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, 1 insertions, 5 deletions

diff --git a/src/lisp.h b/src/lisp.h
index f27c2ad2dd5..19788ef07cc 100644
--- a/src/lisp.h
+++ b/src/lisp.h
@@ -3337,9 +3337,6 @@ union specbinding
       ENUM_BF (specbind_tag) kind : CHAR_BIT;
       /* `where' is not used in the case of SPECPDL_LET.  */
       Lisp_Object symbol, old_value, where;
-      /* Normally this is unused; but it is set to the symbol's
-	 current value when a thread is swapped out.  */
-      Lisp_Object saved_value;
     } let;
     struct {
       ENUM_BF (specbind_tag) kind : CHAR_BIT;
@@ -4453,8 +4450,7 @@ extern void set_unwind_protect (specpdl_ref, void (*) (Lisp_Object),
 				Lisp_Object);
 extern void set_unwind_protect_ptr (specpdl_ref, void (*) (void *), void *);
 extern Lisp_Object unbind_to (specpdl_ref, Lisp_Object);
-extern void rebind_for_thread_switch (void);
-extern void unbind_for_thread_switch (struct thread_state *);
+void specpdl_unrewind (union specbinding *pdl, int distance, bool vars_only);
 extern AVOID error (const char *, ...) ATTRIBUTE_FORMAT_PRINTF (1, 2);
 extern AVOID verror (const char *, va_list)
   ATTRIBUTE_FORMAT_PRINTF (1, 0);