Avoid a parser allocation using non-flat buffer:

When the parser is presented with an input buffer
sequence with length greater than one, the input is
flattened using stack space instead of dynamic
allocation if the buffer size is no greater than 8192.

CHANGELOG.md

@@ -2,6 +2,7 @@ Version 55:
 
 * Don't allocate memory to handle obs-fold
 * Add Beast CMake interface target
+* Avoid a parser allocation using non-flat buffer
 
 --------------------------------------------------------------------------------
 

include/beast/http/basic_parser.hpp

@@ -74,6 +74,9 @@ class basic_parser
     // limit on the size of the obs-fold buffer
     static std::size_t constexpr max_obs_fold = 4096;
 
+    // limit on the size of the stack flat buffer
+    static std::size_t constexpr max_stack_buffer = 8192;
+
     // Message will be complete after reading header
     static unsigned constexpr flagSkipBody              = 1<<  0;
 
@@ -363,9 +366,10 @@ private:
     }
 
     template<class ConstBufferSequence>
-    string_view
-    maybe_flatten(
-        ConstBufferSequence const& buffers);
+    std::size_t
+    put_from_stack(std::size_t size,
+        ConstBufferSequence const& buffers,
+            error_code& ec);
 
     void
     parse_header(char const*& p,

include/beast/http/impl/basic_parser.ipp

@@ -88,9 +88,35 @@ put(ConstBufferSequence const& buffers,
     static_assert(is_const_buffer_sequence<
         ConstBufferSequence>::value,
             "ConstBufferSequence requirements not met");
-    auto const buffer = maybe_flatten(buffers);
+    using boost::asio::buffer_cast;
+    using boost::asio::buffer_copy;
+    using boost::asio::buffer_size;
+    auto const p = buffers.begin();
+    auto const last = buffers.end();
+    if(p == last)
+        return 0;
+    if(std::next(p) == last)
+    {
+        // single buffer
+        auto const b = *p;
+        return put(boost::asio::const_buffers_1{
+            buffer_cast<char const*>(b),
+            buffer_size(b)}, ec);
+    }
+    auto const size = buffer_size(buffers);
+    if(size <= max_stack_buffer)
+        return put_from_stack(size, buffers, ec);
+    if(size > buf_len_)
+    {
+        // reallocate
+        buf_.reset(new char[size]);
+        buf_len_ = size;
+    }
+    // flatten
+    buffer_copy(boost::asio::buffer(
+        buf_.get(), buf_len_), buffers);
     return put(boost::asio::const_buffers_1{
-        buffer.data(), buffer.size()}, ec);
+        buf_.get(), buf_len_}, ec);
 }
 
 template<bool isRequest, class Derived>
@@ -209,37 +235,18 @@ put_eof(error_code& ec)
 
 template<bool isRequest, class Derived>
 template<class ConstBufferSequence>
-inline
-string_view
+std::size_t
 basic_parser<isRequest, Derived>::
-maybe_flatten(
-    ConstBufferSequence const& buffers)
+put_from_stack(std::size_t size,
+    ConstBufferSequence const& buffers,
+        error_code& ec)
 {
-    using boost::asio::buffer_cast;
+    char buf[max_stack_buffer];
+    using boost::asio::buffer;
     using boost::asio::buffer_copy;
-    using boost::asio::buffer_size;
-    auto const p = buffers.begin();
-    auto const last = buffers.end();
-    if(p == last)
-        return {nullptr, 0};
-    if(std::next(p) == last)
-    {
-        // single buffer
-        auto const b = *p;
-        return {buffer_cast<char const*>(b),
-            buffer_size(b)};
-    }
-    auto const len = buffer_size(buffers);
-    if(len > buf_len_)
-    {
-        // reallocate
-        buf_.reset(new char[len]);
-        buf_len_ = len;
-    }
-    // flatten
-    buffer_copy(boost::asio::buffer(
-        buf_.get(), buf_len_), buffers);
-    return {buf_.get(), len};
+    buffer_copy(buffer(buf, sizeof(buf)), buffers);
+    return put(boost::asio::const_buffers_1{
+        buf, size}, ec);
 }
 
 template<bool isRequest, class Derived>