Handle thread-signal towards the main thread (Bug#32502)

* doc/lispref/threads.texi (Basic Thread Functions):
* etc/NEWS: Document thread-signal towards the main thread.

* lisp/emacs-lisp/thread.el: New package.

* src/keyboard.c (read_char): Check for Qthread_event.
(kbd_buffer_get_event, make_lispy_event): Handle THREAD_EVENT.
(syms_of_keyboard): Declare Qthread_event.
(keys_of_keyboard): Add thread-handle-event to special-event-map.

* src/termhooks.h (enum event_kind): Add THREAD_EVENT.

* src/thread.c: Include "keyboard.h".
(poll_suppress_count) Don't declare extern.
(Fthread_signal): Raise event if THREAD is the main thread.  (Bug#32502)

* test/src/thread-tests.el (thread): Require it.
(threads-signal-main-thread): New test.

1 files changed, 21 insertions, 0 deletions

diff --git a/test/src/thread-tests.el b/test/src/thread-tests.el
index 364f6d61f05..cc1dff8a281 100644
--- a/test/src/thread-tests.el
+++ b/test/src/thread-tests.el
@@ -19,6 +19,8 @@
 
 ;;; Code:
 
+(require 'thread)
+
 ;; Declare the functions in case Emacs has been configured --without-threads.
 (declare-function all-threads "thread.c" ())
 (declare-function condition-mutex "thread.c" (cond))
@@ -320,6 +322,25 @@
     (should-not (thread-alive-p thread))
     (should (equal (thread-last-error) '(error)))))
 
+(ert-deftest threads-signal-main-thread ()
+  "Test signaling the main thread."
+  (skip-unless (featurep 'threads))
+  ;; We cannot use `ert-with-message-capture', because threads do not
+  ;; know let-bound variables.
+  (with-current-buffer "*Messages*"
+    (let (buffer-read-only)
+      (erase-buffer))
+    (let ((thread
+           (make-thread #'(lambda () (thread-signal main-thread 'error nil)))))
+      (while (thread-alive-p thread)
+        (thread-yield))
+      (read-event nil nil 0.1)
+      ;; No error has been raised, which is part of the test.
+      (should
+       (string-match
+        (format-message "Error %s: (error nil)" thread)
+        (buffer-string ))))))
+
 (defvar threads-condvar nil)
 
 (defun threads-test-condvar-wait ()