The subtleties of API contracts, or how enabling HTTP/2 broke Go clients

August 10, 2018

A few weeks ago, having finally retired some old crypto needed to support pre-1.2 TLS, we realized we had an opportunity to upgrade OpenSSL on the servers terminating our HTTPS connections, which gave us the option to add support for HTTP/2 on api.stripe.com.

Given that HTTP/2 is pretty obviously the direction that internet communication is going, we turned it on enthusiastically. Every client still had the option to negotiate the old style of HTTP if it didn’t support HTTP/2, so the feature seemed to be strictly an enhancement, and one with low likelihood of risk.

It took a few days, but we eventually had a strange bug report opened on stripe-go, the official Go language library. To make user integrations more robust, our libraries will idempotently retry API requests that look like they failed due to an intermittent problem like a read/write timeout, and it was inside this retry mechanism that the problem presented itself. The first request would execute as expected, but subsequent retries would send invalid bodies which our servers would reject.

HTTP/2 in Go

To understand the problem’s true nature, we have to know a little bit about the history of HTTP/2 in Go. Go picked up HTTP/2 support impressively quickly with an initial low-level implementation landing in golang.org/x/net/http2. That package was then leveraged to retrofit HTTP/2 directly into the existing APIs of the standard library’s net/http for both clients and servers, and shipped with Go 1.6.

This left Go with an interesting property – one which as far as I know is still unique among today’s programming languages. HTTP clients in any Go program compiled with 1.6+ can support HTTP/2 automatically as long as a server can offer it, and with no changes to user code. For over two years since Go 1.6’s release Stripe’s API couldn’t offer HTTP/2, so every Go program happily fell back to speaking plain old HTTP(S).

The upgrade

But then the day came when we turned on HTTP/2, and exactly as intended, our Go clients out in the wild automatically started using it. This is also when the bug made its appearance: every initial request that stripe-go made continued to work, but subsequent ones made by the library’s retry mechanism were broken.

It took some digging to understand why. The retry code was written to be memory efficient so that it would initialize just one http.Request, then reuse that request in a loop until it either got back a successful response, or gave up:

req, err := s.NewRequest(method, path, key, "application/x-www-form-urlencoded", params)
...

for retry := 0; ; {
    res, err = s.HTTPClient.Do(req)
    ...


    // Break on success or if we've retried too many times
    // already.
    if !s.shouldRetry(err, res, retry) {
        break
    }
}

The package’s documentation didn’t comment on whether it was safe to reuse an http.Request 1, but pre-HTTP/2, it was. With HTTP/2, it’s now only sometimes safe. When a request’s body is nil (say for a GET request) the struct can be reused safely, but if not, there’s a danger that a Goroutine elsewhere could still be using it, and the struct can only be reused by waiting for that Goroutine to close it.

Contracts beyond function signatures

Go had added HTTP/2 support in a way that didn’t change the APIs in net/http, but that doesn’t necessarily mean that the package’s contract wasn’t changed. Hyrum’s Law was coined to define this problem:

With a sufficient number of users of an API, it does not matter what you promise in the contract: all observable behaviors of your system will be depended on by somebody.

Go had previously not defined whether it was safe to reuse a request, but it was. Go 1.6 still didn’t define whether it was safe to reuse a request, but it wasn’t, and in the meantime users started to implicitly depend on the behavior. The slight shift in contract is about as subtle as things get, but it was a change in contract nonetheless, and demonstrates how it’s possible to introduce a breaking change even if every function signature stays the same.

In case you’re curious, we fixed the problem with a bit of a hammer by creating a new reader for the same sequence of bytes with every request and setting its Body directly:

{
    ...

    if body != nil {
        // We can safely reuse the same buffer that we used
        // to encode our body, but return a new reader to
        // it every time so that each read is from the 
        // beginning.
        reader := bytes.NewReader(body.Bytes())

        req.Body = nopReadCloser{reader}
    }

    res, err = s.HTTPClient.Do(req)
    ...
}

//
// And elsewhere ...
//

// nopReadCloser's sole purpose is to give us a way to turn
// an `io.Reader` into an `io.ReadCloser` by adding a no-op
// implementation of the `Closer` interface.
//
// (We need this because `http.Request`'s `Body` takes an
// `io.ReadCloser` instead of a `io.Reader`.)
type nopReadCloser struct {
	io.Reader
}

func (nopReadCloser) Close() error { return nil }

Lastly, I’ll note that we also got lucky with this one. The retry mechanic in Go had been added somewhat recently, and wasn’t turned on by default, so few users were broken by HTTP/2 being enabled.

1 It technically does today, but it’s still somewhat difficult to find.

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The subtleties of API contracts, or how enabling HTTP/2 broke Go clients

Published
August 10, 2018

Find me on Twitter at @brandur.

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