#3169 reverse trace order conflicts trace delete safety

[Don Porter]

The Active*Trace mechanism for
protecting against trace deletion
during trace execution is based on
a hard and fast assumption that
the trace lists are traversed in
a single direction.

However, "leave" and "enterstep"
execution traces attempt to
fire in the opposite direction.

This leaves us open to a disaster like:

% proc foo {} {}
% proc x args {trace remove execution foo leave y}
% trace add execution foo leave x
% trace add execution foo leave y
% foo

Program received signal EXC_BAD_ACCESS, Could not
access memory.
0x0007027c in TclCheckExecutionTraces
(interp=0x1803200, command=0x60f5d0 "foo\n",
numChars=3, cmdPtr=0x6054d0, code=0, traceFlags=2,
objc=1, objv=0x1803404) at ../generic/tclCmdMZ.c:4206
4206 while (tracePtr->nextPtr !=
lastTracePtr) {
(gdb) bt
#0 0x0007027c in TclCheckExecutionTraces
(interp=0x1803200, command=0x60f5d0 "foo\n",
numChars=3, cmdPtr=0x6054d0, code=0, traceFlags=2,
objc=1, objv=0x1803404) at ../generic/tclCmdMZ.c:4206
#1 0x0002468c in TclEvalObjvInternal
(interp=0x1803200, objc=1, objv=0x1803404,
command=0x60f5d0 "foo\n", length=3, flags=0) at
../generic/tclBasic.c:3101
#2 0x0007346c in TclExecuteByteCode (interp=0x1803200,
codePtr=0x60f620) at ../generic/tclExecute.c:1417
#3 0x000722d8 in TclCompEvalObj (interp=0x1803200,
objPtr=0x1801138) at ../generic/tclExecute.c:981
#4 0x00025e50 in Tcl_EvalObjEx (interp=0x1803200,
objPtr=0x1801138, flags=131072) at
../generic/tclBasic.c:4051
#5 0x0004942c in Tcl_RecordAndEvalObj
(interp=0x1803200, cmdPtr=0x1801138, flags=131072) at
../generic/tclHistory.c:142
#6 0x00005cac in Tcl_Main (argc=1, argv=0xbffffd60,
appInitProc=0x39c0 <Tcl_AppInit>) at
../generic/tclMain.c:390
#7 0x000039a4 in main (argc=1, argv=0xbffffd60) at
../unix/tclAppInit.c:90

It appears the reverse direction
implementation never even considered
the possibility of the trace list changing
out from under it.

[Don Porter]

[Don Porter]

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attached patch makes the
Active*Trace mechanism
aware of the scan direction.

[Don Porter]

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patch committed for 8.4.11

report open for port to HEAD

[Don Porter]

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fix ported to HEAD