Middleware [retired]

You've been asked to add a checksum to all outgoing HTTP responses, as a custom HTTP header.

Task

Write an HTTP handler function, which conforms to Go HTTP middleware conventions, that wraps an HTTP response and modifies it to add the checksum. It should get the response from the HTTP handler it wraps, compute a checksum, add the checksum as an HTTP header called X-Checksum, add a list of headers as X-Checksum-Headers, and send the response. The checksum is the SHA1 hash (hex encoded string) of a "canonical response".

A canonical response is constructed as follows (in pseudo-code):

 stringified http status code + "\r\n" +
 <for each lexicographically sorted (by name) http header>:
     header name + ": " + header value + "\r\n" +
 "X-Checksum-Headers: " + name[0] + ";" + name[1] + ";" + ... + ";" + name[n] + "\r\n\r\n" +
 body

X-Checksum-Headers is a list of the HTTP header names that were encountered in the original response, so that a downstream client can deterministically recreate the canonical response. (HTTP proxies between here and there may add arbitrary headers.)

Once you've written the middleware, run the server (go run middleware.go), and make an HTTP GET request to the root '/' path, using curl or a similar tool. Your answer is the value of the HTTP header X-Checksum in the response. It's a hex-encoded SHA-1 hash, so it will be a 40-character string.

You must write Go code in the place indicated in middleware.go. There is a unit test included in the file middleware_test.go: you can run go test locally to try out your solution before submitting the answer.

Answer: <checksum string goes here>

Notes

There are downloads and instructions for installing go at: https://golang.org/doc/install

This code should work with Go version 1.3 or greater. You can compile and run the middleware.go file by running:

go run middleware.go

from this directory. Make sure the go executable is in your path.

Don't assume the unit tests provided are exhaustive. You should also CURL against the running program.

Take care to get the right concatenation of status, headers, and body — that's a CRLF, or "\r" and "\n", between each component, and 2 of them between the last header and the body.

If you're unfamiliar with writing middlewares in Go, read these blog posts:

• http://www.alexedwards.net/blog/making-and-using-middleware
• https://justinas.org/writing-http-middleware-in-go/

If you get an error message trying to run the server (go run middleware.go), like "listen tcp :8080: bind: address already in use", you can pass a flag to the program invocation instead, to have it listen on a port of your choosing:

go run middleware.go -http 127.0.0.1:7070

If you're stuck and need a hint, base64-decode this string:

VHJ5IHVzaW5nIGh0dHA6Ly9nb2xhbmcub3JnL3BrZy9uZXQvaHR0cC9odHRwdGVzdC8jUmVzcG9uc2VSZWNvcmRlciBpbiB5b3VyIHNvbHV0aW9uLg=

Please gofmt your code before submitting!