basic_parser::content_length is stable (API Change):

fix #1375

* The value returned from `basic_parser::content_length`
  no longer changes as the body of the message is received.

Actions Required:

* Call `basic_parser::content_length_remaining` instead
  of `basic_parser::content_length`.

include/boost/beast/http/basic_parser.hpp

@@ -72,6 +72,7 @@ class basic_parser
     std::uint64_t body_limit_ =
         default_body_limit(is_request{});   // max payload body
     std::uint64_t len_ = 0;                 // size of chunk or body
+    std::uint64_t len0_ = 0;                // content length if known
     std::unique_ptr<char[]> buf_;           // temp storage
     std::size_t buf_len_ = 0;               // size of buf_
     std::size_t skip_ = 0;                  // resume search here
@@ -231,6 +232,18 @@ public:
     boost::optional<std::uint64_t>
     content_length() const;
 
+    /** Returns the remaining content length if known
+
+        If the message header specifies a Content-Length,
+        the return value will be the number of bytes remaining
+        in the payload body have not yet been parsed.
+
+        @note The return value is undefined unless
+              @ref is_header_done would return `true`.
+    */
+    boost::optional<std::uint64_t>
+    content_length_remaining() const;
+
     /** Returns `true` if the message semantics require an end of file.
 
         Depending on the contents of the header, the parser may

include/boost/beast/http/impl/basic_parser.ipp

@@ -48,6 +48,17 @@ template<bool isRequest>
 boost::optional<std::uint64_t>
 basic_parser<isRequest>::
 content_length() const
+{
+    BOOST_ASSERT(is_header_done());
+    if(! (f_ & flagContentLength))
+        return boost::none;
+    return len0_;
+}
+
+template<bool isRequest>
+boost::optional<std::uint64_t>
+basic_parser<isRequest>::
+content_length_remaining() const
 {
     BOOST_ASSERT(is_header_done());
     if(! (f_ & flagContentLength))
@@ -461,6 +472,7 @@ finish_header(error_code& ec, std::true_type)
     else
     {
         len_ = 0;
+        len0_ = 0;
         state_ = state::complete;
     }
 
@@ -796,6 +808,7 @@ do_field(field f,
 
         ec = {};
         len_ = v;
+        len0_ = v;
         f_ |= flagContentLength;
         return;
     }