Mangling of binary response bodies

[na--]

While benchmarking my fixes to #1041, I found another issue that has been lurking in k6 for a similar amount of time now 🤦‍♂️ Specifically, it has to do with the buffer pools that are used for storing response bodies when the specified responseType is binary: https://github.com/loadimpact/k6/blob/607b63c4400c8b6852c7f51a0a6151f0bb8ec6b7/lib/netext/httpext/request.go#L367-L379

Notice how we get a bytes.Buffer from a the buffer pool BPool, reset it, read the response body into it, save the result as binary or a string to be exported to the JS script and continue. The buffer is then returned back to the pool (defer state.BPool.Put(buf)).

The problem with that piece of code is that buf.Bytes() doesn't copy the data - it references the same underlying byte slice that the buffer uses. So, if a buffer gets reused before the current iteration has ended (which can easily happen, especially with http.batch()), it can still have references pointing to it. So, the old response body may get mangled... buf.String() doesn't have the same problem, since Go strings are immutable: https://play.golang.org/p/RLoDrXnVRiK

Here's a script demonstrating the issue:

import http from 'k6/http';

const testServer = "https://httpbin.test.loadimpact.com"
const batchSize = 150;

function bin2String(array) {
    var result = "";
    for (var i = 0; i < array.length; i++) {
        result += String.fromCharCode(array[i]);
    }
    return result;
}

export default function () {
    let requests = [];
    for (let i = 0; i < batchSize; i++) {
        requests.push(["GET", `${testServer}/get?req=${i}`, null, { responseType: "binary" }]);
    }

    let responses = http.batch(requests);

    for (let i = 0; i < batchSize; i++) {
        try {
            let reqNumber = responses[i].json().args.req;
            if (i != reqNumber) {
                console.log(`[${__VU}:${__ITER}:${i}] got ${reqNumber}, expected ${i}...`)
            }
        } catch (err) {
            console.log(`[${__VU}:${__ITER}:${i}] Error: \n${err}\n\nBody: \n` + bin2String(responses[i].body));
        }
    }
};

And while the buffer pools have been present since k6 v0.17.2 (added with commit b002f2c), the actual issue only occurred with the addition of binary response bodies in k6 v0.23.0.