Tcl Source Code

<h1>Issue</h1>
The trailing close in the following sequence throws an error but no error message:
<verbatim>
% set sock [socket -async localhost 30001]
sock420
% fconfigure $sock -blocking 0
% puts $sock ok
% flush $sock
% fileevent $sock writable {set x 1}
% vwait x
% catch {close $sock} e d
1
% set e
% set d
-code 1 -level 0 -errorstack {INNER {invokeStk1 close sock420}} -errorcode NONE
-errorinfo {
    while executing
"close $sock"} -errorline 1
</verbatim>

This arises on the following platforms:
   *   TCL 8.5.15 on Windows Vista 32 bit 
   *   TCL 8.6.1 on Windows Vista 32 bit
   *   TCL 8.6.1 on Linux too (Reported bei Reinhard Max)

<h1>Possible reasons</h1>

The error is generated by a backround flush sheduled by the writable event.
The error code is stored in unreportedError.
Eventually, the error message is missing in unreportedMsg.

A debug trace on tclIO.c:

<h2>flush $sock</h2>
   *   tclIO.c:FlushChannel()
   *   calls tclWinSock.c:TclOutputProc("ok"), which returns error EWOULDBLOCK
   *   Sets flag BG_FLUSH_SCHEDULED
   *   Clears error

<h2>vwait x</h2>
   *   tclIO.c:FlushChannel(calledFromAsyncFlush=1)
   *   calls tclWinSock.c:TclOutputProc("ok"), which tries to write and returns error 126
   *   Sets statePtr->unreportedError = errorCode (126)
   *   statePtr->unreportedMsg = msg (NULL)
   *   Calls DiscardOutputQueued(statePtr)
   *   Flag BG_FLUSH_SCHEDULED is cleared (no data)

<h2>close $sock</h2>
   *   tclIO.c:Tcl_Close()
   *   > flushcode = FlushChannel(interp, chanPtr, 0); -> flushcode = 126
   *   >> errorCode = CloseChannel(interp, chanPtr, errorCode); -> errorCode = 126
   *   >>> result = ChanClose(chanPtr, interp); -> result = 0
   *   >>>> tclWinSock.c:TclCloseProc(), which succeeds
   *   >>> errorCode = statePtr->unreportedError; (126)
   *   >>> Tcl_SetChannelErrorInterp(interp, statePtr->unreportedMsg); (no action, unreportedMsg = NULL)
   *   >>> CloseChannel: return errorCode (126)
   *   >> FLushChannel: return errorCode (126)
   *   > if ((flushcode != 0) || (result != 0)) { return TCL_ERROR; }

close returns empty error message if failed non-blocking write flush pending