MultiPartParser fails to find the first boundary if the boundary string starts with dashes (e.g., Firefox/Gecko)

🐛 Bug Report: MultiPartParser Fails with Dash-Prefixed Boundaries (Firefox Compatibility)

📄 Description

The MultiPartParser fails to correctly identify the first part of a multipart/form-data request when the boundary string in the Content-Type header begins with multiple dashes.

This is a common scenario with Firefox (Gecko-based) browsers, which generate boundaries like:

----geckoformboundary...

---

⚙️ Technical Root Cause

In lib/src/MultiPart.cc, the method:

parse(const HttpRequestPtr &req)

extracts the raw boundary string from the header and searches for it directly in the request body.

However, per RFC 7578, the actual delimiter in the body is:

"--" + boundary

Example:

• Boundary: ----ABC
• Actual delimiter: ------ABC

Because of this:

• The parser searches for ----ABC
• But the body contains ------ABC
• The match is found starting at offset +2 instead of index 0

This introduces a 2-byte offset error.

As a result:

• parseEntity() begins parsing from the wrong position
• Expected \r\n sequences are missed
• Header parsing (e.g., Content-Disposition) fails
• The entire multipart parsing fails

---

🔁 Steps to Reproduce

Save the following as repro.cc and run it:

#include <drogon/MultiPart.h>
#include <drogon/HttpRequest.h>
#include <iostream>

int main() {
    // 1. Define a boundary starting with dashes (common in Firefox)
    std::string boundary = "----geckoformboundary7805dba873e5a74dd0aa7640be4f989";
    
    // 2. Construct valid RFC 7578 body (Delimiter = "--" + boundary)
    std::string body = 
        "--" + boundary + "\r\n"
        "Content-Disposition: form-data; name=\"file\"; filename=\"test.txt\"\r\n"
        "\r\n"
        "Hello World\r\n"
        "--" + boundary + "--\r\n";

    auto req = drogon::HttpRequest::newHttpRequest();
    req->setMethod(drogon::Post);
    req->addHeader("Content-Type", "multipart/form-data; boundary=" + boundary);
    req->setBody(std::move(body));

    drogon::MultiPartParser parser;
    if (parser.parse(req) == 0 && parser.getFiles().size() > 0) {
        std::cout << "SUCCESS: Parsed " << parser.getFiles().size() << " files." << std::endl;
    } else {
        std::cerr << "FAILURE: MultiPartParser could not parse the request." << std::endl;
        return 1;
    }
    return 0;
}

---

###Expected Behavior

• The parser should correctly detect the delimiter
• parser.parse(req) should return 0
• Uploaded files should be successfully parsed

---

Actual Behavior

• The parser returns -1
• Multipart data is not parsed due to internal offset mismatch

---

💡 Suggested Fix

Ensure the parser searches for the full delimiter ("--" + boundary) instead of the raw boundary string.

Proposed Change (MultiPart.cc):

// Within MultiPartParser::parse(const HttpRequestPtr &req)
// ...

std::string fixed_boundary = "--" + boundary;
return parse(req, fixed_boundary.data(), fixed_boundary.size());

---

Additional Notes

• This issue primarily affects clients like Firefox that prepend dashes to boundary values.
• The fix aligns the implementation with RFC 7578 and improves compatibility across browsers.