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#1576 traces on array misbehaving
This shouldn't core-dump!
% info patch
8.4a4
% set x(bar) 0
0
% trace variable x r {set x(foo) 1 ;#}
% trace variable x r {unset -nocomplain x(bar) ;#}
% trace vinfo x
{r {unset -nocomplain x(bar) ;#}} {r {set x(foo) 1 ;#}}
% array get
x
": no such element in array
% array get x
Segmentation fault (core dumped)
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Bug is also present in 8.3.3 and 8.0.5.
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Note, a very similar script causes a segfault
in Tcl 8.3.3. (Change [unset -nocomplain ...]
to [catch {unset ... )
patch to tclVar.c and trace.test
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It is difficult to define what the correct behaviour of
[array get]
should be when there are read traces that modify the array
structure.
In discussions on the Tcl'ers Chat with Kevin Kenny and
Don Porter we agreed that a (the only ?) reasonable
behaviour
is given by
proc array-get { aName } {
set result [list]
upvar 1 $aName array
foreach s [array names array] {
if { [info exists array($s)] } {
# because a trace could have deleted it
lappend result $s $array($s)
}
}
return $result
}
The enclosed patch implements this behaviour and adds three
tests
for it. These tests cause illegal memory behaviour without
the patch
to tclVar.c - an access to a previously disposed Tcl_Obj is
detected
by TCL_MEM_DEBUG.
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Patch committed to HEAD.
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the patch included no doc changes. Is there a
best place to document the particular behavior?
In the [array get] docs, perhaps?
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Re docs: or with [trace]? Or both?
It is a really something that is relevant to the interplay
between [array get] and [trace] - when there are read traces
on an array that either add or remove an element of the
array.
I'm also not 100% happy with the error message when the
trace unsets the whole array (see test trace-1.13 ): should
it rather return {} and no error as does [array get
nonExistingArray] ?
Reopening the ticket, with lower priority now that the
segfault is gone ...
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Docs added to array.n