Remove BodyReader::is_deferred (API Change):

The is_deferred nested type is removed from the BodyReader
requirements.

Performance for sending messages with `file_body` was cut
almost in half with is_deferred as `std::true_type` since
it caused double the number of socket writes. If the
deferred behavior is absolutely necessary, callers can get
the same effect by manually sending the headers first.

Actions Required:

* Callers who need the behavior of is_deferred as `std::true_type`
  should manually construct a serializer and serialize the header
  first, followed by the body.

include/beast/http/buffer_body.hpp

@@ -96,8 +96,6 @@ struct buffer_body
         value_type const& body_;
 
     public:
-        using is_deferred = std::false_type;
-
         using const_buffers_type =
             boost::asio::const_buffers_1;
 

include/beast/http/dynamic_body.hpp

@@ -45,8 +45,6 @@ struct basic_dynamic_body
         DynamicBuffer const& body_;
 
     public:
-        using is_deferred = std::false_type;
-
         using const_buffers_type =
             typename DynamicBuffer::const_buffers_type;
 

include/beast/http/empty_body.hpp

@@ -49,8 +49,6 @@ struct empty_body
 #else
     struct reader
     {
-        using is_deferred = std::false_type;
-
         using const_buffers_type =
             boost::asio::null_buffers;
 

include/beast/http/impl/serializer.ipp

@@ -74,12 +74,10 @@ get(error_code& ec, Visit&& visit)
         if(ec)
             return;
         auto result = rd_->get(ec);
+        if(ec == error::need_more)
+            goto go_header_only;
         if(ec)
-        {
-            // Can't use need_more when ! is_deferred
-            BOOST_ASSERT(ec != error::need_more);
             return;
-        }
         if(! result)
             goto go_header_only;
         more_ = result->second;
@@ -103,10 +101,12 @@ get(error_code& ec, Visit&& visit)
         break;
 
     case do_body:
-        BOOST_ASSERT(! rd_);
-        rd_.emplace(m_, ec);
-        if(ec)
-            return;
+        if(! rd_)
+        {
+            rd_.emplace(m_, ec);
+            if(ec)
+                return;
+        }
         s_ = do_body + 1;
         BOOST_FALLTHROUGH;
 
@@ -139,12 +139,10 @@ get(error_code& ec, Visit&& visit)
         if(ec)
             return;
         auto result = rd_->get(ec);
+        if(ec == error::need_more)
+            goto go_header_only_c;
         if(ec)
-        {
-            // Can't use need_more when ! is_deferred
-            BOOST_ASSERT(ec != error::need_more);
             return;
-        }
         if(! result)
             goto go_header_only_c;
         more_ = result->second;
@@ -179,10 +177,12 @@ get(error_code& ec, Visit&& visit)
         break;
 
     case do_body_c:
-        BOOST_ASSERT(! rd_);
-        rd_.emplace(m_, ec);
-        if(ec)
-            return;
+        if(! rd_)
+        {
+            rd_.emplace(m_, ec);
+            if(ec)
+                return;
+        }
         s_ = do_body_c + 1;
         BOOST_FALLTHROUGH;
 

include/beast/http/serializer.hpp

@@ -144,16 +144,11 @@ class serializer
         do_complete         = 110
     };
 
-    void split(bool, std::true_type) {}
-    void split(bool v, std::false_type) { split_ = v; }
     void frdinit(std::true_type);
     void frdinit(std::false_type);
 
     using reader = typename Body::reader;
 
-    using is_deferred =
-        typename reader::is_deferred;
-
     using ch_t = consuming_buffers<typename
         Fields::reader::const_buffers_type>;        // header
 
@@ -190,7 +185,7 @@ class serializer
     boost::variant<boost::blank,
         ch_t, cb0_t, cb1_t, ch0_t, ch1_t, ch2_t> v_;
     int s_ = do_construct;
-    bool split_ = is_deferred::value;
+    bool split_ = false;
     bool header_done_ = false;
     bool chunked_;
     bool close_;
@@ -227,14 +222,12 @@ public:
         When the split feature is enabled, the implementation will
         write only the octets corresponding to the serialized header
         first. If the header has already been written, this function
-        will have no effect on output. This function should be called
-        before retrieving any buffers using @ref get, otherwise the
-        behavior is undefined.
+        will have no effect on output.
     */
     void
     split(bool v)
     {
-        split(v, is_deferred{});
+        split_ = v;
     }
 
     /** Return `true` if serialization of the header is complete.

include/beast/http/string_body.hpp

@@ -47,8 +47,6 @@ struct string_body
         value_type const& body_;
 
     public:
-        using is_deferred = std::false_type;
-
         using const_buffers_type =
             boost::asio::const_buffers_1;
 

include/beast/http/string_view_body.hpp

@@ -48,8 +48,6 @@ struct string_view_body
         string_view body_;
 
     public:
-        using is_deferred = std::false_type;
-
         using const_buffers_type =
             boost::asio::const_buffers_1;