protocol-native: add message sending capability

This patch adds the PA_COMMAND_SEND_OBJECT_MESSAGE command to protocol-native
so that clients can use the messaging feature introduced in the previous patch.

Sending messages can in effect replace the extension system for modules. The
approach is more flexible than the extension interface because a generic string
format is used to exchange information. Furthermore the messaging system can be
used for any object, not only for modules, and is easier to implement than
extensions.

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

PROTOCOL

@@ -435,6 +435,23 @@ sink, source and card ports):
     string availability_group
     uint32 type
 
+## v35, implemented by >= 15.0
+
+Added new command for communication with objects.
+
+PA_COMMAND_SEND_OBJECT_MESSAGE:
+sends a message to an object identified by an object path
+
+parameters:
+    string object_path - unique path identifying the object
+    string message - message name
+    string message_parameters - additional parameters if required (may be
+                                NULL, which should be treated the same as an
+                                empty string)
+
+The command returns a string, which may be empty or NULL (NULL should be
+treated the same as an empty string).
+
 #### If you just changed the protocol, read this
 ## module-tunnel depends on the sink/source/sink-input/source-input protocol
 ## internals, so if you changed these, you might have broken module-tunnel.

configure.ac

@@ -42,7 +42,7 @@ AC_SUBST(PA_MINOR, pa_minor)
 AC_SUBST(PA_MAJORMINOR, pa_major.pa_minor)
 
 AC_SUBST(PA_API_VERSION, 12)
-AC_SUBST(PA_PROTOCOL_VERSION, 34)
+AC_SUBST(PA_PROTOCOL_VERSION, 35)
 
 # The stable ABI for client applications, for the version info x:y:z
 # always will hold x=z

meson.build

@@ -19,7 +19,7 @@ endif
 pa_version_major_minor = pa_version_major + '.' + pa_version_minor
 
 pa_api_version = 12
-pa_protocol_version = 34
+pa_protocol_version = 35
 
 # The stable ABI for client applications, for the version info x:y:z
 # always will hold x=z

src/map-file

@@ -87,6 +87,7 @@ pa_context_remove_autoload_by_name;
 pa_context_remove_sample;
 pa_context_rttime_new;
 pa_context_rttime_restart;
+pa_context_send_message_to_object;
 pa_context_set_card_profile_by_index;
 pa_context_set_card_profile_by_name;
 pa_context_set_default_sink;

src/pulse/introspect.c

@@ -2205,3 +2205,67 @@ pa_operation* pa_context_suspend_source_by_index(pa_context *c, uint32_t idx, in
 
     return o;
 }
+
+/** Object response string processing **/
+
+static void context_string_callback(pa_pdispatch *pd, uint32_t command, uint32_t tag, pa_tagstruct *t, void *userdata) {
+    pa_operation *o = userdata;
+    const char *response;
+    int success = 1;
+
+    pa_assert(pd);
+    pa_assert(o);
+    pa_assert(PA_REFCNT_VALUE(o) >= 1);
+
+    if (!o->context)
+        goto finish;
+
+    if (command != PA_COMMAND_REPLY) {
+        if (pa_context_handle_error(o->context, command, t, false) < 0)
+            goto finish;
+
+        success = 0;
+        response = "";
+    } else if (pa_tagstruct_gets(t, &response)  < 0 ||
+               !pa_tagstruct_eof(t)) {
+        pa_context_fail(o->context, PA_ERR_PROTOCOL);
+        goto finish;
+    }
+
+    if (!response)
+        response = "";
+
+    if (o->callback) {
+        pa_context_string_cb_t cb = (pa_context_string_cb_t) o->callback;
+        cb(o->context, success, response, o->userdata);
+    }
+
+finish:
+    pa_operation_done(o);
+    pa_operation_unref(o);
+}
+
+pa_operation* pa_context_send_message_to_object(pa_context *c, const char *object_path, const char *message, const char *message_parameters, pa_context_string_cb_t cb, void *userdata) {
+    pa_operation *o;
+    pa_tagstruct *t;
+    uint32_t tag;
+
+    pa_assert(c);
+    pa_assert(PA_REFCNT_VALUE(c) >= 1);
+
+    PA_CHECK_VALIDITY_RETURN_NULL(c, !pa_detect_fork(), PA_ERR_FORKED);
+    PA_CHECK_VALIDITY_RETURN_NULL(c, c->state == PA_CONTEXT_READY, PA_ERR_BADSTATE);
+
+    o = pa_operation_new(c, NULL, (pa_operation_cb_t) cb, userdata);
+
+    t = pa_tagstruct_command(c, PA_COMMAND_SEND_OBJECT_MESSAGE, &tag);
+
+    pa_tagstruct_puts(t, object_path);
+    pa_tagstruct_puts(t, message);
+    pa_tagstruct_puts(t, message_parameters);
+
+    pa_pstream_send_tagstruct(c->pstream, t);
+    pa_pdispatch_register_reply(c->pdispatch, tag, DEFAULT_TIMEOUT, context_string_callback, pa_operation_ref(o), (pa_free_cb_t) pa_operation_unref);
+
+    return o;
+}

src/pulse/introspect.h

@@ -204,6 +204,12 @@
  * Server modules can be remotely loaded and unloaded using
  * pa_context_load_module() and pa_context_unload_module().
  *
+ * \subsection message_subsec Messages
+ *
+ * Server objects like sinks, sink inputs or modules can register a message
+ * handler to communicate with clients. A message can be sent to a named
+ * message handler using pa_context_send_message_to_object().
+ *
  * \subsection client_subsec Clients
  *
  * The only operation supported on clients is the possibility of kicking
@@ -489,6 +495,16 @@ pa_operation* pa_context_unload_module(pa_context *c, uint32_t idx, pa_context_s
 
 /** @} */
 
+/** @{ \name Messages */
+
+/** Callback prototype for pa_context_send_message_to_object() \since 15.0 */
+typedef void (*pa_context_string_cb_t)(pa_context *c, int success, const char *response, void *userdata);
+
+/** Send a message to an object that registered a message handler. \since 15.0 */
+pa_operation* pa_context_send_message_to_object(pa_context *c, const char *recipient_name, const char *message, const char *message_parameters, pa_context_string_cb_t cb, void *userdata);
+
+/** @} */
+
 /** @{ \name Clients */
 
 /** Stores information about clients. Please note that this structure

src/pulsecore/native-common.h

@@ -187,6 +187,9 @@ enum {
      * BOTH DIRECTIONS */
     PA_COMMAND_REGISTER_MEMFD_SHMID,
 
+    /* Supported since protocol v34 (14.0) */
+    PA_COMMAND_SEND_OBJECT_MESSAGE,
+
     PA_COMMAND_MAX
 };
 

src/pulsecore/pdispatch.c

@@ -199,6 +199,9 @@ static const char *command_names[PA_COMMAND_MAX] = {
     /* Supported since protocol v31 (9.0) */
     /* BOTH DIRECTIONS */
     [PA_COMMAND_REGISTER_MEMFD_SHMID] = "REGISTER_MEMFD_SHMID",
+
+    /* Supported since protocol v35 (15.0) */
+    [PA_COMMAND_SEND_OBJECT_MESSAGE] = "SEND_OBJECT_MESSAGE",
 };
 
 #endif

src/pulsecore/protocol-native.c

@@ -47,6 +47,7 @@
 #include <pulsecore/namereg.h>
 #include <pulsecore/core-scache.h>
 #include <pulsecore/core-subscribe.h>
+#include <pulsecore/message-handler.h>
 #include <pulsecore/log.h>
 #include <pulsecore/mem.h>
 #include <pulsecore/strlist.h>
@@ -4721,6 +4722,55 @@ static void command_extension(pa_pdispatch *pd, uint32_t command, uint32_t tag,
         protocol_error(c);
 }
 
+/* Send message to an object which registered a handler. Result must be returned as string. */
+static void command_send_object_message(pa_pdispatch *pd, uint32_t command, uint32_t tag, pa_tagstruct *t, void *userdata) {
+    pa_native_connection *c = PA_NATIVE_CONNECTION(userdata);
+    const char *object_path = NULL;
+    const char *message = NULL;
+    const char *message_parameters = NULL;
+    const char *client_name;
+    char *response = NULL;
+    int ret;
+    pa_tagstruct *reply;
+
+    pa_native_connection_assert_ref(c);
+    pa_assert(t);
+
+    if (pa_tagstruct_gets(t, &object_path) < 0 ||
+        pa_tagstruct_gets(t, &message) < 0 ||
+        pa_tagstruct_gets(t, &message_parameters) < 0 ||
+        !pa_tagstruct_eof(t)) {
+        protocol_error(c);
+        return;
+    }
+
+    CHECK_VALIDITY(c->pstream, c->authorized, tag, PA_ERR_ACCESS);
+    CHECK_VALIDITY(c->pstream, object_path != NULL, tag, PA_ERR_INVALID);
+    CHECK_VALIDITY(c->pstream, pa_utf8_valid(object_path), tag, PA_ERR_INVALID);
+    CHECK_VALIDITY(c->pstream, message != NULL, tag, PA_ERR_INVALID);
+    CHECK_VALIDITY(c->pstream, pa_utf8_valid(message), tag, PA_ERR_INVALID);
+    if (message_parameters)
+        CHECK_VALIDITY(c->pstream, pa_utf8_valid(message_parameters), tag, PA_ERR_INVALID);
+
+    client_name = pa_strnull(pa_proplist_gets(c->client->proplist, PA_PROP_APPLICATION_PROCESS_BINARY));
+    pa_log_debug("Client %s sent message %s to path %s", client_name, message, object_path);
+    if (message_parameters)
+        pa_log_debug("Message parameters: %s", message_parameters);
+
+    ret = pa_message_handler_send_message(c->protocol->core, object_path, message, message_parameters, &response);
+
+    if (ret < 0) {
+        pa_pstream_send_error(c->pstream, tag, -ret);
+        return;
+    }
+
+    reply = reply_new(tag);
+    pa_tagstruct_puts(reply, response);
+    pa_xfree(response);
+
+    pa_pstream_send_tagstruct(c->pstream, reply);
+}
+
 static void command_set_card_profile(pa_pdispatch *pd, uint32_t command, uint32_t tag, pa_tagstruct *t, void *userdata) {
     pa_native_connection *c = PA_NATIVE_CONNECTION(userdata);
     uint32_t idx = PA_INVALID_INDEX;
@@ -4972,6 +5022,8 @@ static const pa_pdispatch_cb_t command_table[PA_COMMAND_MAX] = {
 
     [PA_COMMAND_REGISTER_MEMFD_SHMID] = command_register_memfd_shmid,
 
+    [PA_COMMAND_SEND_OBJECT_MESSAGE] = command_send_object_message,
+
     [PA_COMMAND_EXTENSION] = command_extension
 };