channel/msgpack_rpc: Refactor API dispatching

This is how API dispatching worked before this commit:

- The generated `msgpack_rpc_dispatch` function receives a the `msgpack_packer`
  argument.
- The response is incrementally built while validating/calling the API.
- Return values/errors are also packed into the `msgpack_packer` while the
  final response is being calculated.

Now the `msgpack_packer` argument is no longer provided, and the
`msgpack_rpc_dispatch` function returns `Object`/`Error` values to
`msgpack_rpc_call`, which will use those values to build the response in a
single pass.

This was done because the new `channel_send_call` function created the
possibility of having recursive API invocations, and this wasn't possible when
sharing a single `msgpack_sbuffer` across call frames(it was shared implicitly
through the `msgpack_packer` instance).

Since we only start to build the response when the necessary information has
been computed, it's now safe to share a single `msgpack_sbuffer` instance
across all channels and API invocations.

Some other changes also had to be performed:

- Handling of the metadata discover was moved to `msgpack_rpc_call`
- Expose more types as subtypes of `Object`, this was required to forward the
  return value from `msgpack_rpc_dispatch` to `msgpack_rpc_call`
- Added more helper macros for casting API types to `Object`
  any

src/nvim/api/private/defs.h

@@ -65,8 +65,16 @@ typedef enum {
   kObjectTypeInteger,
   kObjectTypeFloat,
   kObjectTypeString,
+  kObjectTypeBuffer,
+  kObjectTypeWindow,
+  kObjectTypeTabpage,
   kObjectTypeArray,
-  kObjectTypeDictionary
+  kObjectTypeDictionary,
+  kObjectTypePosition,
+  kObjectTypeStringArray,
+  kObjectTypeBufferArray,
+  kObjectTypeWindowArray,
+  kObjectTypeTabpageArray,
 } ObjectType;
 
 struct object {
@@ -76,8 +84,16 @@ struct object {
     Integer integer;
     Float floating;
     String string;
+    Buffer buffer;
+    Window window;
+    Tabpage tabpage;
     Array array;
     Dictionary dictionary;
+    Position position;
+    StringArray stringarray;
+    BufferArray bufferarray;
+    WindowArray windowarray;
+    TabpageArray tabpagearray;
   } data;
 };
 

src/nvim/api/private/helpers.c

@@ -426,6 +426,8 @@ bool object_to_vim(Object obj, typval_T *tv, Error *err)
       }
       tv->vval.v_dict->dv_refcount++;
       break;
+    default:
+      abort();
   }
 
   return true;

src/nvim/api/private/helpers.h

@@ -14,7 +14,9 @@
     err->set = true;                               \
   } while (0)
 
-#define BOOL_OBJ(b) ((Object) {                                               \
+#define OBJECT_OBJ(o) o
+
+#define BOOLEAN_OBJ(b) ((Object) {                                            \
   .type = kObjectTypeBoolean,                                                 \
   .data.boolean = b                                                           \
   })
@@ -26,26 +28,59 @@
 
 #define STRING_OBJ(s) ((Object) {                                             \
   .type = kObjectTypeString,                                                  \
-  .data.string = cstr_to_string(s)                                            \
+  .data.string = s                                                            \
   })
 
-#define STRINGL_OBJ(d, s) ((Object) {                                         \
-  .type = kObjectTypeString,                                                  \
-  .data.string = (String) {                                                   \
-    .size = s,                                                                \
-    .data = xmemdup(d, s)                                                     \
-  }})
+#define BUFFER_OBJ(s) ((Object) {                                             \
+  .type = kObjectTypeBuffer,                                                  \
+  .data.buffer = s                                                            \
+  })
+
+#define WINDOW_OBJ(s) ((Object) {                                             \
+  .type = kObjectTypeWindow,                                                  \
+  .data.window = s                                                            \
+  })
+
+#define TABPAGE_OBJ(s) ((Object) {                                            \
+  .type = kObjectTypeTabpage,                                                 \
+  .data.tabpage = s                                                           \
+  })
 
 #define ARRAY_OBJ(a) ((Object) {                                              \
   .type = kObjectTypeArray,                                                   \
   .data.array = a                                                             \
   })
 
+#define STRINGARRAY_OBJ(a) ((Object) {                                        \
+  .type = kObjectTypeStringArray,                                             \
+  .data.stringarray = a                                                       \
+  })
+
+#define BUFFERARRAY_OBJ(a) ((Object) {                                        \
+  .type = kObjectTypeBufferArray,                                             \
+  .data.bufferarray = a                                                       \
+  })
+
+#define WINDOWARRAY_OBJ(a) ((Object) {                                        \
+  .type = kObjectTypeWindowArray,                                             \
+  .data.windowarray = a                                                       \
+  })
+
+#define TABPAGEARRAY_OBJ(a) ((Object) {                                       \
+  .type = kObjectTypeTabpageArray,                                            \
+  .data.tabpagearray = a                                                      \
+  })
+
 #define DICTIONARY_OBJ(d) ((Object) {                                         \
   .type = kObjectTypeDictionary,                                              \
   .data.dictionary = d                                                        \
   })
 
+#define POSITION_OBJ(p) ((Object) {                                           \
+  .type = kObjectTypePosition,                                                \
+  .data.position = p                                                          \
+  })
+
 #define NIL ((Object) {.type = kObjectTypeNil})
 
 #define PUT(dict, k, v)                                                       \