Re: libcurl 8.16.0 spawning large number of getaddrinfo threads?

[Stefan Eissing via curl-library]

> Am 17.10.2025 um 01:45 schrieb Vadim Grinshpun via curl-library 
> <curl-library@lists.haxx.se>:
> 
> Hi folks,
> I'm working on a project that makes use of libcurl.
> After upgrading libcurl from 8.14.1 to 8.16.0, I've noticed an unexpected 
> behavior change.
> Specifically, I'm observing a large number of curl_thread_create_thunk 
> threads, all of which 
> appear to have a backtrace like the one shown below. 
> 
> Since the only thing that changed is the libcurl version,
> I want to double-check a few things:
> - are there any known issues in 8.16.0 that might have symptoms like this?
> - has anything changed between 8.14.1 and 8.16.1 that might lead to this 
> behavior?
> - is it possible to misuse libcurl in a way that can lead to this behavior?

curl 8.16.0 uses pthread_cancel() to make threads terminate early when the 
results are no longer needed. This was reverted in 
de3fc1d7adb78c078e4cc7ccc48e550758094ad3, as it proved to be unreliable due to 
unsoundness of libc's implementations. See 
https://eissing.org/icing/posts/rip_pthread_cancel/ for my analysis of this.

What you see in the stacktraces seem to be exactly that. libc holds a lock and 
does not release it on a pthread_cancel, leading to all other started resolver 
threads to wait on this.

The workarounds are either to apply the mentioned patch or to build curl using 
c-ares for resolving. Or try our recent release candidate https://curl.se/rc/ 
for curl 8.17 due on November 5.

- Stefan

> 
> Thanks for any suggestions!
> -Vadim
> 
> 
> Stack traces:
> Hundreds of threads look like this, providing a couple of examples below:
> 
> Thread 953 (Thread 0x7f620bbf6640 (LWP 4084852) "ddc_beacon_rece"):
> #0  futex_wait (private=0, expected=2, futex_word=0x7f640ac23040 <lock>) at 
> ../sysdeps/nptl/futex-internal.h:146
> #1  __GI___lll_lock_wait_private (futex=futex@entry=0x7f640ac23040 <lock>) at 
> ./nptl/lowlevellock.c:34
> #2  0x00007f640ab43dc4 in __check_pf 
> (seen_ipv4=seen_ipv4@entry=0x7f620bbf1516, 
> seen_ipv6=seen_ipv6@entry=0x7f620bbf1517, in6ai=in6ai@entry=0x7f620bbf1528, 
> in6ailen=in6ailen@entry=0x7f620bbf1530) at 
> ../sysdeps/unix/sysv/linux/check_pf.c:307
> #3  0x00007f640ab0bd61 in __GI_getaddrinfo (name=<optimized out>, 
> service=<optimized out>, service@entry=0x7f620bbf1cfc "8482", 
> hints=<optimized out>, hints@entry=0x7f638ebb1008, 
> pai=pai@entry=0x7f620bbf1c10) at ../sysdeps/posix/getaddrinfo.c:2446
> #4  0x0000561dbba888b8 in Curl_getaddrinfo_ex (nodename=<optimized out>, 
> servname=servname@entry=0x7f620bbf1cfc "8482", 
> hints=hints@entry=0x7f638ebb1008, result=result@entry=0x7f638ebb1000) at 
> curl_addrinfo.c:122
> #5  0x0000561dbba7a808 in getaddrinfo_thread (arg=arg@entry=0x7f638ebb0fc0) 
> at asyn-thrdd.c:245
> #6  0x0000561dbba88fff in curl_thread_create_thunk (arg=<optimized out>) at 
> curl_threads.c:57
> #7  0x00007f640aa94ac3 in start_thread (arg=<optimized out>) at 
> ./nptl/pthread_create.c:442
> #8  0x00007f640ab269d0 in clone3 () at 
> ../sysdeps/unix/sysv/linux/x86_64/clone3.S:81
> 
> ...
> Thread 11 (Thread 0x7f63f4572640 (LWP 2016127) "ddc_beacon_rece"):
> #0  futex_wait (private=0, expected=2, futex_word=0x7f640ac23040 <lock>) at 
> ../sysdeps/nptl/futex-internal.h:146
> #1  __GI___lll_lock_wait_private (futex=futex@entry=0x7f640ac23040 <lock>) at 
> ./nptl/lowlevellock.c:34
> #2  0x00007f640ab43dc4 in __check_pf 
> (seen_ipv4=seen_ipv4@entry=0x7f63f456d516, 
> seen_ipv6=seen_ipv6@entry=0x7f63f456d517, in6ai=in6ai@entry=0x7f63f456d528, 
> in6ailen=in6ailen@entry=0x7f63f456d530) at 
> ../sysdeps/unix/sysv/linux/check_pf.c:307
> #3  0x00007f640ab0bd61 in __GI_getaddrinfo (name=<optimized out>, 
> service=<optimized out>, service@entry=0x7f63f456dcfc "8482", 
> hints=<optimized out>, hints@entry=0x7f6406422348, 
> pai=pai@entry=0x7f63f456dc10) at ../sysdeps/posix/getaddrinfo.c:2446
> #4  0x0000561dbba888b8 in Curl_getaddrinfo_ex (nodename=<optimized out>, 
> servname=servname@entry=0x7f63f456dcfc "8482", 
> hints=hints@entry=0x7f6406422348, result=result@entry=0x7f6406422340) at 
> curl_addrinfo.c:122
> #5  0x0000561dbba7a808 in getaddrinfo_thread (arg=arg@entry=0x7f6406422300) 
> at asyn-thrdd.c:245
> #6  0x0000561dbba88fff in curl_thread_create_thunk (arg=<optimized out>) at 
> curl_threads.c:57
> #7  0x00007f640aa94ac3 in start_thread (arg=<optimized out>) at 
> ./nptl/pthread_create.c:442
> #8  0x00007f640ab269d0 in clone3 () at 
> ../sysdeps/unix/sysv/linux/x86_64/clone3.S:81
> 
> Possibly relevant: there's another thread that's in the process of destroying 
> an object,
> which leads to this (abbreviated) stack trace:
> Thread 3 (Thread 0x7f64093ff640 (LWP 1898221) "ddc_beacon_rece"):
> #0  __futex_abstimed_wait_common64 (private=128, cancel=true, abstime=0x0, 
> op=265, expected=3821886, futex_word=0x7f622fb54910) at 
> ./nptl/futex-internal.c:57
> #1  __futex_abstimed_wait_common (cancel=true, private=128, abstime=0x0, 
> clockid=0, expected=3821886, futex_word=0x7f622fb54910) at 
> ./nptl/futex-internal.c:87
> #2  __GI___futex_abstimed_wait_cancelable64 
> (futex_word=futex_word@entry=0x7f622fb54910, expected=3821886, 
> clockid=clockid@entry=0, abstime=abstime@entry=0x0, 
> private=private@entry=128) at ./nptl/futex-internal.c:139
> #3  0x00007f640aa96624 in __pthread_clockjoin_ex (threadid=140059683931712, 
> thread_return=thread_return@entry=0x0, clockid=clockid@entry=0, 
> abstime=abstime@entry=0x0, block=block@entry=true) at 
> ./nptl/pthread_join_common.c:105
> #4  0x00007f640aa964c3 in ___pthread_join (threadid=<optimized out>, 
> thread_return=thread_return@entry=0x0) at ./nptl/pthread_join.c:24
> #5  0x0000561dbba8910a in Curl_thread_join (hnd=hnd@entry=0x7f638eb9ac80) at 
> curl_threads.c:95
> #6  0x0000561dbba7acbf in asyn_thrdd_await (data=data@entry=0x7f6407663800, 
> addr_ctx=0x7f638eb9ac80, entry=entry@entry=0x0) at asyn-thrdd.c:557
> #7  0x0000561dbba7ad00 in Curl_async_thrdd_destroy 
> (data=data@entry=0x7f6407663800) at asyn-thrdd.c:586
> #8  0x0000561dbba7a41d in Curl_async_destroy (data=data@entry=0x7f6407663800) 
> at asyn-base.c:214
> #9  0x0000561dbba6678e in Curl_close (datap=datap@entry=0x7f64093fa8b8) at 
> url.c:303
> #10 0x0000561dbba4610c in curl_easy_cleanup (ptr=<optimized out>) at 
> easy.c:870
> (additional application-specific stack levels not shown).
> 
> 
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