API: Refactor: Duplicate/free string arguments coming from msgpack

When receiving strings *from* msgpack, we don't need to duplicate/free since
the data only lives in the msgpack parse buffer until the end of the call.

But in order to reuse `msgpack_rpc_free_object` when sending event data(which is
sent *to* msgpack), Strings must be freed, which means they must also be
allocated separately.

src/nvim/api/buffer.c

@@ -307,7 +307,7 @@ Integer buffer_get_number(Buffer buffer, Error *err)
 
 String buffer_get_name(Buffer buffer, Error *err)
 {
-  String rv = {.size = 0, .data = ""};
+  String rv = STRING_INIT;
   buf_T *buf = find_buffer(buffer, err);
 
   if (!buf || buf->b_ffname == NULL) {

src/nvim/api/private/defs.h

@@ -6,6 +6,7 @@
 #include <string.h>
 
 #define ARRAY_DICT_INIT {.size = 0, .items = NULL}
+#define STRING_INIT {.data = NULL, .size = 0}
 #define REMOTE_TYPE(type) typedef uint64_t type
 
 #define TYPED_ARRAY_OF(type)                                                  \

src/nvim/api/private/helpers.c

@@ -319,7 +319,7 @@ tabpage_T * find_tab(Tabpage tabpage, Error *err)
 
 String cstr_to_string(const char *str) {
     if (str == NULL) {
-        return (String) { .data = NULL, .size = 0 };
+        return (String) STRING_INIT;
     }
 
     size_t len = strlen(str);

src/nvim/os/msgpack_rpc.c

@@ -147,9 +147,13 @@ bool msgpack_rpc_to_float(msgpack_object *obj, Float *arg)
 
 bool msgpack_rpc_to_string(msgpack_object *obj, String *arg)
 {
-  arg->data = (char *)obj->via.raw.ptr;
+  if (obj->type != MSGPACK_OBJECT_RAW) {
+    return false;
+  }
+
+  arg->data = xmemdup(obj->via.raw.ptr, obj->via.raw.size);
   arg->size = obj->via.raw.size;
-  return obj->type == MSGPACK_OBJECT_RAW;
+  return true;
 }
 
 bool msgpack_rpc_to_object(msgpack_object *obj, Object *arg)
@@ -328,6 +332,15 @@ void msgpack_rpc_from_dictionary(Dictionary result, msgpack_packer *res)
   }
 }
 
+void msgpack_rpc_free_string(String value)
+{
+  if (!value.data) {
+    return;
+  }
+
+  free(value.data);
+}
+
 void msgpack_rpc_free_object(Object value)
 {
   switch (value.type) {

src/nvim/os/msgpack_rpc.h

@@ -81,7 +81,7 @@ void msgpack_rpc_from_dictionary(Dictionary result, msgpack_packer *res);
 #define msgpack_rpc_init_integer
 #define msgpack_rpc_init_float
 #define msgpack_rpc_init_position
-#define msgpack_rpc_init_string
+#define msgpack_rpc_init_string = STRING_INIT
 #define msgpack_rpc_init_buffer
 #define msgpack_rpc_init_window
 #define msgpack_rpc_init_tabpage
@@ -100,9 +100,7 @@ void msgpack_rpc_from_dictionary(Dictionary result, msgpack_packer *res);
 #define msgpack_rpc_free_integer(value)
 #define msgpack_rpc_free_float(value)
 #define msgpack_rpc_free_position(value)
-// Strings are not copied from msgpack and so don't need to be freed(they
-// probably "live" in the msgpack streaming buffer)
-#define msgpack_rpc_free_string(value)
+void msgpack_rpc_free_string(String value);
 #define msgpack_rpc_free_buffer(value)
 #define msgpack_rpc_free_window(value)
 #define msgpack_rpc_free_tabpage(value)