Tcl Source Code

bschofield reports on clt that the script 'crash'
(attached) crashes tcl8.4.9. I verified the crash in
both HEAD and 8.4-branch.

The immediate cause of the crash is that tracePtr is
NULL on line 1492 in TclCheckInterpTraces. The stack
trace is

#0  0x0808cdcf in TclCheckInterpTraces
(interp=0x81432c8, command=0x8183bd5 "set x 1\n",
numChars=7,
    cmdPtr=0x81484f0, code=0, traceFlags=1, objc=3,
objv=0x8144260) at ../generic/tclTrace.c:1492
#1  0x080b9d9b in TclEvalObjvInternal
(interp=0x81432c8, objc=3, objv=0x8144260,
    command=0x8183bd5 "set x 1\n", length=7, flags=0)
at ../generic/tclBasic.c:3339
#2  0x080f7001 in TclExecuteByteCode (interp=0x81432c8,
codePtr=0x8185d48) at ../generic/tclExecute.c:1721
#3  0x080f5758 in TclCompEvalObj (interp=0x81432c8,
objPtr=0x8167440) at ../generic/tclExecute.c:1030
#4  0x08081ee9 in TclObjInterpProc
(clientData=0x8161ff8, interp=0x81432c8, objc=1,
objv=0xbfffee50)
    at ../generic/tclProc.c:1328

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fix ported to HEAD

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patch committed for 8.4.11

report still open to deal with
it in the HEAD.

File Added - 139168: 1201035.patch

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Attached patch (for 8.4.11) appears to
stop the crashes.  Please test.

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The deletion of "interp" traces
lacks the walk of the activeTrace
list that's done for command and
variable traces.  That's at least related,
and possibly the cause of this.

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Note that adding a trace
during an enterstep or
leavestep callback can't
impace the current command
evaluation, but (inconsistently?)
removing a trace takes effect
immediately:

% proc foo {} {
    lappend ::x foo
    lappend ::x foo
}
% proc bar args {   
    lappend ::x bar
trace remove execution foo enterstep bar
trace add execution foo enterstep bar
}
% trace add execution foo enterstep bar
% foo
bar foo foo

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Simpler script that make same crash:

proc x {} {
    puts ACTION
}
proc es {args} {
    puts $args
    trc remove
    trc add
    puts done
}
proc trc {m} {
    trace $m execution ::x leavestep es
    trace $m execution ::x enterstep es
}
trc add
x

Reverse the two commands in the [trc] body,
and there's no crash, but the trace result
is incorrect.

File Added - 134265: crash