[ruby-core:116041] [Ruby master Bug#20155] Using value of rb_fiber_scheduler_current() crashes Ruby

Issue #20155 has been reported by paddor (Patrik Wenger).

----------------------------------------
Bug #20155: Using value of rb_fiber_scheduler_current() crashes Ruby
https://bugs.ruby-lang.org/issues/20155

* Author: paddor (Patrik Wenger)
* Status: Open
* Priority: Normal
* ruby -v: ruby 3.3.0 (2023-12-25 revision 5124f9ac75) [x86_64-linux]
* Backport: 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN
----------------------------------------
While trying to manually block/unblock fibers from an extension using the Fiber Scheduler,
I noticed that using the return value of `rb_fiber_scheduler_current()` crashes Ruby.

I've created a minimal extension gem called "fiber_blocker". Its test suite shows the behavior. See https://github.com/paddor/fiber_blocker, especially the lines containing `FIXME`.

Passing `Fiber.scheduler` to the extension functions works. But letting it get the current scheduler itself does not seem to work.

Is `rb_fiber_scheduler_current()` not the equivalent to `Fiber.scheduler`?
Even just printing the its return value with `#p` will crash Ruby.


Ruby either crashes like this:
```
# Running:

T1 BEGIN
T2 BEGIN
T1 END
..T1 BEGIN
ext: blocking fiber
passed scheduler = #<Scheduler:0x00007fc5f22d39e8 @readable={}, @writable={}, @waiting={}, @closed=false, @lock=#<Thread::Mutex:0x00007fc5f22ec8d0>, @blocking={}, @ready=[], @urgent=[#<IO:fd 5>, #<IO:fd 6>]>
T2 BEGIN
ext: unblocking fiber
T1 END
.E

Finished in 1.007014s, 3.9721 runs/s, 2.9791 assertions/s.

  1) Error:
TestFiberBlocker#test_fiber_blocker_current_fiber:
fatal: machine stack overflow in critical region
    No backtrace
```

Or with a segfault:
```
# Running:

FiberBlocker.test works.
.T1 BEGIN
T2 BEGIN
T1 END
.T1 BEGIN
ext: blocking fiber
/home/user/dev/oss/async_ruby_test/rbnng/fiber_blocker/test/test_fiber_blocker.rb:40: [BUG] Segmentation fault at 0x00000000390d8f98
ruby 3.3.0 (2023-12-25 revision 5124f9ac75) [x86_64-linux]

-- Control frame information -----------------------------------------------
c:0003 p:---- s:0012 e:000011 CFUNC  :block_fiber
c:0002 p:0014 s:0006 e:000005 BLOCK  /home/user/dev/oss/async_ruby_test/rbnng/fiber_blocker/test/test_fiber_blocker.rb:40 [FINISH]
c:0001 p:---- s:0003 e:000002 DUMMY  [FINISH]

-- Ruby level backtrace information ----------------------------------------
/home/user/dev/oss/async_ruby_test/rbnng/fiber_blocker/test/test_fiber_blocker.rb:40:in `block in test_fiber_blocking_in_ext'
/home/user/dev/oss/async_ruby_test/rbnng/fiber_blocker/test/test_fiber_blocker.rb:40:in `block_fiber'

-- Threading information ---------------------------------------------------
Total ractor count: 1
Ruby thread count for this ractor: 4

-- Machine register context ------------------------------------------------
 RIP: 0x00007f1554f17ad8 RBP: 0x00000000390d8f90 RSP: 0x00007f153a79e280
 RAX: 0x00007f1554addba8 RBX: 0x00007f153a79eab0 RCX: 0x0000000000000000
 RDX: 0x00007f1554ade600 RDI: 0x00007f15551e8788 RSI: 0x0000000000000ae1
  R8: 0x000000000000002b  R9: 0x00007f153a79f038 R10: 0x00007f1554c0b9b0
 R11: 0x00007f153a79e490 R12: 0x0000000000000ae1 R13: 0x0000000000000000
 R14: 0x0000000000000000 R15: 0x000055ab732d7df0 EFL: 0x0000000000010206

-- C level backtrace information -------------------------------------------
/home/user/.rubies/ruby-3.3.0/lib/libruby.so.3.3(rb_print_backtrace+0x14) [0x7f1554f24961] /home/user/src/ruby-3.3.0/vm_dump.c:820
/home/user/.rubies/ruby-3.3.0/lib/libruby.so.3.3(rb_vm_bugreport) /home/user/src/ruby-3.3.0/vm_dump.c:1151
/home/user/.rubies/ruby-3.3.0/lib/libruby.so.3.3(rb_bug_for_fatal_signal+0x104) [0x7f1554d1c214] /home/user/src/ruby-3.3.0/error.c:1065
/home/user/.rubies/ruby-3.3.0/lib/libruby.so.3.3(sigsegv+0x4f) [0x7f1554e700df] /home/user/src/ruby-3.3.0/signal.c:926
/lib/x86_64-linux-gnu/libc.so.6(0x7f1554842520) [0x7f1554842520]
/home/user/.rubies/ruby-3.3.0/lib/libruby.so.3.3(RBASIC_CLASS+0x0) [0x7f1554f17ad8] ./include/ruby/internal/globals.h:178
/home/user/.rubies/ruby-3.3.0/lib/libruby.so.3.3(gccct_method_search) /home/user/src/ruby-3.3.0/vm_eval.c:475
/home/user/.rubies/ruby-3.3.0/lib/libruby.so.3.3(rb_funcallv_scope) /home/user/src/ruby-3.3.0/vm_eval.c:1063
/home/user/.rubies/ruby-3.3.0/lib/libruby.so.3.3(rb_funcallv) /home/user/src/ruby-3.3.0/vm_eval.c:1084
/home/user/.rubies/ruby-3.3.0/lib/libruby.so.3.3(rb_inspect+0x19) [0x7f1554dc1569] /home/user/src/ruby-3.3.0/object.c:697
/home/user/.rubies/ruby-3.3.0/lib/libruby.so.3.3(ruby__sfvextra+0x11a) [0x7f1554e7223a] /home/user/src/ruby-3.3.0/sprintf.c:1119
/home/user/.rubies/ruby-3.3.0/lib/libruby.so.3.3(BSD_vfprintf+0xa69) [0x7f1554e73059] /home/user/src/ruby-3.3.0/vsnprintf.c:830
/home/user/.rubies/ruby-3.3.0/lib/libruby.so.3.3(RBASIC_SET_CLASS_RAW+0x0) [0x7f1554e75b56] /home/user/src/ruby-3.3.0/sprintf.c:1168
/home/user/.rubies/ruby-3.3.0/lib/libruby.so.3.3(ruby_vsprintf0) /home/user/src/ruby-3.3.0/sprintf.c:1169
/home/user/.rubies/ruby-3.3.0/lib/libruby.so.3.3(rb_enc_vsprintf+0x5d) [0x7f1554e75ecd] /home/user/src/ruby-3.3.0/sprintf.c:1195
/home/user/.rubies/ruby-3.3.0/lib/libruby.so.3.3(rb_sprintf+0x9d) [0x7f1554e7607d] /home/user/src/ruby-3.3.0/sprintf.c:1225
/home/user/dev/oss/async_ruby_test/rbnng/fiber_blocker/lib/fiber_blocker/fiber_blocker.so(block_fiber+0x4a) [0x7f1554ad430a] ../../../../ext/fiber_blocker/fiber_blocker.c:29
/home/user/.rubies/ruby-3.3.0/lib/libruby.so.3.3(vm_cfp_consistent_p+0x0) [0x7f1554ef64b4] /home/user/src/ruby-3.3.0/vm_insnhelper.c:3490
/home/user/.rubies/ruby-3.3.0/lib/libruby.so.3.3(vm_call_cfunc_with_frame_) /home/user/src/ruby-3.3.0/vm_insnhelper.c:3492
/home/user/.rubies/ruby-3.3.0/lib/libruby.so.3.3(vm_call_cfunc_with_frame) /home/user/src/ruby-3.3.0/vm_insnhelper.c:3518
/home/user/.rubies/ruby-3.3.0/lib/libruby.so.3.3(vm_call_cfunc_other) /home/user/src/ruby-3.3.0/vm_insnhelper.c:3544
/home/user/.rubies/ruby-3.3.0/lib/libruby.so.3.3(vm_sendish+0x9e) [0x7f1554f06f87] /home/user/src/ruby-3.3.0/vm_insnhelper.c:5581
/home/user/.rubies/ruby-3.3.0/lib/libruby.so.3.3(vm_exec_core) /home/user/src/ruby-3.3.0/insns.def:834
/home/user/.rubies/ruby-3.3.0/lib/libruby.so.3.3(rb_vm_exec+0x19a) [0x7f1554f0d1fa] /home/user/src/ruby-3.3.0/vm.c:2486
/home/user/.rubies/ruby-3.3.0/lib/libruby.so.3.3(rb_vm_invoke_proc+0x5f) [0x7f1554f12e0f] /home/user/src/ruby-3.3.0/vm.c:1728
/home/user/.rubies/ruby-3.3.0/lib/libruby.so.3.3(rb_fiber_start+0x1ba) [0x7f1554cf098a] /home/user/src/ruby-3.3.0/cont.c:2536
/home/user/.rubies/ruby-3.3.0/lib/libruby.so.3.3(fiber_entry+0x20) [0x7f1554cf0d00] /home/user/src/ruby-3.3.0/cont.c:847
/home/user/.rubies/ruby-3.3.0/lib/libruby.so.3.3(rb_threadptr_root_fiber_setup) (null):0
```

I hope I'm not missing something obvious. My C isn't very good.



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