Don't signal during backtrace unrewind (Bug#40088)
backtrace_eval_unrewind is used to temporarily reverse let-bindings (it's called with a positive argument to reverse bindings, and then a negative argument to re-apply them) by backtrace--locals and backtrace-eval. For the SPECPDL_LET_DEFAULT and SPECPDL_LET_LOCAL cases (which occur for let-bindings on buffer-local variables), the code calls Fdefault_value and Fbuffer_local_value on the symbol. For symbols which are unbound at top-level, the first (with positive argument) call to backtrace_eval_unrewind will set the symbol's value to unbound (putting the current value in the specpdl's "old value" slot). On the second (with negative argument) call, backtrace_eval_unrewind attempts to retrieve the symbol's value with Fdefault_value or Fbuffer_local_value, but that raises a void-variable signal. This interrupts the restoration of the let-bindings, so any other variables more recent on the stack will now have the wrong value. * src/data.c (default_value): Make non-static. * src/lisp.h: Declare it. * src/eval.c (backtrace_eval_unrewind): Replace the calls to Fdefault_value and Fbuffer_local_value with default_value and buffer_local_value, respectively. The latter do exactly the same as the former, except if the symbol's value is Qunbound they just return it instead of signaling void-variable.
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