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

[Dave (NK7Z)]

Dennis,

Thanks you for the information, you answered my timeout-- where does it 
go, question.  Plus the references to the RFCs...  THANK YOU!

I see you have a  ham ticket, as I do, this script logs into an AR 
cluster, and watches the incoming data stream...  Parses it, and alerts 
me to calls I have tagged to look for...

Your answers have been very helpful, again, thank you sir!!!

73, and thanks,
Dave (NK7Z)
https://www.nk7z.net
ARRL Volunteer Examiner
ARRL Technical Specialist, RFI
ARRL Asst. Director, NW Division, Technical Resources

On 2/28/23 20:38, Dennis Lee Bieber wrote:
> 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).
> 
>