read_size replaces read_size_helper:

* detail::read_size_helper is removed

* public function read_size helps determine the optimum
  size for calls to a DynamicBuffer's prepare function.

* special case for Asio types which don't conform to
  the concept but have their own helper function.

include/beast/core/impl/read_size.ipp

@@ -0,0 +1,75 @@
+//
+// Copyright (c) 2013-2017 Vinnie Falco (vinnie dot falco at gmail dot com)
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+//
+
+#ifndef BEAST_IMPL_READ_SIZE_IPP
+#define BEAST_IMPL_READ_SIZE_IPP
+
+namespace beast {
+
+namespace detail {
+
+template<class T, class = void>
+struct has_read_size_helper : std::false_type {};
+
+template<class T>
+struct has_read_size_helper<T, decltype(
+    read_size_helper(std::declval<T&>(), 512),
+    (void)0)> : std::true_type
+{
+};
+
+template<class DynamicBuffer>
+std::size_t
+read_size(DynamicBuffer& buffer,
+    std::size_t max_size, std::true_type)
+{
+    return read_size_helper(buffer, max_size);
+}
+
+template<class DynamicBuffer>
+std::size_t
+read_size(DynamicBuffer& buffer,
+    std::size_t max_size, std::false_type)
+{
+    static_assert(is_dynamic_buffer<DynamicBuffer>::value,
+        "DynamicBuffer requirements not met");
+    BOOST_ASSERT(max_size >= 1);
+    auto const size = buffer.size();
+    auto const limit = buffer.max_size() - size;
+    BOOST_ASSERT(size <= buffer.max_size());
+    return std::min<std::size_t>(
+        std::max<std::size_t>(512, buffer.capacity() - size),
+        std::min<std::size_t>(max_size, limit));
+}
+
+} // detail
+
+template<class DynamicBuffer>
+inline
+std::size_t
+read_size(
+    DynamicBuffer& buffer, std::size_t max_size)
+{
+    return detail::read_size(buffer, max_size,
+        detail::has_read_size_helper<DynamicBuffer>{});
+}
+
+template<class DynamicBuffer>
+std::size_t
+read_size_or_throw(
+    DynamicBuffer& buffer, std::size_t max_size)
+{
+    auto const n = read_size(buffer, max_size);
+    if(n == 0)
+        BOOST_THROW_EXCEPTION(std::length_error{
+            "buffer overflow"});
+    return n;
+}
+
+} // beast
+
+#endif