Commits on Jun 12, 2022

1. Disable Mach exception handlers when read barriers in place …

   The GC compaction mechanism implements a kind of read barrier by marking
   some (OS) pages as unreadable, and installing a SIGBUS/SIGSEGV handler
   to detect when they're accessed and invalidate an attempt to move the
   object.
   
   Unfortunately, when a debugger is attached to the Ruby interpreter on
   Mac OS, the debugger will trap the EXC_BAD_ACCES mach exception before
   the runtime can transform that into a SIGBUS signal and dispatch it.
   Thus, execution gets stuck; any attempt to continue from the debugger
   re-executes the line that caused the exception and no forward progress
   can be made.
   
   This makes it impossible to debug either the Ruby interpreter or a C
   extension whilst compaction is in use.
   
   To fix this, we disable the EXC_BAD_ACCESS handler when installing the
   SIGBUS/SIGSEGV handlers, and re-enable them once the compaction is done.
   The debugger will still trap on the attempt to read the bad page, but it
   will be trapping the SIGBUS signal, rather than the EXC_BAD_ACCESS mach
   exception. It's possible to continue from this in the debugger, which
   invokes the signal handler and allows forward progress to be made.

   

   KJTsanaktsidis committed Jun 12, 2022
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