Merg [3148192] implementation fix, and caveat comment

1 files changed, 10 insertions, 0 deletions

diff --git a/generic/tclTrace.c b/generic/tclTrace.c
index a60a80b..13359ee 100644
--- a/generic/tclTrace.c
+++ b/generic/tclTrace.c
@@ -2883,6 +2883,16 @@ Tcl_UntraceVar2(
      * The code below makes it possible to delete traces while traces are
      * active: it makes sure that the deleted trace won't be processed by
      * TclCallVarTraces.
+     *
+     * Caveat (Bug 3062331): When an unset trace handler on a variable
+     * tries to delete a different unset trace handler on the same variable,
+     * the results may be surprising.  When variable unset traces fire, the
+     * traced variable is already gone.  So the TclLookupVar() call above
+     * will not find that variable, and not finding it will never reach here
+     * to perform the deletion.  This means callers of Tcl_UntraceVar*()
+     * attempting to delete unset traces from within the handler of another
+     * unset trace have to account for the possibility that their call to
+     * Tcl_UntraceVar*() is a no-op.
      */
 
     for (activePtr = iPtr->activeVarTracePtr;  activePtr != NULL;