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disable ldb at runtime in sbcl

If sbcl suffers a low-level problem and crashes, it normally enters a low-level debugger called "ldb" and waits for you to start troubleshooting. That's not so nice if you want the application to simply crash and restart automatically and unattended.

You can disable ldb by tweaking *FEATURES* at build time, but that's a little inconvenient. So here's a way to do it at runtime, inspired by a hint from David Lichteblau:

(funcall (sb-alien:define-alien-routine 
          ("disable_lossage_handler" cl-user::disable-sbcl-ldb) 

Thanks to Stas Boukarev for helping me test it.

update Pierre Mai suggested a better sb-alien technique:

    (sb-alien:extern-alien "disable_lossage_handler" (function sb-alien:void)))

He has more helpful info in his comment. And the original author of ldb in CMUCL also chipped in.



More direct version and alternatives

I think this is a good hint, and possibly more should be done to make disabling just the ldb easier out of the box. FWIW, I don't think the return value of sb-alien:define-alien-routine is actually specified, and for a one-off call d-a-r isn't actually needed or useful (creating a new throw-away function), so the shorter use of alien-funcall might be better, i.e.:
    (sb-alien:extern-alien "disable_lossage_handler" (function sb-alien:void)))
should do the same in a cleaner way. Of course as a library definition it might make more sense to do
  (sb-alien:define-alien-routine ("disable_lossage_handler" disable-sbcl-ldb)
  (sb-alien:define-alien-routine ("enable_lossage_handler" enable-sbcl-ldb)
which also makes enable-sbcl-ldb available, for re-enabling the ldb, if that is wanted. That said, you can get the same behaviour, plus a disabled lisp debugger by just calling the slightly underdocumented
which mirrors the behaviour of passing --disable-debugger to the sbcl runtime. Reenabling is via
which I think is even less documented.

Re: More direct version and alternatives

Thanks for the improved sb-alien code!

In my case, I only want the "disable ldb" aspect of --disable-debugger, so sb-ext:disable-debugger doesn't quite do what I need.