pactl: Implement list message-handlers

For better readability, "pactl list message-handlers" is introduced which
prints a formatted output of "pactl send-message /core list-handlers".

The patch also adds the functions pa_message_params_read_raw() and
pa_message_params_read_string() for easy parsing of the message response
string. Because the functions need to modify the parameter string,
the message handler and the pa_context_string_callback function now
receive a char* instead of a const char* as parameter argument.

Part-of: <https://gitlab.freedesktop.org/pulseaudio/pulseaudio/-/merge_requests/51>

src/pulsecore/core.c

@@ -86,7 +86,7 @@ static char *message_handler_list(pa_core *c) {
     return pa_strbuf_to_string_free(buf);
 }
 
-static int core_message_handler(const char *object_path, const char *message, const char *message_parameters, char **response, void *userdata) {
+static int core_message_handler(const char *object_path, const char *message, char *message_parameters, char **response, void *userdata) {
     pa_core *c;
 
     pa_assert(c = (pa_core *) userdata);

src/pulsecore/message-handler.c

@@ -90,6 +90,8 @@ void pa_message_handler_unregister(pa_core *c, const char *object_path) {
 /* 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;
+    int ret;
+    char *parameter_copy;
 
     pa_assert(c);
     pa_assert(object_path);
@@ -101,9 +103,14 @@ int pa_message_handler_send_message(pa_core *c, const char *object_path, const c
     if (!(handler = pa_hashmap_get(c->message_handlers, object_path)))
         return -PA_ERR_NOENTITY;
 
+    parameter_copy = pa_xstrdup(message_parameters);
+
     /* 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);
+    ret = handler->callback(handler->object_path, message, parameter_copy, response, handler->userdata);
+
+    pa_xfree(parameter_copy);
+    return ret;
 }
 
 /* Set handler description */

src/pulsecore/message-handler.h

@@ -26,7 +26,7 @@
 typedef int (*pa_message_handler_cb_t)(
         const char *object_path,
         const char *message,
-        const char *message_parameters,
+        char *message_parameters,
         char **response,
         void *userdata);