* src/thread.c (lisp_mutex_init) [MPS]: Allocate condition variable.
(lisp_mutex_lock_for_thread):
(lisp_mutex_unlock):
(lisp_mutex_unlock_for_wait): Adjust to pointer type.
(lisp_mutex_destroy) [MPS]: Also free condition variable.
(Fmake_condition_variable) [MPS]: Allocate condition variable.
(condition_wait_callback):
(condition_notify_callback): Adjust to pointer type.
(finalize_one_condvar) [MPS]: Also free condition variable.
* src/thread.h (lisp_mutex_t, struct Lisp_CondVar): Make 'condition'
and 'cond' a pointer or single-element array.

On GNU/Linux, glibc expects condvars and mutexes to be in memory which
is always accessible.

* src/thread.c (main_thread_condvar): New variable.
(main_thread): Add reference to it.
(run_thread): Adjust.
(finalize_one_thread) [MPS]: Free 'thread_condvar'.
(Fmake_thread) [MPS]: Allocate 'thread_condvar'.
(thread_join_callback):
(init_threads): Adjust.
* src/thread.h (struct thread_state): Turn 'thread_condvar' into a
pointer or single-element array.
* test/src/igc-tests.el (igc-test--list-length): New variable.
(igc-test--trigger-incremental-gc): New function.
(igc-test-thread-incremental-gc): New test.

This struct cannot live in MPS-managed memory because it's required
for scanning the bytecode stack.

* src/bytecode.c (free_bc_thread) [MPS]: Free bytecode structure.
(Finternal_stack_stats, exec_byte_code):
* src/igc.c (scan_bc): Adjust.
(root_create_bc): Save 'bc' pointer.
* src/lisp.h (get_act_rec, set_act_rec):
* src/thread.c (mark_one_thread, finalize_one_thread): Adjust.
(Fmake_thread) [MPS]:
(init_threads) [MPS]: Allocate bytecode structure.
* src/thread.h (struct thread_state): Turn 'bc' member into a
pointer (MPS) or single-member array.

This replaces the AMS pool with an AMC pool + pinning.

* src/igc.c (struct igc_pins): New.
(struct igc): Add member pins.
(pin, unpin, amke_pins, count_pins, enlarge_pins): New
functions.
(make_fake_entry_pins): New function.
(Figc_info): Use make_fake_entry_pins.
(igc_thread_remove): Unpin thread_state.
(igc_alloc_global_ref): Pin allocated global ref.
(igc_free_global_ref): New function.
(make_pool_ams): Remove.
(make_igc): Make immovable pool and AMC pool.
* src/igc.h (igc_free_global_ref): Declare.
* src/emacs-module.c (module_free_global_ref): Call
igc_free_global_ref.
* src/lisp.h (struct thread_state): Add member pin_index.
* src/thread.h (GCALIGNED_STRUCT): Add member pin_index.
* src/igc.c

* src/thread.c (Fmake_thread): Add new argument (bug#76969).
(thread_set_error): New function, extracted from thread-signal.
(Fthread_buffer_disposition): Add getter.
(Fthread_set_buffer_disposition): And setter.
(thread_check_current_buffer): Check the values of threads'
buffer_disposition.
(thread_all_before_buffer_killed): New function.
(init_threads): Set buffer_disposition to nil for the main thread.
(syms_of_threads): Add new symbols and define the error.

* src/thread.h (thread_state): New field buffer_disposition.
(thread_all_before_buffer_killed): Declare.

* src/buffer.c (Fkill_buffer): Call thread_check_current_buffer
one more time after all hooks and after that call
thread_all_before_buffer_killed.

* src/comp.c (ABI_VERSION): Increase the value.

* test/src/thread-tests.el (thread-buffer-disposition-t)
(thread-buffer-disposition-nil)
(thread-buffer-disposition-silently)
(thread-set-buffer-disposition)
(thread-set-buffer-disposition-main-thread): New tests.

* doc/lispref/threads.texi (Basic Thread Functions): Document
buffer-disposition in make-thread and its getter and setter.

* etc/NEWS: Add entry.

This makes it necessary to allocate the buffer indirectly, as it needs
to be an MPS root rather than part of a normal pseudovector.

* src/thread.c (main_thread_getcjmp): New variable.
(main_thread): Use it.
(run_thread): Free setjmp buffer when tearing down a thread.
(Fmake_thread): Allocate setjmp buffer when starting a new thread.
* src/thread.h (m_getcjmp): Point to sys_jmp_buf indirectly.
* src/igc.c (root_create_main_thread): Create an ambiguous root for
'main_thread_getjmp'.