Re: using trap function to cleanup and exit?

[Bart Schaefer <schaefer@xxxxxxxxxxxxxxxx>]

On Thu, Apr 14, 2022 at 2:29 PM Bart Schaefer <schaefer@xxxxxxxxxxxxxxxx> wrote:
>
> [I] would prefer that another set of eyes review
> the code change.

To expand on that a bit ...

The BIN_RETURN branch checks that (trap_return == -2) which is a
pretty specific number.  exec.c says:
 * This is only active if we are inside a trap, else its value
 * is irrelevant.  It is initialised to -1 for a function trap and
 * -2 for a non-function trap and if negative is decremented as
 * we go deeper into functions and incremented as we come back up.
 * The value is used to decide if an explicit "return" should cause
 * a return from the caller of the trap; it does this by setting
 * trap_return to a status (i.e. a non-negative value).

My interpretation is that, since we are in an explicit "exit" rather
than an explicit "return", we don't really care how trap_return is
set; we're going to force the caller to return, period.

However, the test for whether to increment trap_return again on the
way out is dependent on trap_state == TRAP_STATE_PRIMED, so if we
change trap_state we're never going to increment (or decrement,
actually) trap_return again.  (It may, however, get restored from a
stacked value.)  I can imagine there being some side-effect of this
from using a mix of "return" and "exit" from traps or from functions
that might be called by traps, but have no good idea how to exercise
such a code path.

I guess I'll go ahead and push this and if someone spots a problem we
can do a revision.