[TCLCORE] TIP #361: Releasing Channel Buffers

[Wayne C. <wc...@us...>]

 TIP #361: RELEASING CHANNEL BUFFERS 
=====================================
 Version:      $Revision: 1.1 $
 Author:       Wayne Cuddy <wcuddy_at_useunix.net>
 State:        Draft
 Type:         Project
 Tcl-Version:  8.7
 Vote:         Pending
 Created:      Wednesday, 03 February 2010
 URL:          http://purl.org/tcl/tip/361.html
 WebEdit:      http://purl.org/tcl/tip/edit/361
 Post-History: 

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 ABSTRACT 
==========

 Tcl should provide a mechanism by which a channel's output buffer can 
 be released without requiring that Tcl flush any remaining data in the 
 buffer to the operating system. This is of particular interest with 
 output mechanisms which can block indefinitely causing the interpreter 
 to consume unnecessary resources or prevent the interpreter from 
 exiting. 

 PROBLEM 
=========

 When working with processes that handle multiplexing/non-blocking I/O 
 it is not uncommon to write, or call *puts*, with more data than the 
 operating system will accept. Thus Tcl begins to buffer this data using 
 internal buffers at the application level and flushes the data in the 
 background. Problems arise when the consumer of this data, be it the 
 other end of a socket, pipe or FIFO, refuse to read data and do not 
 close the channel. 

 This has adverse effects on the *close* function and consequently the 
 interpreter/process when it attempts to flush and close channels during 
 finalization. 

 CONSEQUENCES 
--------------

    1. When *close* is issued on a non-blocking channel Tcl removes the 
       channel from pool of accessible channels from the current 
       interpreter and attempts to flush any remaining data in the 
       output buffers before closing the channel. Since the other end of 
       the channel is not consuming data Tcl will never be able to flush 
       it's internal buffers nor close the lower level channel driver. 
       In a multiplexing server which opens new channels this will 
       eventually result in resource starvation or denial of service. 

    2. When *exit* is called Tcl attempts to flush all output buffers, 
       if any open channel blocks the interpreter will be unable exit 
       and must be forcibly terminated by OS specific mechanisms. 

 PROPOSE API ADDITIONS 
=======================

 Provide a function and/or flags to existing functions which can be used 
 to clear internal buffers without flushing. 

     * *chan clear* /channelId/ 

     * *close -noflush* /channelId/ 

 COPYRIGHT 
===========

 This document has been placed in the public domain. 

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