I don’t think the promise chain is really needed here.

I used this script:

import Axios from 'axios'
import OldFS from 'fs'
import { PromiseChain } from '@feather-ink/ts-utils'

const fs = OldFS.promises

const image = process.argv[2]
const destination = `http://${process.argv[3]}/vfs/ota`
const now = process.argv[4] === 'now'
const once = process.argv[4] === 'once'

async function triggerUpdate(): Promise<void> {
  console.log('Uploading new binary')
  const file = await fs.readFile(image)

  await Axios({
    method: 'POST',
    url: destination,
    headers: {
      'Content-Type': 'application/octet-stream',
      'Content-Length': file.byteLength
    },
    data: file
  })
  console.log('Finished uploading')
}

(async () => {
  const updateChain = new PromiseChain()
  console.log(`Watching file '${image}' for changes\nWill upload to '${destination}'!`)
  if (once) {
    await triggerUpdate()
    return
  }
  if (now)
    await updateChain.enqueue(triggerUpdate)
  OldFS.watch(image, async (eventType) => {
    if (eventType !== 'change')
      return
    let succ = false
    do {
      try {
        console.log('Change detected')
        await updateChain.enqueue(triggerUpdate)
        succ = true
      } catch (e) {
        console.error(e)
        console.log('Retrying upload')
      }
    } while (!succ)
    console.log('Upload finished')
  })
})()

Relevent code on the esp:

You can ignore my cpp stuff and just put this in the handler of the stock webserver.

auto ota = vfs->addHandler(makeDirectory("ota"));
        {
          ota->addHandler(makeDirect([](auto &con) {
            if (con.req->method != HTTP_POST)
              return HandlerReturn::UNHANDLED;

            // https://github.com/espressif/esp-idf/tree/master/examples/system/ota/native_ota_example/main
            // https://docs.espressif.com/projects/esp-idf/en/latest/esp32/api-reference/system/ota.html
            auto updatePartition = esp_ota_get_next_update_partition(nullptr);
            if (updatePartition == nullptr)
              return sendError(con,500, "No free ota partition found!");
            esp_ota_handle_t otaHandle;
            auto err = esp_ota_begin(updatePartition, con.req->content_len, &otaHandle);
            if (err != ESP_OK)
              return sendError(con, 500, std::string{"Can't start ota update: "} + esp_err_to_name(err), true);

            int receivedBytes = 0;
            do {
              auto end = httpd_req_recv(con.req, buf.data(), buf.size());
              // ESP_LOGE(TAG, "Received %d", receivedBytes);
              // hexDump("RECV:", buf.data(), end);
              if (end <= 0) {
                esp_ota_abort(otaHandle);
                return sendError(con, 500, "Error receiving", true);
              }
              err = esp_ota_write(otaHandle, buf.data(), end);
              if (err != ESP_OK) {
                esp_ota_abort(otaHandle);
                return sendError(con, 500, std::string{"Error writing: "} + esp_err_to_name(err), true);
              }
              receivedBytes += end;
            } while (receivedBytes < con.req->content_len);

            err = esp_ota_end(otaHandle);
            if (err != ESP_OK)
              return sendError(con, 500, std::string{"Failed to end: "} + esp_err_to_name(err), true);

            err = esp_ota_set_boot_partition(updatePartition);
            if (err != ESP_OK)
              return sendError(con, 500, std::string{"esp_ota_set_boot_partition failed: "} + esp_err_to_name(err), true);
            auto ret = sendOK(con);
            FactoryResetServiceCon().reboot(1000 / portTICK_PERIOD_MS);
            return ret;
          }));
        }

I also used a custom partition table for 2 partitions so that when my program crashes it can just go back to boot the previous version.

Here it is for reference:

partitions.csv

# Name,   Type, SubType, Offset,  Size, Flags
# Note: if you change the phy_init or app partition offset, make sure to change the offset in Kconfig.projbuild
nvs,      data, nvs,     0x011000, 0x006000,
otadata,  data, ota,     0x017000, 0x002000,
phy_init, data, phy,     0x019000, 0x001000,
ota_0,    app,  ota_0,   0x020000, 0x1F0000,
ota_1,    app,  ota_1,   0x210000, 0x1F0000,

Note: This partition table is for a special model of the ESP32 though.

Also another disclaimer: This code does not represent my current coding abilities and may be outdated - it worked well though.

Hahahah. Awesome. Have fun! You just need a simple webserver. The builtin one will do and then you use the ota functions of the ESP IDF.