Subject: bug#35644: emacs module support doesn't work

From: Caleb Ristvedt <[email-masked]>
Date: Wed, 08 May 2019 21:16:48 -0500

While attempting to package libegit2
(https://github.com/magit/libegit2), I found that any attempt at loading
a dynamic module from emacs would seemingly work (it returns t), but not
actually work: no module api functions invoked from C seem to have any
effect, including defining functions and printing messages. Here's an
example

-------------------------------
#include <stdio.h>
#include <stdlib.h>
#include <emacs-module.h>

int plugin_is_GPL_compatible;

static emacs_value
hello(emacs_env *env, ptrdiff_t nargs, emacs_value args[], void *data)
{
        const char str[] = "Hello Emacs";
        return env->make_string(env, str, sizeof(str) - 1);
}

/* Module init function.  */
int
emacs_module_init(struct emacs_runtime *ert)
{
        emacs_env *env = ert->get_environment(ert);

        emacs_value hellofn = env->make_function(env, 0, 0, hello, "return hello string", NULL);

        // Bind NAME to FUN.
        // (defalias 'hello-c '(lambda () "Hello Emacs"))
        emacs_value defalias_sym = env->intern(env, "defalias");
        emacs_value hello_sym = env->intern(env, "hello-c");
        emacs_value defalias_args[] = { hello_sym, hellofn };

        env->funcall(env, defalias_sym, 2, defalias_args);

        // Provide FEATURE to Emacs.
        // (provide 'hello-core)
        emacs_value feature_sym = env->intern(env, "hello-module");
        emacs_value provide_sym = env->intern(env, "provide");
        emacs_value provide_args[] = { feature_sym };

        env->funcall(env, provide_sym, 1, provide_args);

	char message[] = "TEST MESSAGE!";
	emacs_value message_str = env->make_string(env, message, sizeof(message) - 1);
	emacs_value message_sym = env->intern(env, "message");
	emacs_value message_args[] = {message_str};

	// spam a bit just to be sure
	env->funcall(env, message_sym, 1, message_args);
	env->funcall(env, message_sym, 1, message_args);
	env->funcall(env, message_sym, 1, message_args);

        return 0;
}
-------------------------------

Assuming the above file is named emacs-module-test.c:

-------------------------------
$ gcc -fPIC -shared emacs-module-test.c -o hello-module.so
$ emacs -Q -L . --batch -l hello-module --eval "(message (hello-c))"

=> Symbol's function definition is void: hello-c
-------------------------------

As far as I can tell, this should work. Our emacs is built with
'--with-modules', and 'MODULES' is in system-configuration-features. Any
idea what's going wrong?

- reepca