[TCLCORE] Help about writable fileevent on connect error and non-blocking flush

[Harald O. <har...@el...>]

Dear TCL'ers,

I am sorry to ask for help.
There is something I am stuck with.

What happens:

- take current bug-13d3af3ad5 branch
https://core.tcl.tk/tcl/info/cb1a6a0924
on windows (Vista for me)

- start tclsh and do the following, while there is no server on port
9999, socket should report "connection refused":

set s [socket -async localhost 9999]
fconfigure $s -blocking 0
puts $s ok
fileevent $s writable { set x 1; puts "w" }
vwait x
close $s

This works well. Now insert a flush:

set s [socket -async localhost 9999]
fconfigure $s -blocking 0
puts $s ok
flush $s
fileevent $s writable { set x 1; puts "w" }
vwait x
close $s

The fileevent is never delivered and thus the program stalls

What happens inside:
- when the socket connect code knows that the connect will fail, it
invokes (Line 1409 of tclWinSock.c):
	Tcl_NotifyChannel(infoPtr->channel, TCL_WRITABLE);

This is implemented in tclIO.c and comes to the line 8039:

    /*
     * If we are flushing in the background, be sure to call
FlushChannel for
     * writable events. Note that we have to discard the writable event
so we
     * don't call any write handlers before the flush is complete.
     */

    if (GotFlag(statePtr, BG_FLUSH_SCHEDULED) && (mask & TCL_WRITABLE)) {
	FlushChannel(NULL, chanPtr, 1);
	mask &= ~TCL_WRITABLE;
    }

Well, it happens exactly what is described. The writable event is not
executed due to a pending flush.
The flush will never succeed as this is a callback to report an error.

How is this supposed to work ?

Thank you all for any light in the dark.

Harald