Re: Return value of finished threads

[Michael Albinus <michael.albinus@gmx.de>]

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Tom Tromey <tom@tromey.com> writes:

Hi Tom,

> Michael> So I'm wondering if we could extend thread support by an
> Michael> indication of the thread result, when finished. One idea
> Michael> would be that `thread-join' returns with that value.
>
> This doesn't seem all that hard to implement.

Yep. The appended patch does it for me. Maybe you could check whether
I'm missing something?

> Also if the thread exited due to an exception of some kind, this could
> be propagated by thread-join as well.

Yeah, this would be a bonus. I've failed to implement it, so I let it to
somebody else. I don't need it first-hand.

> Michael> The documentation recommends the use of global variables,
> Michael> which would be inconvenient for many threads to supervise.
>
> Or you can use a queue of some kind and have each worker push its
> result there.

Yes, but you must indicate the worker threads where to push the
result. Imagine find-file would support threads for wildcard file names,
and somebody calls find-file this way twice, both running concurrently.
Every worker thread must know, whether to push the result into the
global variable associated with the first call or the second one. Still
overhead to implement; I'll prefer that thread-join returns the result.

> Tom

Best regards, Michael.


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diff --git a/src/thread.c b/src/thread.c
index 754d286e9f..e2d16df7cb 100644
--- a/src/thread.c
+++ b/src/thread.c
@@ -681,7 +681,7 @@ invoke_thread_function (void)
 {
   ptrdiff_t count = SPECPDL_INDEX ();
 
-  Ffuncall (1, &current_thread->function);
+  current_thread->result = Ffuncall (1, &current_thread->function);
   return unbind_to (count, Qnil);
 }
 
@@ -789,6 +789,7 @@ If NAME is given, it must be a string; it names the new thread.  */)
   new_thread->m_last_thing_searched = Qnil; /* copy from parent? */
   new_thread->m_saved_last_thing_searched = Qnil;
   new_thread->m_current_buffer = current_thread->m_current_buffer;
+  new_thread->result = Qnil;
   new_thread->error_symbol = Qnil;
   new_thread->error_data = Qnil;
   new_thread->event_object = Qnil;
@@ -933,9 +934,9 @@ thread_join_callback (void *arg)
 
 DEFUN ("thread-join", Fthread_join, Sthread_join, 1, 1, 0,
        doc: /* Wait for THREAD to exit.
-This blocks the current thread until THREAD exits or until
-the current thread is signaled.
-It is an error for a thread to try to join itself.  */)
+This blocks the current thread until THREAD exits or until the current
+thread is signaled.  It returns the result of the THREAD function.  It
+is an error for a thread to try to join itself.  */)
   (Lisp_Object thread)
 {
   struct thread_state *tstate;
@@ -949,7 +950,7 @@ It is an error for a thread to try to join itself.  */)
   if (thread_alive_p (tstate))
     flush_stack_call_func (thread_join_callback, tstate);
 
-  return Qnil;
+  return tstate->result;
 }
 
 DEFUN ("all-threads", Fall_threads, Sall_threads, 0, 0, 0,
@@ -1017,6 +1018,7 @@ init_main_thread (void)
   main_thread.m_saved_last_thing_searched = Qnil;
   main_thread.name = Qnil;
   main_thread.function = Qnil;
+  main_thread.result = Qnil;
   main_thread.error_symbol = Qnil;
   main_thread.error_data = Qnil;
   main_thread.event_object = Qnil;
@@ -1090,8 +1092,7 @@ syms_of_threads (void)
   DEFSYM (Qmutexp, "mutexp");
   DEFSYM (Qcondition_variable_p, "condition-variable-p");
 
-  DEFVAR_LISP ("main-thread",
-	       Vmain_thread,
+  DEFVAR_LISP ("main-thread", Vmain_thread,
     doc: /* The main thread of Emacs.  */);
 #ifdef THREADS_ENABLED
   XSETTHREAD (Vmain_thread, &main_thread);
diff --git a/src/thread.h b/src/thread.h
index c10e5ecb75..922eea6217 100644
--- a/src/thread.h
+++ b/src/thread.h
@@ -52,6 +52,9 @@ struct thread_state
   /* The thread's function.  */
   Lisp_Object function;
 
+  /* The thread's result, if function has finished.  */
+  Lisp_Object result;
+
   /* If non-nil, this thread has been signaled.  */
   Lisp_Object error_symbol;
   Lisp_Object error_data;
diff --git a/test/src/thread-tests.el b/test/src/thread-tests.el
index a447fb3914..ddf9e987cc 100644
--- a/test/src/thread-tests.el
+++ b/test/src/thread-tests.el
@@ -100,9 +100,9 @@ threads-test-thread1
    (progn
      (setq threads-test-global nil)
      (let ((thread (make-thread #'threads-test-thread1)))
-       (thread-join thread)
-       (and threads-test-global
-	    (not (thread-alive-p thread)))))))
+       (and (= (thread-join thread) 23)
+            (= threads-test-global 23)
+            (not (thread-alive-p thread)))))))
 
 (ert-deftest threads-join-self ()
   "Cannot `thread-join' the current thread."