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

> Am 16.06.2026 um 16:48 schrieb Bryan Henderson via curl-library <[email protected]>:
> 
>>>> 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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