gRPC on workers

index.js

// Turn on gRPC for domain using 
// https://developers.cloudflare.com/support/network/understanding-cloudflare-grpc-support/#enable-grpc, then CF 
// rewrites grpc requests to grpc-web (like a reverse envoy filter, see https://blog.cloudflare.com/road-to-grpc/#converting-to-http-1-1)
// which can be handled by workers. Does not work with workers.dev.
export default {
	async fetch(request, env, context) {
		// Use a stream so CF doesn't add content-lenght
		// which would prevent grpc-web -> grpc conversion.
		const { readable, writable } = new TransformStream();
		let writer = writable.getWriter();
		let enc = new TextEncoder();
		let responseString = enc.encode(`request content type was ${request.headers.get('content-type')}`);
		context.waitUntil(
			writer.write(envelope(new Uint8Array([0x0a /* field 1 varint tag */, ...uVarInt(responseString.length), ...responseString])))
		);
		context.waitUntil(
			new Promise(resolve => setTimeout(resolve, 5000)).then(() => {
				// Note: no space between header and value, CF grpc-web -> grpc 
				// conversion inserts one (but really shouldn't).
				writer.write(envelope(enc.encode("Grpc-Status:9\r\nGrpc-Message:ohoh\r\n"), true))
			}).then(() => writer.close())
		);
		return new Response(readable, {
				status: 200,
				headers: new Headers({
					'content-type': 'application/grpc-web',
				})
			})
	},
}

// poor mans varint encoding for pb message
function uVarInt(value) {
	let buf = [];
	while (value > 0x7f) {
		buf.push((value & 0x7f) | 0x80);
		value = value >>> 7;
	}
	buf.push(value);
	return buf;
}

// write a grpc-web envelope
function envelope(bytes, trailers = false) {
	let size = Uint8Array.of(
		(bytes.length & 0xff000000) >> 24,
		(bytes.length & 0x00ff0000) >> 16,
		(bytes.length & 0x0000ff00) >> 8,
		(bytes.length & 0x000000ff) >> 0);
	return new Uint8Array([trailers ? 0x80 : 0x00, ...size, ...bytes]);
}