Use byte_slice for sending zmq messages - removes data copy within zmq

contrib/epee/include/byte_slice.h

@@ -42,7 +42,12 @@ namespace epee
 
   struct release_byte_slice
   {
-    void operator()(byte_slice_data*) const noexcept;
+    //! For use with `zmq_message_init_data`, use second arg for buffer pointer.
+    static void call(void*, void* ptr) noexcept;
+    void operator()(byte_slice_data* ptr) const noexcept
+    {
+      call(nullptr, ptr);
+    }
   };
 
   /*! Inspired by slices in golang. Storage is thread-safe reference counted,
@@ -140,6 +145,9 @@ namespace epee
         \throw std::out_of_range If `size() < end`.
         \return Slice starting at `data() + begin` of size `end - begin`. */
     byte_slice get_slice(std::size_t begin, std::size_t end) const;
+
+    //! \post `empty()` \return Ownership of ref-counted buffer.
+    std::unique_ptr<byte_slice_data, release_byte_slice> take_buffer() noexcept;
   };
 } // epee
 

contrib/epee/src/byte_slice.cpp

@@ -49,12 +49,16 @@ namespace epee
     std::atomic<std::size_t> ref_count;
   };
 
-  void release_byte_slice::operator()(byte_slice_data* ptr) const noexcept
+  void release_byte_slice::call(void*, void* ptr) noexcept
   {
-    if (ptr && --(ptr->ref_count) == 0)
+    if (ptr)
     {
-      ptr->~byte_slice_data();
-      free(ptr);
+      byte_slice_data* self = static_cast<byte_slice_data*>(ptr);
+      if (--(self->ref_count) == 0)
+      {
+        self->~byte_slice_data();
+        free(self);
+      }
     }
   }
 
@@ -206,4 +210,11 @@ namespace epee
       return {};
     return {storage_.get(), {portion_.begin() + begin, end - begin}};
   }
+
+  std::unique_ptr<byte_slice_data, release_byte_slice> byte_slice::take_buffer() noexcept
+  {
+    std::unique_ptr<byte_slice_data, release_byte_slice> out{std::move(storage_)};
+    portion_ = nullptr;
+    return out;
+  }
 } // epee