Hangs on easy cleanup when aborted by callback

[Togtja]

I did this

Using CURLOPT_XFERINFODATA and CURLOPT_XFERINFOFUNCTION that send in a struct with an abort bool.
Where the function essentially has this:

    if (meta->abort) {
        return 1;
    }

The download is an sftp/ftp download

During the perform step it return CURLE_ABORTED_BY_CALLBACK

And then it goes to clean up, and it hangs there for about 2 minutes.
curl_easy_cleanup(curl);

I expected the following

That the cleanup would take a very short time, as it happens for most returns.

curl/libcurl version

curl 7.74.0 (Linux) libcurl/7.74.0 OpenSSL/1.1.1g zlib/1.2.11 libssh2/1.9.0_DEV
Release-Date: 2020-12-09
Protocols: dict file ftp ftps gopher http https imap imaps mqtt pop3 pop3s rtsp scp sftp smb smbs smtp smtps telnet tftp
Features: alt-svc AsynchDNS HTTPS-proxy IPv6 Largefile NTLM SSL UnixSockets

I also tried this on 7.73, and same behaviour.

EDIT: Downloaded source-code.zip, instead of 7.74.0.zip, but fix that
EDIT 2: Forgot it ran it on my arm emulator, so fixed the versioning

operating system

Custom OS made using buildroot
Linux Togtja 4.20.0-OS_NAME #1 SMP PREEMPT Wed Dec 16 12:52:46 CET 2020 armv7l GNU/Linux

[Togtja]

Running more tests, it seems to be very accurately stuck for 2 minutes.
From my log

2020-12-16 11:05:58.744416136;I;19831;easy cleanup
2020-12-16 11:07:58.746969519;I;19831;global cleanup

2020-12-16 11:18:47.769916181;I;21527;easy cleanup
2020-12-16 11:20:47.771713916;I;21527;global cleanup

2020-12-16 11:21:42.455842282;I;21915;easy cleanup
2020-12-16 11:23:42.456945581;I;21915;global cleanup

[bagder]

Can you create a small example source code for us that reproduces this?

[Togtja]

Sorry for taking so long time to get back to you with a source code. I released that I actually compiled for ARM, on a custom Linux OS, forgot what it was based of, anyway the "uname -a" is:
Linux Togtja 4.20.0-OS_NAME #1 SMP PREEMPT Wed Dec 16 12:52:46 CET 2020 armv7l GNU/Linux

Where OS_NAME is what we have called the OS, uncertain if I can disclose it, but don't think it matters stoo much
Now, I have made a simple source code for this (in C++, as my C is very rusty):

size_t serverToFile(char* buffer, size_t size, size_t nmemb, void* null) {
    return size * nmemb;
}
struct prog {
    bool abort = false;
};

int progress_callback(prog* meta, curl_off_t _, curl_off_t _1, curl_off_t _2, curl_off_t _3) {
    if (meta->abort) {
        std::cout << "aborted\n";
        return 1;
    }
    return 0;
}

void curl_stuff(CURL* curl) {
    prog p;
    curl_easy_setopt(curl, CURLOPT_NOPROGRESS, false);
    curl_easy_setopt(curl, CURLOPT_XFERINFOFUNCTION, progress_callback);
    curl_easy_setopt(curl, CURLOPT_XFERINFODATA, &p);

    curl_easy_setopt(curl, CURLOPT_USERNAME, "root");
    curl_easy_setopt(curl, CURLOPT_PASSWORD, "root");

    auto url = "ftp://host/file.zip";
    curl_easy_setopt(curl, CURLOPT_URL, url);
    curl_easy_setopt(curl, CURLOPT_PORT, 21);

    curl_easy_setopt(curl, CURLOPT_MAX_RECV_SPEED_LARGE, 10000); // so it not downloaded too fast
    curl_easy_setopt(curl, CURLOPT_BUFFERSIZE, 10000);

    curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, serverToFile);
    curl_easy_setopt(curl, CURLOPT_WRITEDATA, nullptr);

    curl_easy_setopt(curl, CURLOPT_RESUME_FROM_LARGE, 100);
    auto thread1 = std::thread([&p]() {
        std::this_thread::sleep_for(std::chrono::seconds(10));

        p.abort = true;
        return;
    });
    CURLcode res = curl_easy_perform(curl);

    thread1.join();
    std::cout << "response: " << curl_easy_strerror(res) << " for " << url << "\n";
}

int main(int argc, char** argv) {
    curl_global_init(CURL_GLOBAL_DEFAULT);
    CURL* curl = curl_easy_init();
    if (!curl) {
        // Failed to init curl
        curl_global_cleanup();
        std::cout << "failed to init curl, probably memory issues\n";
        return 0;
    }

    curl_stuff(curl);
    auto start = std::chrono::steady_clock::now();
    std::cout << "easy cleanup\n";
    curl_easy_cleanup(curl);
    auto time = std::chrono::steady_clock::now() - start;
    std::cout << "global cleanup easy took:" << std::chrono::duration_cast<std::chrono::seconds>(time).count() << "s\n";
    curl_global_cleanup();
    return 0;
}

Where it logs:

# ./downloader        
aborted
response: Operation was aborted by an application callback for ftp://host/file.zip
easy cleanup
global cleanup easy took:120s

[bagder]

So to be clear, you see this problem with FTP only right? not SFTP .

[Togtja]

I thought it was an FTP and SFTP, but further testing seems to prove it ONLY happens on FTP.

[bagder]

I'm pretty sure my fix in #6337 solves this for you.

[Togtja]

After compiling and running, it does indeed seem to have fixed it 👍
Thank you for such a fast PR!