GOPROXY implementation that allows Go modules to live in subdirectories

Overview

Gosub is a module proxy server for the Go language. It's designed to solve this issue in the go command:

• cmd/go: allow serving module under the subdirectory of git repository

It's a small (~10KB) standalone JS module, ideal for an edge runtime like Cloudflare, Netlify or Vercel. You're also welcome to use my shared server at gosub.moreplease.com.

Gosub currently only supports Github, but if you're interested in using it on Gitlab, Bitbucket, etc, please get in touch.

Installation & usage

You'll need to set up an indirection, AKA "vanity import" for your Go module. This is a <meta name="go-import" ...> tag on your web server that tells the Go toolchain how to find your module.

You can point your vanity import at Gosub in three different ways:

• Deploy the goproxy function, configured to serve a specific Go module;
• Deploy the gosub function, which can dynamically serve any Go module;
• Use my shared server: gosub.moreplease.com

Note that by default, all Go modules are cached by proxy.golang.org; most users will get your module from there, rather than directly from your proxy server.

I'll use my own "UTF-64" module as a worked example:

• Module name: utf64.moreplease.com
• Go source: github.com/more-please/more-stuff/utf64/go

goproxy mode (static config)

type GoproxyConfig = {
  url: string; // Repo URL (currently only Github is supported)
  module?: string; // If set, Go module must match this
  directory?: string; // Subdirectory within the git repo
  tagPrefix?: string; // Prefix for version tags in git (default is "V")
  tagSuffix?: string; // Suffix for version tags in git
};

type GoproxyEnv = {
  GITHUB_TOKEN?: string; // Optional API token for GitHub
}

type Goproxy = (request: Request) => Promise<Response | undefined>

function goproxy(
  base: string,
  config: GoproxyConfig,
  env?: GoproxyEnv
): Goproxy;

The goproxy factory returns an async handler function for a Request (from the Fetch API). The handler returns:

• undefined on requests that don't match its base path;
• 404 on matching but invalid requests;
• GOPROXY data on valid requests.

We call the GitHub API internally to fetch data. Any GitHub error responses are passed through without modification.

To avoid being rate-limited, you should pass a GitHub API token in the GITHUB_TOKEN environment variable. By default we'll try to use the system environment (we try both process.env and Deno.env). To set the environment explicitly, use the env parameter; to prevent any environment lookup, use the value {}.

Example / demo

The root page for my module's domain has the "vanity import" tag:

  <meta
    name="go-import"
    content="utf64.moreplease.com mod https://utf64.moreplease.com/go"
  />

This instructs the Go toolchain to look for a mod (module server) at /go on the same domain. I deploy the goproxy function as follows:

import { goproxy } from "gosub-goproxy";

const handler = goproxy("/go", {
  url: "https://github.com/more-please/more-stuff",
  module: "utf64.moreplease.com",
  directory: "utf64/go",
  tagPrefix: "utf64-go-",
});

The repo has version tags like utf64-go-0.0.11, etc. Goproxy ignores tags without the utf64-go- prefix.

The GOPROXY path to list module versions is $base/$module/@v/list, or in this case:

• utf64.moreplease.com/go/utf64.moreplease.com/@v/list

gosub mode (dynamic config)

function gosubEncode(config: GoproxyConfig): string;
function gosubDecode(path: string): GoproxyConfig | undefined;
function gosub(base: string = "/", env?: GoproxyEnv): Goproxy;

This is a wrapper for goproxy that takes its configuration from the URL. Call the gosubEncode function to convert a config struct into a URL pathname that gosub understands.

Example / demo

Recall the configuration for utf64.moreplease.com:

{
  url: "https://github.com/more-please/more-stuff",
  module: "utf64.moreplease.com",
  directory: "utf64/go",
  tagPrefix: "utf64-go-",
}

This encodes to:

github.com/more-please/more-stuff:d=utf64%2Fgo&m=utf64.moreplease.com&p=utf64-go-;

Note that we can omit module from the config, in which case we get:

github.com/more-please/more-stuff:d=utf64%2Fgo&p=go-;

The only advantage of setting module explicitly is that it blocks nonsense lookups like wrong-module-name/@v/list.

Putting it all together, the @v/list command for utf64.moreplease.com using my gosub server deployed at gosub.moreplease.com is:

• gosub.moreplease.com/github.com/more-please/more-stuff:d=utf64%2Fgo&p=utf64-go-;/utf64.moreplease.com/@v/list

Shared server

As noted above, you're welcome to use the gosub server at gosub.moreplease.com. That site also has a handy web form where you can plug in your repo URL and other config details to generate the proxy path.

All users share the same GitHub API key, so it may get overloaded. However, once your Go module has been cached by proxy.golang.org your users mostly won't be hitting my site directly.

Acknowledgements

Partly inspired by stumbling across GitPkg, which solves a very similar problem in the NPM ecosystem. In particular, I copied the nifty idea of having a shared server that's programmable via URL parameters.