Add detail::dynamic_buffer_prepare:

This function acts as a bottleneck for handling
exceptions thrown by dynamic buffers.

include/boost/beast/core/detail/buffer.hpp

@@ -0,0 +1,83 @@
+//
+// Copyright (c) 2018 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)
+//
+// Official repository: https://github.com/boostorg/beast
+//
+
+#ifndef BOOST_BEAST_CORE_DETAIL_BUFFER_HPP
+#define BOOST_BEAST_CORE_DETAIL_BUFFER_HPP
+
+#include <boost/beast/core/error.hpp>
+#include <boost/optional.hpp>
+#include <stdexcept>
+
+namespace boost {
+namespace beast {
+namespace detail {
+
+template<
+    class DynamicBuffer,
+    class ErrorValue>
+auto
+dynamic_buffer_prepare_noexcept(
+    DynamicBuffer& buffer,
+    std::size_t size,
+    error_code& ec,
+    ErrorValue ev) ->
+        boost::optional<typename
+        DynamicBuffer::mutable_buffers_type>
+{
+    if(buffer.max_size() - buffer.size() < size)
+    {
+        // length error
+        ec = ev;
+        return boost::none;
+    }
+    boost::optional<typename
+        DynamicBuffer::mutable_buffers_type> result;
+    result.emplace(buffer.prepare(size));
+    ec.assign(0, ec.category());
+    return result;
+}
+
+template<
+    class DynamicBuffer,
+    class ErrorValue>
+auto
+dynamic_buffer_prepare(
+    DynamicBuffer& buffer,
+    std::size_t size,
+    error_code& ec,
+    ErrorValue ev) ->
+        boost::optional<typename
+        DynamicBuffer::mutable_buffers_type>
+{
+#ifndef BOOST_NO_EXCEPTIONS
+    try
+    {
+        boost::optional<typename
+            DynamicBuffer::mutable_buffers_type> result;
+        result.emplace(buffer.prepare(size));
+        ec.assign(0, ec.category());
+        return result;
+    }
+    catch(std::length_error const&)
+    {
+        ec = ev;
+    }
+    return boost::none;
+
+#else
+    return dynamic_buffer_prepare_noexcept(
+        buffer, size, ec, ev);
+#endif
+}
+
+} // detail
+} // beast
+} // boost
+
+#endif