[Tutor] Where does the program pointer go when...

[Dennis Lee Bieber]

On Tue, 28 Feb 2023 10:26:58 -0800, "Dave (NK7Z)" <dave at nk7z.net> declaimed
the following:

>I am trying to get my head around what is a timeout, and what happens 
>when one occurs.  I have a working script that does what I want, but I 
>am now working on trapping errors.  i.e. Ethernet dies, telnet server 
>shuts down, telnet server just stops sending data, telnet server starts 
>sending b'', etc.
>

	The library reference manual is a crucial resource.

>As a result of this effort, I find I have a failed understanding of what 
>happens when a TIMEOUT occurs.  I am fairly sure I don't even know WHAT 
>a TIMEOUT is...  See assumptions list later.
>

"""
Telnet.read_until(expected, timeout=None)

    Read until a given byte string, expected, is encountered or until
timeout seconds have passed.

    When no match is found, return whatever is available instead, possibly
empty bytes. Raise EOFError if the connection is closed and no cooked data
is available.
"""

	Note: "return whatever is available"... Often one buffers the partial
data and retries the operation, combining partials until the complete data
has been transferred or some more serious error occurs.

>
>Script above here initing all variables, etc...
>
>         try:
>             result = tn.read_until(b"\r\n", TIMEOUT)

...	which could mean you get stuff up to a <cr> but not the <lf>, or
anything else that might have been sent.

>             if result == b'':
>                 relogin()
>                 configureCluster()
>                 exitscript()

	Uhm... why "relogin" if you are just going to exit the program?

>         except socket.error:  # If socket error-- exit with error.
>             logdata = 'Socket Error in watchstream(), at main read loop.'
>             logit(logdata)
>             exitscript()
>         except EOFError:
>             failcode = 6
>             logdata = 'EOF at main read loop.'
>             logit(logdata)
>             exitscript()
>         except OSError:
>             logdata = 'OS Error in watchstream().'
>             logit(logdata)
>             exitscript()
>
>Rest of script...
>
>I have at least two assumptions I want to check here:
>
>1.  TIMEOUT is defined as when when the telnet server just stops 
>spending data, but the telnet connection still exists, and TIMEOUT is 
>exceeded.  i.e. the far side data collection died, but left the server 
>connected to my client.  Is this correct?

	No... The far side just doesn't have data to send... Imagine that the
far side is sending stuff being typed in by a user at a console (in
non-buffered mode). The user walks away to get a cup of coffee, and your
end times out with whatever the user last entered. Then they come back and,
maybe, enter a few more characters and hit <enter>.

>
>2.  I assume the run pointer, (where the next instruction is run), just 
>drops down to the "Rest of script..." and the run continues, if TIMEOUT 
>is reached.  Is this correct?
>

	On timeout, the next statement executed will be the "if result == b'':"
>
>Any help would be appreciated, I am new to Python...  I have looked all 

	This is not really Python specific -- you probably need to study the
BSD TCP socket protocol, and maybe telnet protocol(s)
https://www.rfc-editor.org/rfc/rfc854
https://www.rfc-editor.org/rfc/rfc5198

>over and have not located anything that tells me what a TIMEOUT is 
>exactly.  Is it if the telnet server drops, is it if the data flow 
>stops, but the telnet server is still connected, etc...

	For .read_until(), a closed connection raises EOFError. It may not be
immediately detected (at the socket layer, there may be a buffer containing
multiple lines of text and your .read_until() needs to empty that buffer
before an actual socket read is next attempted).


-- 
	Wulfraed                 Dennis Lee Bieber         AF6VN
	wlfraed at ix.netcom.com    http://wlfraed.microdiversity.freeddns.org/