Rewrite thread context switch code (bug#48990)

Make the context switch code handle buffer-local variables more
correctly by reusing the code originally written for `backtrace-eval`.
This has the side benefit of making the `saved_value` field unused.

* src/lisp.h (enum specbind_tag): Remove `saved_value` field.
(rebind_for_thread_switch, unbind_for_thread_switch): Delete decls.
(specpdl_unrewind): Declare function.

* src/eval.c (specpdl_saved_value): Delete function.
(specbind): Delete the code related to `saved_value`, and consolidate
common code between the different branches.
(rebind_for_thread_switch, -unbind_for_thread_switch): Move to `thread.c`.
(specpdl_unrewind): New function, extracted from `backtrace_eval_unrewind`.
Use `SET_INTERNAL_THREAD_SWITCH`.  Skip the buffer & excursion unwinds
depending on new arg `vars_only`.
(backtrace_eval_unrewind): Use it.
(mark_specpdl): Don't mark `saved_value`.

* src/thread.c (rebind_for_thread_switch, unbind_for_thread_switch):
Move from `eval.c` and rewrite using `specpdl_unrewind`.

* test/src/thread-tests.el (threads-test-bug48990): New test.

* test/Makefile.in (test_template): Add a + as suggested by make:
"warning: jobserver unavailable: using -j1.  Add '+' to parent make rule".

1 files changed, 16 insertions, 0 deletions

diff --git a/src/thread.c b/src/thread.c
index 8a6a2de18be..4c98d590b7a 100644
--- a/src/thread.c
+++ b/src/thread.c
@@ -83,6 +83,22 @@ release_global_lock (void)
   sys_mutex_unlock (&global_lock);
 }
 
+static void
+rebind_for_thread_switch (void)
+{
+  ptrdiff_t distance
+    = current_thread->m_specpdl_ptr - current_thread->m_specpdl;
+  specpdl_unrewind (specpdl_ptr, -distance, true);
+}
+
+static void
+unbind_for_thread_switch (struct thread_state *thr)
+{
+  ptrdiff_t distance = thr->m_specpdl_ptr - thr->m_specpdl;
+  specpdl_unrewind (thr->m_specpdl_ptr, distance, true);
+}
+
+
 /* You must call this after acquiring the global lock.
    acquire_global_lock does it for you.  */
 static void