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, 2 insertions, 0 deletions

diff --git a/src/process.h b/src/process.h
index 7884efc5494..a783a31cb86 100644
--- a/src/process.h
+++ b/src/process.h
@@ -300,6 +300,8 @@ extern Lisp_Object remove_slash_colon (Lisp_Object);
 extern void update_processes_for_thread_death (Lisp_Object);
 extern void dissociate_controlling_tty (void);
 
+extern int open_channel_for_module (Lisp_Object);
+
 INLINE_HEADER_END
 
 #endif /* EMACS_PROCESS_H */