Add a module function to open a file descriptor connected to a pipe.

A common complaint about the module API is that modules can't
communicate asynchronously with Emacs.  While it isn't possible to
call arbitrary Emacs functions asynchronously, writing to a pipe
should always be fine and is a pretty low-hanging fruit.

This patch implements a function that adapts an existing pipe
process.  That way, users can use familiar tools like process filters
or 'accept-process-output'.

* src/module-env-28.h: Add 'open_channel' module function.

* src/emacs-module.c (module_open_channel): Provide definition for
'open_channel'.
(initialize_environment): Use it.

* src/process.c (open_channel_for_module): New helper function.
(syms_of_process): Define necessary symbol.

* test/src/emacs-module-tests.el (module/async-pipe): New unit test.

* test/data/emacs-module/mod-test.c (signal_system_error): New helper
function.
(signal_errno): Use it.
(write_to_pipe): New function running in the background.
(Fmod_test_async_pipe): New test module function.
(emacs_module_init): Export it.

* doc/lispref/internals.texi (Module Misc): Document new module
function.

* doc/lispref/processes.texi (Asynchronous Processes): New anchor
for pipe processes.

* etc/NEWS: Document 'open_channel' function.

1 files changed, 14 insertions, 0 deletions

diff --git a/test/src/emacs-module-tests.el b/test/src/emacs-module-tests.el
index 48d2e86a605..1f91795e1e6 100644
--- a/test/src/emacs-module-tests.el
+++ b/test/src/emacs-module-tests.el
@@ -424,4 +424,18 @@ See Bug#36226."
       ;; but at least one.
       (should (> valid-after valid-before)))))
 
+(ert-deftest module/async-pipe ()
+  "Check that writing data from another thread works."
+  (with-temp-buffer
+    (let ((process (make-pipe-process :name "module/async-pipe"
+                                      :buffer (current-buffer)
+                                      :coding 'utf-8-unix
+                                      :noquery t)))
+      (unwind-protect
+          (progn
+            (mod-test-async-pipe process)
+            (should (accept-process-output process 1))
+            (should (equal (buffer-string) "data from thread")))
+        (delete-process process)))))
+
 ;;; emacs-module-tests.el ends here