write_channel("N\n") did not only produce an error, but also a new prompt!

To me, it looks like it was only this new prompt (which was triggered by the "Unrecognized command" error) that actually made _test_channel_read in session_preparation work at all.

Therefore, I think it would only be possible to keep the handling logic for the password change prompt in special_login_handler if either _test_channel_read in session_preparation would be removed completely or self.write_channel(self.RETURN) would be needed to to trigger a new prompt for _test_channel_read to read it. Both of which are not really elegant IMHO. Maybe it would also be possible to read until the prompt by using a lookahead assertion, but I don't know if that's possible with the way read_until_pattern is currently implemented.

Let me know what you think.

Kind regards
Flo

[fharbe]

@ktbyers Fixed the failing tests 😄 Did you already have time to look into my proposed fix? 🙂

[nivaldoinacios]

work here

[ktbyers]

Superseded by:

#2728