Why does Boost-Beast give me a partial message exception - c++

Question

I´m trying to use the boost beast http library for an HTTP Client. It´s working without any issues when I´m using a simulated server, however when I try connecting to the real server, boost::beast::http::read throws an exception saying "partial message".
I´ve been working on this issue for a couple days now but I can´t figure out why. Until now I´ve been using a different http client library and the server communication has been working without any similar issues.
I´d be grateful for any kind of idea or hint as to why this is happening and why it doesn't seem to be an issue when using a different library.

Answer 1

boost::beast::http::read throws an exception saying "partial message".
This happens because the message being parsed wasn't complete. A typical reason for it is when the content-length header is wrong, or the sender abandons the connection prematurely. E.g.:
Live On Compiler Explorer
This is what http::[async_]read ends up doing under the hood, but without the network related stuff:
#include <iostream>
#include <iomanip>
#include <string_view>
#include <boost/beast/http.hpp>
int main() {
using namespace boost::beast::http;
using boost::asio::buffer;
for (std::string_view buf : {
"GET / HTTP/1.1\r\n", // incomplete headers
"GET / HTTP/1.1\r\nHost: example.com\r\nContent-Length: 0\r\n\r\ntrailing data",
"GET / HTTP/1.1\r\nHost: example.com\r\nContent-Length: 42\r\n\r\nshort",
})
{
//std::cout << std::quoted(test) << "\n";
std::cout << "---------------------" << "\n";
request_parser<string_body> parser;
boost::system::error_code ec;
size_t n = parser.put(buffer(buf), ec);
if (n && !ec && !parser.is_done()) {
buf.remove_prefix(n);
n = parser.put(buffer(buf), ec); // body
}
if (!ec)
parser.put_eof(ec);
buf.remove_prefix(n);
std::cout
<< (parser.is_header_done()?"headers ok":"incomplete headers")
<< " / " << (parser.is_done()?"done":"not done")
<< " / " << ec.message() << "\n";
if (parser.is_header_done() && !parser.is_done())
std::cout << parser.content_length_remaining().value_or(0) << " more content bytes expected\n";
if (!buf.empty())
std::cout << "Remaining buffer: " << std::quoted(buf) << "\n";
}
}
Prints
---------------------
incomplete headers / not done / need more
---------------------
headers ok / done / Success
Remaining buffer: "trailing data"
---------------------
headers ok / not done / partial message
37 more content bytes expected
If you're not passing error_code to your calls they will throw the exception system_error with the same code, which is exactly what you see.
Side Note
If another library doesn't have this "problem" there are two options:
the library is sloppy (i.e. bad)
you're using it wrong (maybe you're not checking for errors)