cut messages at 512 bytes, taking into account the UTF-8 codepoints

ref #3067

louloulibs/utils/string.cpp

@@ -1,4 +1,5 @@
 #include <utils/string.hpp>
+#include <utils/encoding.hpp>
 
 bool to_bool(const std::string& val)
 {
@@ -11,8 +12,17 @@ std::vector<std::string> cut(const std::string& val, const std::size_t size)
   std::string::size_type pos = 0;
   while (pos < val.size())
     {
-      res.emplace_back(val.substr(pos, size));
-      pos += size;
+      // Get the number of chars, <= size, that contain only whole
+      // UTF-8 codepoints.
+      std::size_t s = 0;
+      auto codepoint_size = utils::get_next_codepoint_size(val[pos + s]);
+      while (s + codepoint_size <= size)
+        {
+          s += codepoint_size;
+          codepoint_size = utils::get_next_codepoint_size(val[pos + s]);
+        }
+      res.emplace_back(val.substr(pos, s));
+      pos += s;
     }
   return res;
 }