Hi, I found an issue with how Crow parses multipart/form-data requests. Specifically, the parser fails when the final boundary does not end with a CRLF, even though this is allowed by the RFC 2046 specification.

However, Crow seems to require that the final boundary must always be followed by --\r\n. This works with tools like curl, which always include the trailing CRLF, but fails with environments like Node.js's fetch and FormData, which do not append a trailing CRLF after the final boundary.

Here's a minimal reproducible example:

C++ server code (Crow):

#include <sstream>
#include <string>

#include "crow_all.h"

int main() {
  crow::SimpleApp app;

  CROW_ROUTE(app, "/").methods("POST"_method)([](const crow::request &req) {
    crow::multipart::message multipart_req(req);
    std::ostringstream oss;
    for (auto &[key, part] : multipart_req.part_map) {
      oss << key << ": " << part.body << "\n";
    }
    return oss.str();
  });

  app.bindaddr("0.0.0.0").port(3000).multithreaded().run();
}

Node.js client code (does not work):

async function main() {
  const language = "auto";
  const code = 123;
  const targetUrl = "http://127.0.0.1:3000/";

  try {
    const formData = new FormData();
    formData.append("language", language);
    formData.append("code", code.toString());

    const response = await fetch(targetUrl, {
      method: "POST",
      body: formData,
    });

    const result = await response.text();
    console.log("Server response:", result);
  } catch (err) {
    console.error("Error:", err);
  }
}

main();

Output:

Server response: Unable to find delimiter in multipart message. Probably ill-formed body

curl command (works):

curl -X POST -F "language=auto" -F "code=123" http://127.0.0.1:3000/

It would be great if the parser could tolerate the absence of CRLF after the final boundary, as allowed by the spec.

Thanks for your great work on Crow!