Re: transactions stall after March 6 commit to improve wakeup and wait code in multi

From: Stefan Eissing via curl-library <curl-library_at_lists.haxx.se>
Date: Tue, 16 Jun 2026 17:30:49 +0200

> Am 16.06.2026 um 16:48 schrieb Bryan Henderson via curl-library <curl-library_at_lists.haxx.se>:
>
>>>> Any chance you can for example modify an existing example source code to
>>>> reproduce this behavior? Then we would have a good reproducer to work with!
>>
>> I'll see what I can do.
>
> This might be easier than I thought to reproduce.
>
> I use an easy handle with no options set whatsoever, not even a URL.
> Apparently, this is legal and just means performing the transaction is a
> no-op. At least that's what Curl did before that March commit.
>
> But following the commit, if I add that handle to a multi manager,
> 'curl_multi_fdset' says there is a file descriptor which I have to wait for to
> become ready before that transaction can proceed, and that file descriptor
> never becomes ready.

That file descriptor is the multi handles "wakeup" socketpair/eventfd and it
is only handed out while the multi handle has transfers to process.

Your case is rather a surprise which we did not account for. I agree that it
does not make sense to add the wakeup fd if there are no other sockets to wait for.

I opened https://github.com/curl/curl/issues/22050 to track this.

- Stefan

>
> I don't know where to find a published example of using curl_multi_fdset to
> use as a reproducer, but I made a very simple one that must look a lot like
> any such example. Code is below.
>
> When I run this against old libcurl, it prints, "There are no file descriptors
> to wait for" and exits. With new libcurl, it prints, "libcurl supplied a file
> descriptor [...] waiting now ..." and then hangs.
>
> -------------------------------------------------------------------------------
> #include <assert.h>
> #include <stdio.h>
> #include <sys/select.h>
>
> #include <curl/curl.h>
> #include <curl/easy.h>
> #include <curl/multi.h>
>
> int
> main(int , const char ** ) {
>
> CURLMcode rc;
>
> printf("Hello world.\n");
>
> CURL * const curlSessionP = curl_easy_init();
>
> assert(curlSessionP);
>
> CURLM * const curlMultiP = curl_multi_init();
>
> assert(curlMultiP);
>
> rc = curl_multi_add_handle(curlMultiP, curlSessionP);
>
> assert(rc == CURLM_OK);
>
> fd_set readFdSet, writeFdSet, exceptFdSet;
> int maxFd;
>
> rc = curl_multi_fdset(curlMultiP,
> &readFdSet, &writeFdSet, &exceptFdSet,
> &maxFd);
>
> assert(rc == CURLM_OK);
>
> if (maxFd == -1)
> // With libcurl 8.19, this is the case.
> fprintf(stderr, "There are no file descriptors to wait for\n");
> else {
> fprintf(stderr, "libcurl supplied a file descriptor to "
> "wait for (maxFd=%u). Waiting now ...\n", maxFd);
>
> pselect(maxFd + 1, &readFdSet, &writeFdSet, &exceptFdSet,
> NULL /* timeout */, NULL /* signal mask */);
>
> // Problem: This 'pselect never completes
>
> fprintf(stderr, "A file descriptor is ready.\n");
> }
>
> return 0;
> }
> -------------------------------------------------------------------------------
>
>
>
> --
> Bryan Henderson Omak, Washington
> --
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Received on 2026-06-16