🐛 [RUM-2721] fix upload retry

[BenoitZugmeyer]

What and why?

This is another attempt to fix #1129

The file upload retry mechanism is incorrect as it tries to upload the same stream instances multiple times. When a FormData (from the form-data package) is built with a used stream, the resulting stream never ends, so the request never ends, resulting in a "socket hang up" error.

Minimal example with form-data only

import FormData from "form-data";
import fs from "fs";

(async () => {
  const stream = fs.createReadStream("./repro.mjs");
  await sendStream(stream); // This works, prints "Start, Data, Data, End"
  await sendStream(stream); // This does not end, prints only "Start"
})();

function sendStream(stream) {
  return new Promise((resolve, reject) => {
    const formData = new FormData();
    formData.append("file", stream);
    formData.on("error", reject);

    formData.resume();

    console.log("Start");

    formData.on("data", () => {
      console.log("Data");
    });
    formData.on("end", () => {
      console.log("End");
      resolve();
    });
  });
}

Example with an actual request

import https from "https";
import FormData from "form-data";
import fs from "fs";

(async () => {
  const stream = fs.createReadStream("./repro.mjs");
  console.log("Sending first request...");
  console.log("First request files:", (await sendRequest(stream)).files); // succeeds
  console.log("Sending second request...");
  console.log("Second request files:", (await sendRequest(stream)).files); // throws "socket hang up"
})();

async function sendRequest(stream) {
  return new Promise((resolve, reject) => {
    const data = new FormData();
    data.append("file", stream);

    const request = https.request("https://httpbin.org/post", {
      method: "POST",
      headers: data.getHeaders(),
    });

    request.on("response", (response) => {
      const chunks = [];
      response.on("data", (chunk) => {
        chunks.push(chunk);
      });
      response.on("end", () => {
        resolve(JSON.parse(Buffer.concat(chunks).toString()));
      });
    });

    request.on("error", reject);

    data.pipe(request);
  });
}

How?

This PR solves the issue by making sure the MultipartPayload does not contain streams but only file paths, and the upload function is then responsible to create streams every time the request is retried.

Note

While this particular behavior happens when form-data is involved, this is not a form-data bug, but really an issue related to our own usage of streams. Without form-data:

• pipe-ing a closed stream to a standard nodejs https.request sends an empty request
• using a closed stream to the fetch nodejs function throws immediately

Review checklist

• Feature or bugfix MUST have appropriate tests (unit, integration)

[datadog-datadog-prod-us1]

Datadog Report

Branch report: benoit/fix-retry-upload
Commit report: 33732e9
Test service: datadog-ci-tests

✅ 0 Failed, 2952 Passed, 0 Skipped, 2m 36.04s Total duration (33.66s time saved)

[bcaudan]

LGTM