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, 25 insertions, 0 deletions

diff --git a/test/src/thread-tests.el b/test/src/thread-tests.el
index b7ab31120aa..75d67140a90 100644
--- a/test/src/thread-tests.el
+++ b/test/src/thread-tests.el
@@ -393,4 +393,29 @@
   (let ((th (make-thread 'ignore)))
     (should-not (equal th main-thread))))
 
+(defvar threads-test--var 'global)
+
+(ert-deftest threads-test-bug48990 ()
+  (skip-unless (fboundp 'make-thread))
+  (let ((buf1 (generate-new-buffer " thread-test"))
+        (buf2 (generate-new-buffer " thread-test")))
+    (with-current-buffer buf1
+      (setq-local threads-test--var 'local1))
+    (with-current-buffer buf2
+      (setq-local threads-test--var 'local2))
+    (let ((seen nil))
+      (with-current-buffer buf1
+        (should (eq threads-test--var 'local1))
+        (make-thread (lambda () (setq seen threads-test--var))))
+      (with-current-buffer buf2
+        (should (eq threads-test--var 'local2))
+        (let ((threads-test--var 'let2))
+          (should (eq threads-test--var 'let2))
+          (while (not seen)
+            (thread-yield))
+          (should (eq threads-test--var 'let2))
+          (should (eq seen 'local1)))
+        (should (eq threads-test--var 'local2)))
+      (should (eq threads-test--var 'global)))))
+
 ;;; thread-tests.el ends here