support HTTP OPTIONS

[adikabintang]

Support for HTTP OPTIONS. Useful for Cross-Origin Resource Sharing (CORS).

[yhirose]

@adikabintang, thanks for the PR. Yes, it would be nice to support OPTIONS, so that we could handle the HTTP preflight request.
https://developer.mozilla.org/en-US/docs/Glossary/Preflight_request

After I took a look at your code and pondered over how the OPTIONS support could be implemented, I now think it would be better to provide options method on the Server class, so that a user could configure HTTP response headers based on the given path.

svr.options(R"(\*)", [](const auto& req, auto& res) {
    res.set_header("Allow", "GET, POST, HEAD, OPTIONS");
});

svr.options("/resource/foo", [](const auto& req, auto& res) {
    res.set_header("Access-Control-Allow-Origin", req.get_header_value("Origin").c_str());
    res.set_header("Allow", "GET, POST, HEAD, OPTIONS");
    res.set_header("Access-Control-Allow-Headers", "X-Requested-With, Content-Type, Accept, Origin, Authorization");
    res.set_header("Access-Control-Allow-Methods", "OPTIONS, GET, POST, HEAD");
});

When I have time, I'll try to implement the above idea. Thanks again for your valuable feedback!

[adikabintang]

@yhirose This feature would be very useful and I can see that you have a new branch (branch issue-57) to support this. Thank you very much for your work.

[yhirose]

@adikabintang, I have just merged the branch. I changed HTTP verb method names to be the initial caps such as Get, Options from this version of httplib.h. (This had to be done for #57.)

svr.Options(R"(\*)", [](const auto& req, auto& res) {
    res.set_header("Allow", "GET, POST, HEAD, OPTIONS");
});

svr.Options("/resource/foo", [](const auto& req, auto& res) {
    res.set_header("Access-Control-Allow-Origin", req.get_header_value("Origin").c_str());
    res.set_header("Allow", "GET, POST, HEAD, OPTIONS");
    res.set_header("Access-Control-Allow-Headers", "X-Requested-With, Content-Type, Accept, Origin, Authorization");
    res.set_header("Access-Control-Allow-Methods", "OPTIONS, GET, POST, HEAD");
});

Hope this fix will solve your problem!

[adikabintang]

Wow, thank you @yhirose