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

> Am 16.06.2026 um 17:30 schrieb Stefan Eissing via curl-library <[email protected]>:
> 
> 
> 
>> 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.

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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