Fetching Go Sub-Packages on Static Sites

One of my favorite things about Go is that there is no central repository for third-party libraries and code. Instead, import paths resemble URLs and the go get command can fetch packages from wherever it is that they are hosted. There is built-in support for popular services like GitHub and Bitbucket, but you can also import packages from any custom URL by adding a simple <meta> tag on the page. This makes it very straightforward to support, even on statically generated sites, and is what allows me to host all of my personal go packages and tools under the namespace willnorris.com/go. What’s not quite as straightforward is how to support fetching sub-packages1 from a custom URL using go get. Here’s how I did it for my static site using nginx.

Fetching From Remote Import Paths

The procedure for fetching from remote import paths is documented as part of the go command, but here’s a basic example. The import path for my image proxy server is willnorris.com/go/imageproxy. When you run go get willnorris.com/go/imageproxy, go fetches https://willnorris.com/go/imageproxy?go-get=1 and discovers the go-import meta tag:

<meta name="go-import" content="willnorris.com/go/imageproxy git https://github.com/willnorris/imageproxy">

That instructs go that any package with the prefix willnorris.com/go/imageproxy can be found in the git repository at https://github.com/willnorris.com/imageproxy. This is the exact package we were looking for, so go will then checkout the git repository. No problem.

Fetching Sub-Packages

My image proxy also contains a command line utility in the cmd/imageproxy sub-package, which I would like to be directly installable by running:

go get willnorris.com/go/imageproxy/cmd/imageproxy

This instructs go to fetch https://willnorris.com/go/imageproxy/cmd/imageproxy?go-get=1 which, until very recently, would return 404 Not Found and result in the error:

package willnorris.com/go/imageproxy/cmd/imageproxy: unrecognized import path "willnorris.com/go/imageproxy/cmd/imageproxy"

That’s because the page doesn’t actually exist; I only have a page on my site for the main imageproxy package, but not all of its sub-packages. What I need is to serve the same meta tag above on the URL for the cmd/imageproxy package. Interestingly, the URL can still return a 404 so long as it contains the meta tag, though that’s not actually how I implemented it.

It’s also worth noting that the meta import for the cmd/imageproxy package URL should not be modified from the main imageproxy package, even though it’s for a different package. That is, it should still read:

<meta name="go-import" content="willnorris.com/go/imageproxy git https://github.com/willnorris/imageproxy">

That’s because the meta import identifies the package prefix that maps to the root of the source control repository. When you request a sub-package, go will see the above meta tag and verify that the prefix specified is a prefix of the requested package. Since it is not an exact match, go will make a second request to https://willnorris.com/go/imageproxy?go-get=1 to confirm that the same meta import information is found there. Since it is, it will then proceed with checking out the source repository and installing the requested package.

Rewriting Requests with Nginx

As I noted above, all of my go packages are located under willnorris.com/go. The simplest way I found to have requests for sub-packages include the same meta include as the top-level packages is to just rewrite the request inside nginx. And in an attempt to ensure that I don’t have multiple URLs with duplicate content, I only do this for URLs that contain ?go-get=1, which go appends to all go get requests. Ideally these requests would also get a X-Robots-Tag: noindex response header, but I don’t think that’s easily possible. Here’s the nginx configuration I use for this:

# Allow go subpackages to be fetchable with `go get`
location ~* ^/go/\w+/.+ {
  if ($arg_go-get) {
    rewrite ^(/go/\w+) $1? last;
  }
}

As noted on http://wiki.nginx.org/IfIsEvil, this is actually one of the few scenarios where using an if directive inside a location context is safe. What we’re doing here is first matching any requests that start with /go, followed by at least two additional path segments: /\w+ which identifies our top-level package, and /.+ that identifies some sub-package. Then we use nginx’s special $arg_name embedded variables to check if the go-get query parameter is set, and if so, rewrite the request to the page for the top-level package. Otherwise, nginx will return a 404.

This now means that https://willnorris.com/go/imageproxy/cmd/imageproxy (without the go-get parameter) will properly return a 404, but https://willnorris.com/go/imageproxy/cmd/imageproxy?go-get=1 will be rewritten to return the same response as https://willnorris.com/go/imageproxy, meta include and all.

  1. Go doesn’t technically have the notion of “sub-packages” in a formal sense, though I’ll use that term here to refer to packages whose import paths are prefixed with the import path of another package. For the most part, there is nothing special about packages that happen to share a common prefix. The only real exceptions are the rules for importing internal packages introduced in go1.4, and the fact that packages that share a common prefix may be installed by go get together, as noted in this post.