core: add simple message interface

This patch adds a new feature to the core which allows to send messages
to objects. An object can register/unregister a message handler with
pa_message_handler_{register, unregister}() while a message can be sent
to the handler using the pa_message_handler_send_message() function.
A message has 4 arguments (apart from passing the core):

object_path: The path identifying the object that will receive the message
message: message command
message_parameters: A string containing additional parameters
response: Pointer to a response string that will be filled by the
          message handler. The caller is responsible to free the string.

The patch is a precondition for the following patches that allow clients
to send messages to pulseaudio objects.

There is no restriction on object names, except that an object path
always starts with a "/". The intention is to use a path-like syntax,
for example /core/sink_1 for a sink or /name/instances/index for modules.
The exact naming convention still needs to be agreed.

src/pulsecore/message-handler.c

@@ -0,0 +1,104 @@
+/***
+  This file is part of PulseAudio.
+
+  PulseAudio is free software; you can redistribute it and/or modify
+  it under the terms of the GNU Lesser General Public License as published
+  by the Free Software Foundation; either version 2.1 of the License,
+  or (at your option) any later version.
+
+  PulseAudio is distributed in the hope that it will be useful, but
+  WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+  General Public License for more details.
+
+  You should have received a copy of the GNU Lesser General Public License
+  along with PulseAudio; if not, see <http://www.gnu.org/licenses/>.
+***/
+
+#ifdef HAVE_CONFIG_H
+#include <config.h>
+#endif
+
+#include <stdlib.h>
+#include <stdio.h>
+
+#include <pulse/xmalloc.h>
+
+#include <pulsecore/core.h>
+#include <pulsecore/core-util.h>
+#include <pulsecore/log.h>
+#include <pulsecore/macro.h>
+
+#include "message-handler.h"
+
+/* Message handler functions */
+
+/* Register message handler for the specified object. object_path must be a unique name starting with "/". */
+void pa_message_handler_register(pa_core *c, const char *object_path, const char *description, pa_message_handler_cb_t cb, void *userdata) {
+    struct pa_message_handler *handler;
+
+    pa_assert(c);
+    pa_assert(object_path);
+    pa_assert(cb);
+    pa_assert(userdata);
+
+    /* Ensure that the object path is not empty and starts with "/". */
+    pa_assert(object_path[0] == '/');
+
+    handler = pa_xnew0(struct pa_message_handler, 1);
+    handler->userdata = userdata;
+    handler->callback = cb;
+    handler->object_path = pa_xstrdup(object_path);
+    handler->description = pa_xstrdup(description);
+
+    pa_assert_se(pa_hashmap_put(c->message_handlers, handler->object_path, handler) == 0);
+}
+
+/* Unregister a message handler */
+void pa_message_handler_unregister(pa_core *c, const char *object_path) {
+    struct pa_message_handler *handler;
+
+    pa_assert(c);
+    pa_assert(object_path);
+
+    pa_assert_se(handler = pa_hashmap_remove(c->message_handlers, object_path));
+
+    pa_xfree(handler->object_path);
+    pa_xfree(handler->description);
+    pa_xfree(handler);
+}
+
+/* Send a message to an object identified by object_path */
+int pa_message_handler_send_message(pa_core *c, const char *object_path, const char *message, const char *message_parameters, char **response) {
+    struct pa_message_handler *handler;
+
+    pa_assert(c);
+    pa_assert(object_path);
+    pa_assert(message);
+    pa_assert(response);
+
+    *response = NULL;
+
+    if (!(handler = pa_hashmap_get(c->message_handlers, object_path)))
+        return -PA_ERR_NOENTITY;
+
+    /* The handler is expected to return an error code and may also
+       return an error string in response */
+    return handler->callback(handler->object_path, message, message_parameters, response, handler->userdata);
+}
+
+/* Set handler description */
+int pa_message_handler_set_description(pa_core *c, const char *object_path, const char *description) {
+    struct pa_message_handler *handler;
+
+    pa_assert(c);
+    pa_assert(object_path);
+
+    if (!(handler = pa_hashmap_get(c->message_handlers, object_path)))
+        return -PA_ERR_NOENTITY;
+
+    pa_xfree(handler->description);
+    handler->description = pa_xstrdup(description);
+
+    return PA_OK;
+}