Accept GZIP-encoded request body as-is in Firebase Functions - node.js

Question

In a Firebase Function, I'm trying to accept a large string and store it.
exports.store = functions.https.onRequest(async (req, res) => {
const body : string = req.body;
await store(body);
return req.send(...);
});
I have found that using GZIP can improve performance and reduce file size by about 3-4x.
The problem is that if a client sends a GZip-encoded string with content encoding like so:
header("Content-Encoding", "gzip")
Then the call to req.body will 'automagically' decode the string to UTF-8. This both drastically reduces performance and requires me to again compress the string to efficiently store it in the server.
If I don't specify a content encoding then the endpoint will work as expected (i.e. it won't decompress the string and store it as-is, and run faster as a result). The problem is then I can't verify the user has sent a gzip-compressed string instead of a UTF-8 one, unless I implement some hacks to check for patterns in the string.
I am sure there is some simple solution, e.g.
functions.dontInflateTheRequestPleaseThanks();
exports.store = functions.https.onRequest(async (req, res) => {
if (req.headers["content-encoding"] !== "gzip") return res.status(415).send("gzip please");
const body : string = req.body;
await store(body);
return req.send(...);
});

Answer 1

The trick is to not pass Content-Encoding=gzip so express (the underlying framework) won't inflate (decompress) the request. Then to identify the request as a gzip, pass ContentType=application/gzip.
if (req.headers["content-encoding"] === "gzip") {
res.status(415).send("Don't specify gzip as the content-encoding. This trips up firebase.");
return;
}
if (req.headers["content-type"] !== "application/gzip") {
res.status(415).send("must be compressed using gzip");
return;
}
// Then body is guaranteed to not be fiddled with.
const body = req.body;

Answer 2

You have some middleware which fills req.body already and auto-decompresses it along the way. Instead, you can collect the body yourself.
Caution: The middleware that already fills req.body uses the same on("data")/on("end") technique as the code below, therefore you must make sure that this code is not executed together with the body-filling middleware.
To ensure this, pass an entire express app to functions.https.onRequest:
exports.store = functions.https.onRequest(express().use(async (req, res, next) => {
if (req.headers["content-encoding"] !== "gzip")
return res.status(415).send("gzip please");
var buffers = [];
req.on("data", function(data) {
buffers.push(data);
}).on("end", async function() {
await store(Buffer.concat(buffers));
res.send(...);
});
}));