common : add minimalist multi-thread progress bar

[angt]

I intentionally kept the bar simple without specifying part numbers (which ultimately don't matter much) the only thing we care about is tracking progress

[ngxson]

Unless I missed something, I can't find any references to std::thread in download.cpp - do we actually using more than 1 thread here?

And AFAIK, httplib::Client is single-threaded.

Edit: nevermind, see next comment

[ngxson]

This looks like a local variable at first glance, but because it's static, that effectively make it a global variable.

IMO we should avoid global variable if possible. Probably a better way is to make a dedicated class to manage threads and implement the mutex as a member of the class.

[ngxson]

Hmm, nevermind, I found it: the multi-threads is launched by std::async which uses a thread pool under the hood.

In this case, I think it's better to introduce a mutex at common_download_file_multiple level (not global/static level)

This is the code where threads are created:

    std::vector<std::future<bool>> futures_download;
    for (auto const & item : urls) {
        futures_download.push_back(std::async(std::launch::async, [bearer_token, offline](const std::pair<std::string, std::string> & it) -> bool {
            return common_download_file_single(it.first, it.second, bearer_token, offline);
        }, item));
    }

The cleaner idea looks like this (pseudo-code):

    std::mutex mutex_progress;
    std::vector<float> progress(urls.size(), 0); // from 0.0 to 100.0
    auto update_progress = [&mutex_progress, &progress](size_t index, float value) {
        std::lock_guard<std::mutex> lock(mutex_progress);
        progress[index] = value;
        // print all progress here
    };

    std::vector<std::future<bool>> futures_download;
    for (auto const & item : urls) {
        futures_download.push_back(std::async(std::launch::async, [bearer_token, offline, update_progress](const std::pair<std::string, std::string> & it) -> bool {
            return common_download_file_single(it.first, it.second, bearer_token, offline, update_progress);
        }, item));
    }