libcurl does not finish CURLOPT_UPLOAD request after a connection lost without data tranfer

[oleg-jukovec]

I did this

I am trying to use the libcurl to communicate with etcd HTTP stream API. The idea of the API to send and receive JSON messages (transferred with https://en.wikipedia.org/wiki/Chunked_transfer_encoding). Everything functions properly until the connection with etcd is unexpectedly lost. The libcurl does not perform/finishes a request with an error until I actually send some data.

The reproducer:

#include <curl/curl.h>

size_t read_callback(char *ptr, size_t size, size_t nmemb, void *userdata) {
	/* Just ensure that it works:
	 * static int i = 0;
	 * printf("read_callback %d\n", i++);
	*/

	/*
	 *  You could uncomment the line to ensure that a data transfer helps to perform
	 *  a request after a connection lost.
	 */
	/* if (0) */
	{
		return CURL_READFUNC_PAUSE;
	}
	ptr[0] = '\n';
	return 1;
}

int progress_callback(void *clientp,
                      double dltotal,
                      double dlnow,
                      double ultotal,
                      double ulnow) {
	/*
	 * Ensure that the continue does not actually help without a
	 * data transfer.
	 */
	CURL *curl = (CURL*) clientp;
	curl_easy_pause(curl, CURLPAUSE_CONT);
	return 0;
}

int main() {
	CURL *curl = curl_easy_init();
	if(curl) {
  		/* We want to use our own read function. */
  		curl_easy_setopt(curl, CURLOPT_READFUNCTION, read_callback);

		/* It will help us to continue the read function. */
		curl_easy_setopt(curl, CURLOPT_XFERINFOFUNCTION, progress_callback);
		curl_easy_setopt(curl, CURLOPT_XFERINFODATA, curl);
		curl_easy_setopt(curl, CURLOPT_NOPROGRESS, 0L);

		/* It will help us to ensure that keepalive does not help. */
		curl_easy_setopt(curl, CURLOPT_TCP_KEEPALIVE, 1L);
		curl_easy_setopt(curl, CURLOPT_TCP_KEEPIDLE, 1L);
		curl_easy_setopt(curl, CURLOPT_TCP_KEEPINTVL, 1L);

  		/* Enable uploading. */
		curl_easy_setopt(curl, CURLOPT_CUSTOMREQUEST, "POST");
  		curl_easy_setopt(curl, CURLOPT_UPLOAD, 1L);
 
  		curl_easy_setopt(curl, CURLOPT_URL, "http://localhost:2379/v3/watch");
		curl_easy_perform(curl);
	}
}

To reproduce:

1. Download etcd server.
2. Extract it and run: ./etcd
3. Save the reproducer as test.c
4. Compile and run the reproducer: gcc test.c -lcurl && ./a.out
5. Close etcd server: kill -SIGKILL $(pidof etcd)
6. See that the reproducer hangs.

I would happily to use an another server to reproduce the error instead of etcd if you help me choose a relevant software..

I expected the following

I expected that the request finishes (with an error) due to a lost connection. So the reproducer will finish too (it actually happens with a data transfer, see comments in read_callback.

curl/libcurl version

curl 8.2.1 (x86_64-pc-linux-gnu) libcurl/8.2.1 OpenSSL/3.1.2 zlib/1.3 brotli/1.0.9 zstd/1.5.5 libidn2/2.3.4 libpsl/0.21.2 (+libidn2/2.3.4) libssh2/1.11.0 nghttp2/1.55.1
Release-Date: 2023-07-26

operating system

Archlinux

Linux host 6.4.11-arch2-1 #1 SMP PREEMPT_DYNAMIC Sat, 19 Aug 2023 15:38:34 +0000 x86_64 GNU/Linux

[bagder]

I can reproduce. I suspect it is related to the pause state. I have not figured it out yet.

What's curios in this scenario is that when the connection goes down and curl reads from the socket:

curl/lib/cf-socket.c

Line 811 in 3f283e9

nread = sread(ctx->sock, buf, len);

... it gets a -1 back and EAGAIN in errno.

If I #ifdef out the sending from the reproducer so that it sends nothing and just waits for data to arrive (that never comes), it instead properly immediately reads a 0 on the connection break and exits as it should.

[icing]

I have a fix in #11756 for http2 where the HOLD was not cleared for a closed stream and transfer happily kept at it. Maybe this is in the same area?

[bagder]

I have mostly figured it out, and it is not like #11756 since this happens with plain old HTTP/1.

I'm going to work on writing a test case that reproduces then submit a PR.

[oleg-jukovec]

Thank you for the fix!