Failed to convert header keys to lower case due to field name conflict: accept-encoding

[oyvindeh]

I get this error when I use express-http-proxy >= 0.9.1:

Failed to convert header keys to lower case due to field name conflict: accept-encoding
Error: Failed to convert header keys to lower case due to field name conflict: accept-encoding
    at foo/node_modules/nock/lib/common.js:210:13
    at foo/node_modules/lodash/lodash.js:4332:15
    at baseForOwn (foo/node_modules/lodash/lodash.js:2731:24)
    at Function.forOwn (foo/node_modules/lodash/lodash.js:12042:24)
    at Object.headersFieldNamesToLowerCase (foo/node_modules/nock/lib/common.js:207:5)
    at RequestOverrider (foo/node_modules/nock/lib/request_overrider.js:111:30)
    at new OverriddenClientRequest (foo/node_modules/nock/lib/intercept.js:240:23)
    at foo/node_modules/nock/lib/intercept.js:363:13
    at Object.module.request (foo/node_modules/nock/lib/common.js:137:14)
    at runProxy (foo/node_modules/express-http-proxy/index.js:87:43)
    at invokeCallback (foo/node_modules/raw-body/index.js:262:16)
    at done (foo/node_modules/raw-body/index.js:251:7)
    at IncomingMessage.onEnd (foo/node_modules/raw-body/index.js:307:7)
    at emitNone (events.js:86:13)
    at IncomingMessage.emit (events.js:185:7)
    at endReadableNT (_stream_readable.js:975:12)

I see the release notes for 0.9.1 says "Set 'Accept-Encoding' header to match bodyEncoding". Perhaps something goes wrong in relation to this?

[monkpow]

Sounds like it. Can you give me a code snippet that triggers this issue, and I'll fix the bug. Thanks!

[btmurrell]

@monkpow , @oyvindeh I'm hitting this issue too. I cannot tell you how to reproduce it -- express (or possibly helmet which I'm using) is loading up my req.headers by some default behavior... however, i do see where the problem in express-http-proxy is happening.

This may be a suitable fix for express-http-proxy index.js line ~83:

      if (bodyEncoding(options)) {
        var acceptEncodingHeader = reqOpt.headers[ 'Accept-Encoding'.toLowerCase() ];
        if (acceptEncodingHeader) {
          reqOpt.headers[ 'Accept-Encoding'.toLowerCase() ] = acceptEncodingHeader + ', ' + bodyEncoding(options);
        } else {
          reqOpt.headers[ 'Accept-Encoding' ] = bodyEncoding(options);
        }
      }

Edit: That is short-sighted; does not take into account unique values in this header. Various ways to do this; lodash would be my toolkit, but it does not look like you are including it.

[monkpow]

@oyvindeh @btmurrell I'm tentative about fixing without being able to see the problem in action. If your hunch is right, you should be able to skip this chunk of code by adding

reqBodyEnconding: null

to your options.

[oyvindeh]

I'm sorry, I haven't had time to write a snippet that triggers the issue. Hopefully, I'll be able to do so next week.

[oyvindeh]

I have problems reproducing this outside of the environment where I get the error (and I cannot share the full code). But I can confirm that reqBodyEncoding: null does indeed work around the issue.

[monkpow]

@oyvindeh I did find a relevant bug in the code, which is patch here 861b620. This fix is in the most recent version (0.10.1) published to npm.

Let me know if you continue to have issues. Thanks for the report and clarifications so far.