lua/prbuf: introduce entry:data() method

prbuf entry contains pointer to raw memory (char *) and its length.
Let's introduce :data() methods which would convert this memory chunk
to Lua string.

NO_DOC=<Feature for internal usage>
NO_CHANGELOG=<Feature for internal usage>

src/lua/buffer.lua

@@ -290,6 +290,13 @@ local function prbuf_iterator_next(iterator)
     return entry
 end
 
+local function prbuf_entry_data(entry)
+    if not ffi.istype(prbuf_entry_t, entry) then
+        errorf('Attempt to entry:data() without object, use entry:data()')
+    end
+    return ffi.string(entry.ptr, tonumber(entry.size))
+end
+
 local prbuf_iterator_methods = {
     next = prbuf_iterator_next;
 }
@@ -300,6 +307,16 @@ local prbuf_iterator_mt = {
 
 ffi.metatype(prbuf_iterator_t, prbuf_iterator_mt);
 
+local prbuf_entry_methods = {
+    data = prbuf_entry_data;
+}
+
+local prbuf_entry_mt = {
+    __index = prbuf_entry_methods;
+}
+
+ffi.metatype(prbuf_entry_t, prbuf_entry_mt);
+
 local function prbuf_tostring(self)
     return '<prbuf>'
 end

test/app-luatest/prbuf_test.lua

@@ -37,6 +37,9 @@ g.test_object_misc = function()
     while entry ~= nil do
         entry_count = entry_count + 1
         t.assert_equals(entry.size, sample_size)
+        local data = entry:data()
+        t.assert_equals(string.len(data), entry.size)
+        t.assert_equals(data, require('ffi').string(entry.ptr, entry.size))
         t.assert_equals(check_memory(entry.ptr, tonumber(entry.size)), true)
         entry = iter:next()
     end