Send Apple push notification from a Go appengine site

Question

I'm trying to send an Apple push notification from a Go appengine site. I'm using the apns2 library as follows:

cert, err := certificate.FromPemFile(pemFile, "")
if err != nil {
    log.Fatalf("cert error: %v", err)
}
client := apns2.NewClient(cert).Development()
n := &apns2.Notification{...}
if res, err := client.Push(n); err != nil { ... }

On a local development server, it works fine; but in production I'm seeing:

Post https://api.development.push.apple.com/3/device/995aa87050518ca346f7254f3484d7d5c731ee93c35e3c359db9ddf95d035003:
dial tcp: lookup api.development.push.apple.com on [::1]:53: dial udp [::1]:53: socket: operation not permitted

It looks like appengine expects you to use its own urlfetch library when sending outbound requests, so I tried setting the underlying HTTPClient to use that:

client.HTTPClient = urlfetch.Client(ctx)

However, the response from the Apple server is now

@@?HTTP/2 client preface string missing or corrupt. Hex dump for received bytes: 504f5354202f332f6465766963652f393935616138373035

I believe the problem is that Apple push notifications require HTTP/2, but urlfetch only implements HTTP/1.1.

How do I solve this problem? Is there a way for an appengine app to send an HTTP/2 request?

Answer 1

This would require going through the App Engine Sockets API. The documentation states:

Libraries that can accept a net.Conn should work without modification.

You can get a net.Conn from the appengine/socket package and pass it to a lib that will accept one, but in the case of apns2 it doesn't allow you to do this. However another user has submitted a pull request to the apns2 project that adds a distinct GAEClient which can use App Engine sockets.

As of right now it looks like the commits still have not been pulled into the master branch, however you could still merge these updates manually into your own source tree as a workaround for now.