Merge headers

[lilouartz]

These seems to be an issue, where if I send headers using Fastify, then anything that gets added by Remix is overriden, e.g.

Here I am adding link header:

await app.register(async (instance) => {
  instance.decorateRequest('cspNonce', null);

  instance.removeAllContentTypeParsers();

  instance.addContentTypeParser('*', (_request, payload, done) => {
    done(null, payload);
  });

  const handler = createRequestHandler({
    build: serverBuild,
    getLoadContext(request) {
      return {
        nonce: request.cspNonce,
        session: request.session as unknown as Session,
        visitor: request.visitor,
      };
    },
    mode: 'production',
  });

  instance.all('*', async (request, reply) => {    
    const links = getLinks(request.url);

    reply.header(
      'Link',
      [
        '</fonts/gabarito/Gabarito-VariableFont.woff2>; rel=preload; as=font; crossorigin',
        ...links,
      ].join(', '),
    );

    try {
      return handler(request, reply);
    } catch (error) {
      captureException({
        error,
        extra: {
          url: request.url,
        },
        message: 'could not render request',
      });

      return replyWithErrorPage(reply, error);
    }
  });
});

but the loaded route also send link header, which overrides my value from the above.

I would expect headers such as link to be merged.

[lilouartz]

Maybe something like this:

const sendRemixResponse = async <Server extends HttpServer>(
  reply: FastifyReply<Server>,
  nodeResponse: Response,
): Promise<void> => {
  reply.status(nodeResponse.status);

  for (const [key, values] of nodeResponse.headers.entries()) {
    // https://github.com/mcansh/remix-fastify/issues/405#issue-2471580896
    if (key.toLowerCase() === 'link') {
      const existingLinkValues = reply.getHeader('link');

      if (typeof existingLinkValues === 'string') {
        // We prepend Fastify headers rather than appending them because
        // Remix's `Link` header is more likely to contain route-specific
        // links, e.g. images. Meanwhile, Cloudflare appear to limit
        // the number of links that it extracts to Early Hints.
        reply.headers({ [key]: values + ', ' + existingLinkValues });
      } else {
        reply.headers({ [key]: values });
      }
    } else {
      reply.headers({ [key]: values });
    }
  }

  if (nodeResponse.body) {
    const stream = responseToReadable(nodeResponse.clone());

    return reply.send(stream);
  }

  return reply.send(await nodeResponse.text());
};

[mcansh]

hey @lilouartz thanks for info i'll poke around soon

[mcansh]

0.0.0-experimental-1d46df2 looks like there's some edge cases where headers are added multiple times as im not limiting it to just Link headers, but should be good for a test :)

[lilouartz]

If I am reading this correctly, I am not sure if this desired behavior beyond Link example (and maybe select others), e.g. I wouldn't want x-robots-tag to be overriden.