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

[Dave (NK7Z)]

Hello Alan,

Thank you for the very complete reply!  I think I now have abetter 
understanding, and the comment about telnet being deprecated answers 
most if not all of my questions...  I need to switch away from telnet, 
and move to something else...  Can't use SSH, as the servers I am 
interacting with do not use SSH...  Any suggestions on what lib I should 
move to in that case?

I am doing this as a learning experience so this is not a truly large 
issue for me...  I will have to learn more about another library...

Again, thank  you for your very nicely constructed help here!!!

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 16:44, Alan Gauld via Tutor wrote:
> On 28/02/2023 18:26, Dave (NK7Z) wrote:
> 
>> I am trying to get my head around what is a timeout, and what happens
>> when one occurs.
> 
> I'll answer this in general terms because  there is no single
> definition of how timeouts work or are used.
> 
> Basically a timeout just means that an operation is time limited.
> It will start and if it does not complete within the specified
> time it either returns empty handed or raises an error or
> causes some specified handler function to execute.
> 
> There are multiple ways to implement timeouts from software
> to hardare and from simple clock lookups to multi-threaded
> interrupts. So it is impossible to give a single answer to how they
> work. The simplest form is a simple loop and timecheck:
> 
> In spseudo code:
> 
> def timed_op(timeout):
>      start = gettime()
>      while True:
>         time = gettime()
>         if time < start+timeout:
>            result = do_operation()
>            if resultOK(result):
>               return result
>         else: raise TimeoutError
> 
> 
> So we start a timer and then enter an infinite loop.
> Every time round we see if the timer has exceeded
> the timeout period. If not we carry out the desired
> operation. If the operation succeeds (according to
> some arbitrary definition of success!) we return
> the result. Otherwise we go round the loop again.
> If the timeout is exceeded we rraise a timeout error
> and the caller must decide what to do  next.
> 
> But both OS and hardware usually have special
> mechanisms for interrupting a running process
> after a given time, so a process can invoke
> that feature too. This tends to be the case
> with servers and long running processes or tasks
> which use the network (or other fallible resources).
> In that case the OS will step in and stop the process
> and call an interrupt handler function (or signal).
> That function will probably just return an error code
> to the caller or similar.
> 
> 
>> 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.
> 
> For a specific scenario you have to dig deeper into
> how the library in question expects you to proceed.
> It may be that you provide a default value, raise an
> error (or catch an error from the library!)  or you
> may provide the library with an error handlr function
> that the library calls in the event of timeout. It
> all depends on the library. Good documentation/tutorials
> are invaluable here.
> 
>> The snippit below is designed to read one line from a telnet stream that
>> never ends, looking for a \n, as the end of a line, while looking to
>> trap errors, and leave me a message as to what failed via
>> logit(logdata).
> 
> 
>>           try:
>>               result = tn.read_until(b"\r\n", TIMEOUT)
>>               if result == b'':
>>                   relogin()
>>                   configureCluster()
>>                   exitscript()
>>           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 idea. But the telnet module docs say:
> =========
> 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.
> ==========
> 
> So it looks like in this case the timeout causes the function to return
> as much data as it has collected so far, regardless of wherther it found
> your expected terminator.
> 
> Note it also says telnetlib is deprecated and will be removed in 3.13.
> You should probably move to another library - maybe ssh based?
> 
>> 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?
> 
> It depends on how the timeout is built.
> But in your specific case, yes. You get a return
> from the function as normal. The TIMEOUT is purely to prevent
> read_until() locking up and never returning.
> 
>> I am trying to trap all errors, and recover cleanly from them while not
>> hammering the far side telnet server with login attempts...
> 
> Good, but unrelated to timeouts! :-)
> 
>> Any help would be appreciated, I am new to Python...  I have looked all
>> 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...
> 
> Any and all of the above.
> 
> There is a brief wikipedia article on "timeout(computing)" that might
> help. It provides some real-world examples of differet types of
> timeout you might encounter.
>