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.

1 files changed, 12 insertions, 6 deletions

diff --git a/etc/NEWS b/etc/NEWS
index c2b6b500eeb..fc2a5d4c039 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -172,11 +172,6 @@ from a remote host.
 This triggers to search the program on the remote host as indicated by
 'default-directory'.
 
-+++
-** New variable 'main-thread' holds Emacs's main thread.
-This is handy in Lisp programs that run on a non-main thread and want
-to signal the main thread, e.g., when they encounter an error.
-
 
 * Editing Changes in Emacs 27.1
 
@@ -578,7 +573,6 @@ It was obsolete since Emacs 22.1, replaced by customize.
 Use of built-in libgnutls based functionality (described in the Emacs
 GnuTLS manual) is recommended instead.
 
-
 ** Message
 
 +++
@@ -624,6 +618,17 @@ If this option is non-nil, messages appended to an output file by the
 selects the messages to summarize with a regexp that matches the
 sender of the current message.
 
+** Threads
+
++++
+*** New variable 'main-thread' holds Emacs's main thread.
+This is handy in Lisp programs that run on a non-main thread and want
+to signal the main thread, e.g., when they encounter an error.
+
++++
+*** 'thread-join' returns the result of the finished thread now.
+
+
 * New Modes and Packages in Emacs 27.1
 
 +++
@@ -739,6 +744,7 @@ however applications should instead call 'display-buffer-in-side-window'
 is backwards-compatible with versions of Emacs in which the old function
 exists.  See the node "Displaying Buffers in Side Windows" in the ELisp
 manual for more details.
+
 
 * Lisp Changes in Emacs 27.1