feat(msgpack_rpc): support out-of-order responses on msgpack-rpc

Added to support MessagePack-RPC fully compliant clients that do
not return responses in request order.

Although it is currently not an efficient implementation for full
compliance and full compliance cannot be guaranteed, the addition
of the new client type `msgpack-rpc` creates a situation where "if
the client type is `msgpack-rpc`, then backward compatibility is
ignored and full compliance with MessagePack- RPC compliance is
justified even if backward compatibility is ignored if the client
type is `msgpack-rpc`.

src/nvim/eval/funcs.c

@@ -6730,7 +6730,7 @@ static void f_rpcrequest(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
     const char *name = NULL;
     Channel *chan = find_channel(chan_id);
     if (chan) {
-      name = rpc_client_name(chan);
+      name = get_client_info(chan, "name");
     }
     msg_ext_set_kind("rpc_error");
     if (name) {

src/nvim/msgpack_rpc/channel.c

@@ -306,6 +306,17 @@ end:
   channel_decref(channel);
 }
 
+static ChannelCallFrame *find_call_frame(RpcState *rpc, uint32_t request_id)
+{
+  for (size_t i = 0; i < kv_size(rpc->call_stack); i++) {
+    ChannelCallFrame *frame = kv_Z(rpc->call_stack, i);
+    if (frame->request_id == request_id) {
+      return frame;
+    }
+  }
+  return NULL;
+}
+
 static void parse_msgpack(Channel *channel)
 {
   Unpacker *p = channel->rpc.unpacker;
@@ -321,13 +332,15 @@ static void parse_msgpack(Channel *channel)
       }
       arena_mem_free(arena_finish(&p->arena));
     } else if (p->type == kMessageTypeResponse) {
-      ChannelCallFrame *frame = kv_last(channel->rpc.call_stack);
-      if (p->request_id != frame->request_id) {
+      ChannelCallFrame *frame = channel->rpc.client_type == kClientTypeMsgpackRpc
+        ? find_call_frame(&channel->rpc, p->request_id)
+        : kv_last(channel->rpc.call_stack);
+      if (frame == NULL || p->request_id != frame->request_id) {
         char buf[256];
         snprintf(buf, sizeof(buf),
-                 "ch %" PRIu64 " returned a response with an unknown request "
+                 "ch %" PRIu64 " (type=%" PRIu32 ") returned a response with an unknown request "
                  "id %" PRIu32 ". Ensure the client is properly synchronized",
-                 channel->id, p->request_id);
+                 channel->id, (unsigned)channel->rpc.client_type, p->request_id);
         chan_close_with_error(channel, buf, LOGLVL_ERR);
       }
       frame->returned = true;
@@ -691,6 +704,25 @@ void rpc_set_client_info(uint64_t id, Dictionary info)
 
   api_free_dictionary(chan->rpc.info);
   chan->rpc.info = info;
+
+  // Parse "type" on "info" and set "client_type"
+  const char *type = get_client_info(chan, "type");
+  if (type == NULL || strequal(type, "remote")) {
+    chan->rpc.client_type = kClientTypeRemote;
+  } else if (strequal(type, "msgpack-rpc")) {
+    chan->rpc.client_type = kClientTypeMsgpackRpc;
+  } else if (strequal(type, "ui")) {
+    chan->rpc.client_type = kClientTypeUi;
+  } else if (strequal(type, "embedder")) {
+    chan->rpc.client_type = kClientTypeEmbedder;
+  } else if (strequal(type, "host")) {
+    chan->rpc.client_type = kClientTypeHost;
+  } else if (strequal(type, "plugin")) {
+    chan->rpc.client_type = kClientTypePlugin;
+  } else {
+    chan->rpc.client_type = kClientTypeUnknown;
+  }
+
   channel_info_changed(chan, false);
 }
 
@@ -699,14 +731,15 @@ Dictionary rpc_client_info(Channel *chan)
   return copy_dictionary(chan->rpc.info, NULL);
 }
 
-const char *rpc_client_name(Channel *chan)
+const char *get_client_info(Channel *chan, const char *key)
+  FUNC_ATTR_NONNULL_ALL
 {
   if (!chan->is_rpc) {
     return NULL;
   }
   Dictionary info = chan->rpc.info;
   for (size_t i = 0; i < info.size; i++) {
-    if (strequal("name", info.items[i].key.data)
+    if (strequal(key, info.items[i].key.data)
         && info.items[i].value.type == kObjectTypeString) {
       return info.items[i].value.data.string.data;
     }

src/nvim/msgpack_rpc/channel_defs.h

@@ -14,6 +14,16 @@
 typedef struct Channel Channel;
 typedef struct Unpacker Unpacker;
 
+typedef enum {
+  kClientTypeUnknown = -1,
+  kClientTypeRemote = 0,
+  kClientTypeMsgpackRpc = 5,
+  kClientTypeUi = 1,
+  kClientTypeEmbedder = 2,
+  kClientTypeHost = 3,
+  kClientTypePlugin = 4,
+} ClientType;
+
 typedef struct {
   uint32_t request_id;
   bool returned, errored;
@@ -37,6 +47,7 @@ typedef struct {
   uint32_t next_request_id;
   kvec_t(ChannelCallFrame *) call_stack;
   Dictionary info;
+  ClientType client_type;
 } RpcState;
 
 #endif  // NVIM_MSGPACK_RPC_CHANNEL_DEFS_H