ipc: some user-visible changes to prepare for making all commands async

I wanted to put all commands through mpv_command_node_async() instead of
mpv_command_node(). Using synchronous commands over a synchronous
transport doesn't make sense anyway.

This would have used the request_id field in IPC requests as reply ID
for the async commands. But the latter need to be [u]int64, while the
former can be any type. To avoid that we need an extra lookup table for
mapping reply IDs to request_id values, we now require that request_id
fields are integers.

Since this would be an incompatible change, just deprecate non-integers
for now, and plan the change for a later time.

DOCS/interface-changes.rst

@@ -103,6 +103,9 @@ Interface changes
       of 3D content doesn't justify such an option anyway.
     - change cycle-values command to use the current value, instead of an
       internal counter that remembered the current position.
+    - ipc: require that "request_id" fields are integers. Other types are still
+      accepted for compatibility, but this will stop in the future. Also, if no
+      request_id is provided, 0 will be assumed.
  --- mpv 0.28.0 ---
     - rename --hwdec=mediacodec option to mediacodec-copy, to reflect
       conventions followed by other hardware video decoding APIs

DOCS/man/ipc.rst

@@ -108,7 +108,10 @@ Because events can occur at any time, it may be difficult at times to determine
 which response goes with which command. Commands may optionally include a
 ``request_id`` which, if provided in the command request, will be copied
 verbatim into the response. mpv does not intrepret the ``request_id`` in any
-way; it is solely for the use of the requester.
+way; it is solely for the use of the requester. The only requirement is that
+the ``request_id`` field must be an integer (a number without fractional parts
+in the range ``-2^63..2^63-1``). Using other types is deprecated and will
+currently show a warning. In the future, this will raise an error.
 
 For example, this request:
 
@@ -122,6 +125,11 @@ Would generate this response:
 
     { "error": "success", "data": 1.468135, "request_id": 100 }
 
+If you don't specify a ``request_id``, command replies will set it to 0.
+
+Commands may run asynchronously in the future, instead of blocking the  socket
+until a reply is sent.
+
 All commands, replies, and events are separated from each other with a line
 break character (``\n``).
 

input/ipc.c

@@ -207,6 +207,10 @@ static char *json_execute_command(struct mpv_handle *client, void *ta_parent,
     }
 
     reqid_node = node_map_get(&msg_node, "request_id");
+    if (reqid_node && reqid_node->format != MPV_FORMAT_INT64) {
+        mp_warn(log, "'request_id' must be an integer. Using other types is "
+                "deprecated and will trigger an error in the future!\n");
+    }
 
     mpv_node *cmd_node = node_map_get(&msg_node, "command");
     if (!cmd_node ||
@@ -404,6 +408,8 @@ error:
      */
     if (reqid_node) {
         mpv_node_map_add(ta_parent, &reply_node, "request_id", reqid_node);
+    } else {
+        mpv_node_map_add_int64(ta_parent, &reply_node, "request_id", 0);
     }
 
     mpv_node_map_add_string(ta_parent, &reply_node, "error", mpv_error_string(rc));