[workers] Add github-discord-notifier (#1150)

Start migrating workers to the monorepo. This PR brings in
github-discord-notifier.

infra/workers/.gitignore

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+node_modules/
+.DS_Store
+.wrangler/
+
+# Workers mostly have dev dependencies, and usually only wrangler, so exact
+# version details and the resulting `yarn.lock` is unnecessary noise.
+#
+# If we need to pin some runtime dependency, we can pin it to an exact version
+# in the package.json itself.
+yarn.lock

infra/workers/.prettierrc.json

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+{
+    "tabWidth": 4
+}

infra/workers/README.md

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+# Cloudflare Workers
+
+Source code for our [Cloudflare
+Workers](https://developers.cloudflare.com/workers/).
+
+Each worker is a self contained directory with its each `package.json`.
+
+## Deploying
+
+* Switch to a worker directory, e.g. `cd github-discord-notifier`.
+
+* Install dependencies (if needed) with `yarn`
+
+* Login into wrangler (if needed) using `yarn wrangler login`
+
+* Deploy! `yarn wrangler deploy`
+
+Wrangler is the CLI provided by Cloudflare to manage workers. Apart from
+deploying, it also allows us to stream logs from running workers by using `yarn
+wrangler tail`.
+
+## Creating a new worker
+
+Copy paste an existing one. Unironically this is a good option because
+Cloudflare's template has a lot of unnecessary noise, but if really do want to
+create one from scratch, use `npm create cloudflare@latest`.
+
+To import an existing worker from the Cloudflare dashboard, use
+
+```sh
+npm create cloudflare@2 existing-worker-name -- --type pre-existing --existing-script existing-worker-name
+```

infra/workers/github-discord-notifier/package.json

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+{
+    "name": "github-discord-notifier",
+    "private": true,
+    "devDependencies": {
+        "@cloudflare/workers-types": "^4.20240314.0",
+        "typescript": "^5",
+        "wrangler": "^3"
+    }
+}

infra/workers/github-discord-notifier/src/index.ts

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+/**
+ * Forward notifications from GitHub to Discord.
+ *
+ * This worker receives webhooks from GitHub, filters out the ones we don't
+ * need, and forwards them to a Discord webhook.
+ *
+ * [Note: GitHub specific Discord Webhooks]
+ *
+ * By appending `/github` to the end of the webhook URL, we can get Discord to
+ * automatically parse the payload sent by GitHub.
+ * https://discord.com/developers/docs/resources/webhook#execute-githubcompatible-webhook
+ *
+ * Note that this doesn't work for all events. And sadly, the events it doesn't
+ * work for get silently ignored (Discord responds with a 204).
+ * https://github.com/discord/discord-api-docs/issues/6203#issuecomment-1608151265
+ */
+export default {
+    async fetch(request: Request, env: Env) {
+        return handleRequest(request, env.DISCORD_WEBHOOK_URL);
+    },
+} satisfies ExportedHandler<Env>;
+
+interface Env {
+    DISCORD_WEBHOOK_URL: string;
+}
+
+const handleRequest = async (request: Request, targetURL: string) => {
+    const requestBody = await request.text();
+    let sender = JSON.parse(requestBody)["sender"]["login"];
+    if (sender === "cloudflare-pages[bot]" || sender === "CLAassistant") {
+        // Ignore pings from CF bot
+        return new Response(null, { status: 200 });
+    }
+
+    const response = await fetch(targetURL, {
+        method: request.method,
+        headers: request.headers,
+        body: requestBody,
+    });
+
+    const responseBody = await response.text();
+    const newResponse = new Response(responseBody, {
+        status: response.status,
+        statusText: response.statusText,
+        headers: response.headers,
+    });
+
+    return newResponse;
+};

infra/workers/github-discord-notifier/tsconfig.json

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+{ "extends": "../tsconfig.base.json", "include": ["src/**/*.ts"] }

infra/workers/github-discord-notifier/wrangler.toml

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+name = "github-discord-notifier"
+main = "src/index.ts"
+compatibility_date = "2024-03-14"
+
+[vars]
+# Added as a secret via the Cloudflare dashboard
+# DISCORD_WEBHOOK_URL = "https://discord.com/api/webhooks/{webhook.id}/{webhook.token}/github"

infra/workers/tsconfig.base.json

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+{
+    /* A shared TSConfig for use by TypeScript Cloudflare Workers */
+    /* TSConfig docs: https://aka.ms/tsconfig.json */
+    "compilerOptions": {
+        /* tsc is used for by us for type checking, not compilation (the
+           Cloudflare workers runtime natively supports TypeScript) */
+        "noEmit": true,
+
+        /* The Workers runtime supports the latest and greatest */
+        /* https://developers.cloudflare.com/workers/reference/languages/#javascript--typescript */
+        "lib": ["esnext"],
+        "target": "esnext",
+        "module": "esnext",
+
+        /* Types that are implicitly available */
+        /* https://www.npmjs.com/package/@cloudflare/workers-types */
+        "types": ["@cloudflare/workers-types"],
+
+        /* Tell TypeScript how to lookup the file for a given import */
+        "moduleResolution": "node",
+
+        /* Speed things up by not type checking `node_modules` */
+        "skipLibCheck": true,
+        /* Require the `type` modifier when importing types */
+        "verbatimModuleSyntax": true,
+        /* Enable importing .json files */
+        "resolveJsonModule": true,
+
+        /* strict and then some */
+        "strict": true,
+        "noImplicitReturns": true,
+        "noUnusedParameters": true,
+        "noUnusedLocals": true,
+        "noFallthroughCasesInSwitch": true,
+        "noUncheckedIndexedAccess": true,
+        "exactOptionalPropertyTypes": true
+    }
+}