Restore fixupBoehmStackPointer

[edolstra]

Motivation

This was removed in #11152. However, we need it for the multi-threaded evaluator, because otherwise Boehm GC will crash while scanning the thread stack:

#0  GC_push_all_eager (bottom=<optimized out>, top=<optimized out>) at extra/../mark.c:1488
#1  0x00007ffff74691d5 in GC_push_all_stack_sections (lo=<optimized out>, hi=<optimized out>, traced_stack_sect=0x0) at extra/../mark_rts.c:704
#2  GC_push_all_stacks () at extra/../pthread_stop_world.c:876
#3  GC_default_push_other_roots () at extra/../os_dep.c:2893
#4  0x00007ffff746235c in GC_mark_some (cold_gc_frame=0x7ffee8ecaa50 "`\304G\367\377\177") at extra/../mark.c:374
#5  0x00007ffff7465a8d in GC_stopped_mark (stop_func=stop_func@entry=0x7ffff7453c80 <GC_never_stop_func>) at extra/../alloc.c:875
#6  0x00007ffff7466724 in GC_try_to_collect_inner (stop_func=0x7ffff7453c80 <GC_never_stop_func>) at extra/../alloc.c:624
#7  0x00007ffff7466a22 in GC_collect_or_expand (needed_blocks=needed_blocks@entry=1, ignore_off_page=ignore_off_page@entry=0, retry=retry@entry=0) at extra/../alloc.c:1688
#8  0x00007ffff746878f in GC_allocobj (gran=<optimized out>, kind=<optimized out>) at extra/../alloc.c:1798
#9  GC_generic_malloc_inner (lb=<optimized out>, k=k@entry=1) at extra/../malloc.c:193
#10 0x00007ffff746cd40 in GC_generic_malloc_many (lb=<optimized out>, k=<optimized out>, result=<optimized out>) at extra/../mallocx.c:477
#11 0x00007ffff746cf35 in GC_malloc_kind (bytes=120, kind=1) at extra/../thread_local_alloc.c:187
#12 0x00007ffff796ede5 in nix::allocBytes (n=<optimized out>, n=<optimized out>) at ../src/libexpr/include/nix/expr/eval-inline.hh:19

This is because it will use the stack pointer of the coroutine, so it will scan a region of memory that doesn't exist, e.g.

Stack for thread 0x7ffea4ff96c0 is [0x7ffe80197af0w,0x7ffea4ffa000)

(where 0x7ffe80197af0 is the sp of the coroutine and 0x7ffea4ffa000 is the base of the thread stack).

We don't scan coroutine stacks, because currently they don't have GC roots (there is no evaluation happening in coroutines). So there is currently no need to restore the other parts of the original patch, such as BoehmGCStackAllocator.

I'm not sure if this is an issue for the single-threaded evaluator, at least I haven't seen these crashes there. That might be luck (a GC is unlikely to be triggered if a coroutine is running, since they don't do any GC allocations) or it might be that the main thread stack is treated differently. Still doesn't hurt to be safe.

Cherry-picked from DeterminateSystems#125.

Context

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[Mic92]

• Revert "Merge pull request #13709 from NixOS/boehm-coroutines-sp" #13731