Jupyterhub Configurable Http Proxy issue - node.js

Question

I have been working with Jupyterhub's Configurable Http Proxy and I have been adding the necessary options for the proxy to handle client's ssl certificates without having to use the command line options.
My main goal is that I want to take in a clients request to the proxy and add their certificate information to the header. Once in the header, I will use jupyterhub's authenticator to craft a username.
My issue is that when I use the proxy.on('proxyReq method available for the http-proxy to set the header, I get this error: [Error: Can't set headers after they are sent.]
I have been looking all over the code to see where a response/request is being written or sent, but I cannot find it.
Here is the ConfigurableProxy function code, I can give you more if needed:
function ConfigurableProxy (options) {
var that = this;
this.options = options || {};
this.trie = new trie.URLTrie();
this.auth_token = this.options.auth_token;
this.includePrefix = options.includePrefix === undefined ? true : options.includePrefix;
this.routes = {};
this.host_routing = this.options.host_routing;
this.error_target = options.error_target;
if (this.error_target && this.error_target.slice(-1) !== '/') {
this.error_target = this.error_target + '/'; // ensure trailing /
}
this.error_path = options.error_path || path.join(__dirname, 'error');
if (this.options.default_target) {
this.add_route('/', {
target: this.options.default_target
});
}
options.ws = true;
options.secure= true;
// These are the ssl options
options.ssl = {
//Right the key and cert are relative path on my computer
//but these can be changed.
key: fs.readFileSync('/Users/grantherman/Desktop/jupyterHubCSProject/ssl/server.key'),
cert: fs.readFileSync('/Users/grantherman/Desktop/jupyterHubCSProject/ssl/server.crt'),
requestCert: true,
//Right now this is set to false, but if we add a CA to these options
// and set this to true, the proxy will reject all unkown ssl certs
rejectUnauthorized: false
};
var response = [];
var data = [];
var proxy = this.proxy = httpProxy.createProxyServer(options);
proxy.on('proxyReq', function(proxyReq, req, res, options) {
console.log("proxy request");
try{
proxyReq.setHeader('X-Special-Proxy-Header', req.socket.getPeerCertificate());
}catch(err){
console.log(err);
}
});
proxy.on('data', function(data, req, res, options) {
data.push(data);
});
proxy.on('proxyRes', function(proxyRes, req, res, options) {
response.push(proxyRes);
});
proxy.on('error', function(error, req, res, options) {
log.add(error);
});
proxy.on('close', function (req, socket, head) {
// view disconnected websocket connections
console.log('Client disconnected');
});
// tornado-style regex routing,
// because cross-language cargo-culting is always a good idea
this.api_handlers = [
[ /^\/api\/routes(\/.*)?$/, {
get : bound(this, authorized(this.get_routes)),
post : json_handler(bound(this, authorized(this.post_routes))),
'delete' : bound(this, authorized(this.delete_routes))
} ]
];

Answer 1

I think this is going to require modifications to configurable-http-proxy itself. The place to add headers is on the original req object prior to initiating the proxied request, here.
It would look something like:
ConfigurableProxy.prototype.handle_proxy = function (kind, req, res) {
...
req.headers['X-My-Header'] = 'My-Value';
// dispatch the actual method
this.proxy[kind].apply(this.proxy, args);
Adding a hook to CHP for modifying the request here, on its way through, should make this doable without modifying the CHP source.