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, 25 insertions, 8 deletions

diff --git a/src/thread.c b/src/thread.c
index 1c73d938655..78cb2161993 100644
--- a/src/thread.c
+++ b/src/thread.c
@@ -25,6 +25,7 @@ along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.  */
 #include "process.h"
 #include "coding.h"
 #include "syssignal.h"
+#include "keyboard.h"
 
 static struct thread_state main_thread;
 
@@ -34,7 +35,6 @@ static struct thread_state *all_threads = &main_thread;
 
 static sys_mutex_t global_lock;
 
-extern int poll_suppress_count;
 extern volatile int interrupt_input_blocked;
 
 
@@ -863,7 +863,8 @@ DEFUN ("thread-signal", Fthread_signal, Sthread_signal, 3, 3, 0,
 This acts like `signal', but arranges for the signal to be raised
 in THREAD.  If THREAD is the current thread, acts just like `signal'.
 This will interrupt a blocked call to `mutex-lock', `condition-wait',
-or `thread-join' in the target thread.  */)
+or `thread-join' in the target thread.
+If THREAD is the main thread, just the error message is shown.  */)
   (Lisp_Object thread, Lisp_Object error_symbol, Lisp_Object data)
 {
   struct thread_state *tstate;
@@ -874,13 +875,29 @@ or `thread-join' in the target thread.  */)
   if (tstate == current_thread)
     Fsignal (error_symbol, data);
 
-  /* What to do if thread is already signaled?  */
-  /* What if error_symbol is Qnil?  */
-  tstate->error_symbol = error_symbol;
-  tstate->error_data = data;
+  if (main_thread_p (tstate))
+    {
+      /* Construct an event.  */
+      struct input_event event;
+      EVENT_INIT (event);
+      event.kind = THREAD_EVENT;
+      event.frame_or_window = Qnil;
+      event.arg = list3 (Fcurrent_thread (), error_symbol, data);
+
+      /* Store it into the input event queue.  */
+      kbd_buffer_store_event (&event);
+    }
+
+  else
+    {
+      /* What to do if thread is already signaled?  */
+      /* What if error_symbol is Qnil?  */
+      tstate->error_symbol = error_symbol;
+      tstate->error_data = data;
 
-  if (tstate->wait_condvar)
-    flush_stack_call_func (thread_signal_callback, tstate);
+      if (tstate->wait_condvar)
+        flush_stack_call_func (thread_signal_callback, tstate);
+    }
 
   return Qnil;
 }