proxyReq.setHeader in 'proxyReq' event callback throw 'Cannot set headers after they are sent to the client'

[anson0370]

I did some debug and figured out what's happened.
In my case, this is because I was using curl to post a request with a body size bigger than 1024. curl sends a http request with header expect: 100-continue before sends the post request.

Looks like the problem is in Node http module:

    // _http_client.js: 221
    if (this.getHeader('expect')) {
      if (this._header) {
        throw new ERR_HTTP_HEADERS_SENT('render');
      }
      // here set headers for the request, so when we call proxyReq.setHeader later, it throw an exception
      this._storeHeader(this.method + ' ' + this.path + ' HTTP/1.1\r\n',
                        this[outHeadersKey]);
    }

Maybe we can check is here a expect header before fire proxyReq event, or ignore the setHeader invoke?

[skleeschulte]

I found a workaround that might also help in your case: #1219 (comment)

[jsmylnycky]

Fixed as part of #1447