TIP:            398
Title:          Quickly Exit with Non-Blocking Blocked Channels
Version:        $Revision: 1.7 $
Author:         Alexandre Ferrieux <alexandre.ferrieux@gmail.com>
State:          Final
Type:           Project
Vote:           Done
Created:        24-Feb-2012
Post-History:   
Keywords:       close,exit,flush,blocking,nonblocking
Tcl-Version:    8.6

~ Abstract

This TIP reverts an age-old documented behavior that is useless, and the ill
effects of which cannot be circumvented: Tcl's insistence on flushing even
non-blocking channels on exit.

~ Rationale

The close.n manpage says:

 > Channels are automatically closed when an interpreter is destroyed and when
   the process exits.  Channels are switched to blocking mode, to ensure that
   all output is correctly flushed before the process exits.

So, assuming the application has been using nonblocking IO all the time to
stay responsive even with blocked sockets (network failures) or pipes (stalled
consumer), if it decides to give up and exit, it ''cannot'' do so today: Tcl
will switch those channels back to blocking mode, and sit forever trying to
flush them, until a kind hand hits ^C.

No combination of '''close'''/'''fconfigure'''/'''exit''' will do, thanks to
the above clarification.

Note that the intent of this behavior is to automagically prevent careless
applications from truncating their output. The flaw of this logic is that such
careless applications typically don't use nonblocking channels. Hence the
generalization is abusive.

Hence, the proposal of this TIP is simply to revert this deleterious choice.
This way:

 * simple apps continue to autoflush their blocking channels,

 * smart nonblocking contraptions enjoy a timely exit, and

 * generic code can even guarantee quick death by switching all channels to
   nonblocking mode, like this:

|   proc instantdeath {{status 0}} {
|       foreach ch [chan names] {fconfigure $ch -blocking 0}
|       exit $status
|   }

~ Specification

Nonblocking channels shall no longer be switched to blocking mode
when the process calls '''exit''' or ''Tcl_Exit()''.  Any buffered data
for nonblocking channels will be discarded.

~ Notes

Blocking channels are unaffected by this TIP;  blocking channels shall
still be flushed and closed at '''exit'''-time

~ Related Bugs

Bug 2946474 noted the accidental, non-TIP-warranted suppression of this
strange behavior.  Fixing this bug simply meant to comply with Day One
documentation.  This TIP now judges the substance.

~ Reference Implementation

Branch tip-398-impl holds the code.

~ Copyright

This document has been placed in the public domain.