Add detail::dynamic_buffer_prepare:

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

test/beast/core/buffer.cpp

@@ -0,0 +1,87 @@
+//
+// Copyright (c) 2016-2017 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)
+//
+// Official repository: https://github.com/boostorg/beast
+//
+
+// Test that header file is self-contained.
+//#include <boost/beast/core/buffer.hpp>
+
+#include <boost/beast/core/detail/buffer.hpp>
+#include <boost/beast/core/flat_buffer.hpp>
+#include <boost/beast/unit_test/suite.hpp>
+#include <string>
+
+#include <boost/asio/error.hpp>
+
+namespace boost {
+namespace beast {
+
+// VFALCO No idea why boost::system::errc::message_size fails
+//        to compile, so we use boost::asio::error::eof instead.
+//
+class buffer_test : public beast::unit_test::suite
+{
+public:
+    template<class DynamicBuffer>
+    void
+    testPrepare()
+    {
+    #ifndef BOOST_NO_EXCEPTIONS
+        error_code ec;
+        DynamicBuffer b(32);
+        detail::dynamic_buffer_prepare(b, 20, ec,
+            //boost::system::errc::message_size);
+            boost::asio::error::eof);
+        BEAST_EXPECTS(! ec, ec.message());
+        b.commit(20);
+        auto const result =
+            detail::dynamic_buffer_prepare(b, 20, ec,
+            //boost::system::errc::message_size);
+            boost::asio::error::eof);
+        BEAST_EXPECT(result == boost::none);
+        BEAST_EXPECTS(
+            //ec == boost::system::errc::message_size,
+            ec == boost::asio::error::eof, ec.message());
+    #else
+        fail("exceptions disabled", __FILE__, __LINE__);
+    #endif
+    }
+
+    template<class DynamicBuffer>
+    void
+    testPrepareNoexcept()
+    {
+        error_code ec;
+        DynamicBuffer b(32);
+        detail::dynamic_buffer_prepare_noexcept(b, 20, ec,
+            //boost::system::errc::message_size);
+            boost::asio::error::eof);
+        BEAST_EXPECTS(! ec, ec.message());
+        b.commit(20);
+        auto const result =
+            detail::dynamic_buffer_prepare_noexcept(b, 20, ec,
+            //boost::system::errc::message_size);
+            boost::asio::error::eof);
+        BEAST_EXPECT(result == boost::none);
+        BEAST_EXPECTS(
+            //ec == boost::system::errc::message_size,
+            ec == boost::asio::error::eof, ec.message());
+    }
+
+    void
+    run() override
+    {
+        testPrepare<flat_buffer>();
+        testPrepareNoexcept<flat_buffer>();
+        pass();
+    }
+};
+
+BEAST_DEFINE_TESTSUITE(beast,core,buffer);
+
+} // beast
+} // boost