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, 12 insertions, 0 deletions

diff --git a/src/process.c b/src/process.c
index e4e5e57aeee..07881d6c5d3 100644
--- a/src/process.c
+++ b/src/process.c
@@ -8200,6 +8200,17 @@ restore_nofile_limit (void)
 #endif
 }
 
+int
+open_channel_for_module (Lisp_Object process)
+{
+  CHECK_PROCESS (process);
+  CHECK_TYPE (PIPECONN_P (process), Qpipe_process_p, process);
+  int fd = dup (XPROCESS (process)->open_fd[SUBPROCESS_STDOUT]);
+  if (fd == -1)
+    report_file_error ("Cannot duplicate file descriptor", Qnil);
+  return fd;
+}
+
 
 /* This is not called "init_process" because that is the name of a
    Mach system call, so it would cause problems on Darwin systems.  */
@@ -8446,6 +8457,7 @@ amounts of data in one go.  */);
   DEFSYM (Qinterrupt_process_functions, "interrupt-process-functions");
 
   DEFSYM (Qnull, "null");
+  DEFSYM (Qpipe_process_p, "pipe-process-p");
 
   defsubr (&Sprocessp);
   defsubr (&Sget_process);