thread-join returns the result of finished thread

* doc/lispref/threads.texi (Basic Thread Functions):
* etc/NEWS: Document return value of `thread-join'.

* src/thread.c (invoke_thread_function, Fmake_thread)
(init_main_thread): Set result.
(Fthread_join): Propagate signals, and return result.
(Vmain_thread): New defvar.

* src/thread.h (struct thread_state): Add `result' field.

* test/src/thread-tests.el (threads-join): Test also return value.
(threads-join-error): New test.
(threads-mutex-signal): Check for propagation of `quit' signal.

2 files changed, 18 insertions, 7 deletions

diff --git a/src/thread.c b/src/thread.c
index 754d286e9f8..1c73d938655 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;
@@ -933,12 +934,13 @@ 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;
+  Lisp_Object error_symbol, error_data;
 
   CHECK_THREAD (thread);
   tstate = XTHREAD (thread);
@@ -946,10 +948,16 @@ It is an error for a thread to try to join itself.  */)
   if (tstate == current_thread)
     error ("Cannot join current thread");
 
+  error_symbol = tstate->error_symbol;
+  error_data = tstate->error_data;
+
   if (thread_alive_p (tstate))
     flush_stack_call_func (thread_join_callback, tstate);
 
-  return Qnil;
+  if (!NILP (error_symbol))
+    Fsignal (error_symbol, error_data);
+
+  return tstate->result;
 }
 
 DEFUN ("all-threads", Fall_threads, Sall_threads, 0, 0, 0,
@@ -1017,6 +1025,7 @@ 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 +1099,7 @@ 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 c10e5ecb758..922eea62178 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;