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: Wed, 17 Jun 2026 15:34:33 +0200

> Am 16.06.2026 um 17:30 schrieb Stefan Eissing via curl-library <curl-library_at_lists.haxx.se>:
>
>
>
>> 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.

And propose to fix this via https://github.com/curl/curl/pull/22066

Bryan, if you could verify that this matches your expectations, I'd appreciate it.

- Stefan

>
> - 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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>
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Received on 2026-06-17