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.

4 files changed, 26 insertions, 0 deletions

diff --git a/src/emacs-module.c b/src/emacs-module.c
index 60f16418efa..cdcbe061b53 100644
--- a/src/emacs-module.c
+++ b/src/emacs-module.c
@@ -88,6 +88,7 @@ To add a new module function, proceed as follows:
 #include "dynlib.h"
 #include "coding.h"
 #include "keyboard.h"
+#include "process.h"
 #include "syssignal.h"
 #include "sysstdio.h"
 #include "thread.h"
@@ -977,6 +978,13 @@ module_make_big_integer (emacs_env *env, int sign,
   return lisp_to_value (env, make_integer_mpz ());
 }
 
+static int
+module_open_channel (emacs_env *env, emacs_value pipe_process)
+{
+  MODULE_FUNCTION_BEGIN (-1);
+  return open_channel_for_module (value_to_lisp (pipe_process));
+}
+
 
 /* Subroutines.  */
 
@@ -1391,6 +1399,7 @@ initialize_environment (emacs_env *env, struct emacs_env_private *priv)
   env->make_big_integer = module_make_big_integer;
   env->get_function_finalizer = module_get_function_finalizer;
   env->set_function_finalizer = module_set_function_finalizer;
+  env->open_channel = module_open_channel;
   Vmodule_environments = Fcons (make_mint_ptr (env), Vmodule_environments);
   return env;
 }
diff --git a/src/module-env-28.h b/src/module-env-28.h
index a2479a8f744..5d884c148c4 100644
--- a/src/module-env-28.h
+++ b/src/module-env-28.h
@@ -9,3 +9,6 @@
   void (*set_function_finalizer) (emacs_env *env, emacs_value arg,
                                   void (*fin) (void *) EMACS_NOEXCEPT)
     EMACS_ATTRIBUTE_NONNULL (1);
+
+  int (*open_channel) (emacs_env *env, emacs_value pipe_process)
+    EMACS_ATTRIBUTE_NONNULL (1);
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);
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 */