[RndTbl] Screen in production

Robert Keizer robert at keizer.ca
Wed Oct 10 11:05:59 CDT 2012

On 12-10-10 10:33 AM, Gilbert E. Detillieux wrote:
> On 2012-10-10 09:50, Robert Keizer wrote:
>> I have a production service that prints some debug to stdout. I could
>> make it print to stderr, but it really doesn't matter in this case.
> Are you sure it doesn't matter?  Read on...
>> I want to have some hope of if something goes wrong figuring out what 
>> did.
>> Log files aren't really an option so I ran it in screen. Yes yes I could
>> have made it network log to somewhere else, but again, not really an
>> viable option in this case. Works great as the program dies and the
>> output stops, leaving the last debug messages in the buffer of screen.
> One of the classic C/UNIX lessons about stdout vs stderr is that the C 
> library buffers stdout by default but not stderr, exactly for the 
> reason that you want error messages to be flushed out right away, 
> leaving nothing in the buffer.  This is particularly important if the 
> program dies in a way that prevents the usual exit processing, such as 
> flushing buffers and closing file I/O.
> So, issues with "screen" aside, you probably should either send your 
> debug output to stderr, or set your stdout to unbuffered mode while 
> debugging (or both).


Thought I'd test out stdout vs stderr just to see if this was in fact 
the case. It's not - stderr and stdout have the same effect in this case.

Below is a script the listens for incoming data on a given port ( 
example is 1337 ). When data is received it writes to either stdout or 
stderr and then sends data on to another port ( in this case 1338 ). 
This script also fills up the screen buffer by a simple loop ( tested 
filling with both stderr and stdout ).

Using telnet to feed the script data, and nc ( nc -lk 1338 ) to listen 
on port 1338 for the output, one can test to confirm.

net = require "net"
server = net.createServer ( c ) ->
         c.setEncoding "utf8"
         c.on "data", ( data ) ->
                 _c = net.createConnection 1338, ( ) ->
                         process.stderr.write "Data: " + data + "\n"
                         _c.write data
                 _c.end( )

server.listen 1337, ""
loops = ( ) ->
         process.stderr.write "Loopies: " + Date( ) + "\n"
         setTimeout loops, 100
loops( )

Screenshot is attached - it shows this script in the top left running in 
screen, with copy mode enabled. Right of it is the telnet session 
feeding it, and on the bottom left is the nc or "output". Note that the 
last two inputs did not make it to the output. Ergo, unless I'm totally 
an idiot in my implementation: This is blocking on stderr as well.

Thoughts? Because now I'm officially annoyed that this blocks on stderr 
as well!


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