apt-cache-ng is taking 100% cpu

[Ponnuvel Palaniyappan]

Every once in a while, /usr/sbin/apt-cacher-ng seems to get stuck in a loop and takes up 100% of cpu.

root@magicbox:~# lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 21.04
Release: 21.04
Codename: hirsute

root@magicbox:~# dpkg -l | grep apt-cacher
ii apt-cacher-ng 3.6.3-1 amd64 caching proxy server for software repositories

root@magicbox:~# ps -P 1494
    PID PSR TTY STAT TIME COMMAND
   1494 2 ? Ssl 2674:27 /usr/sbin/apt-cacher-ng -c /etc/apt-cacher-ng ForeGround=1

root@magicbox:~# top -p 1494
    PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
   1494 apt-cac+ 20 0 755256 10244 7392 S 100.0 0.1 2681:35 apt-cacher-ng

Seems one of the threads stuck in a select/pol loop (from gdb):
(gdb) bt
#0 0x00007f8c9495309e in epoll_wait (epfd=3, events=0x55b8c69e4eb0, maxevents=32, timeout=-1) at ../sysdeps/unix/sysv/linux/epoll_wait.c:30
#1 0x00007f8c94ca8719 in ?? () from /lib/x86_64-linux-gnu/libevent-2.1.so.7
#2 0x00007f8c94c9e685 in event_base_loop () from /lib/x86_64-linux-gnu/libevent-2.1.so.7
#3 0x00007f8c94d8455a in acng::evabase::MainLoop() () from /lib/x86_64-linux-gnu/libsupacng.so
#4 0x000055b8c61f5b25 in ?? ()
#5 0x00007f8c94863565 in __libc_start_main (main=0x55b8c61f5af0, argc=4, argv=0x7fff32802fc8, init=<optimised out>, fini=<optimised out>, rtld_fini=<optimised out>, stack_end=0x7fff32802fb8) at ../csu/libc-start.c:332
#6 0x000055b8c61f5ede in ?? ()

(gdb) info threads
  Id Target Id Frame
* 1 LWP 1494 "apt-cacher-ng" 0x00007f8c9495309e in epoll_wait (epfd=3, events=0x55b8c69e4eb0, maxevents=32, timeout=-1) at ../sysdeps/unix/sysv/linux/epoll_wait.c:30
  2 LWP 5052 "apt-cacher-ng" __futex_abstimed_wait_common64 (cancel=true, private=0, abstime=0x0, clockid=0, expected=0, futex_word=0x7f8c94dae090) at ../sysdeps/nptl/futex-internal.c:74
  3 LWP 11108 "apt-cacher-ng" __futex_abstimed_wait_common64 (cancel=true, private=0, abstime=0x0, clockid=0, expected=0, futex_word=0x7f8c8802ba98) at ../sysdeps/nptl/futex-internal.c:74
  4 LWP 18009 "apt-cacher-ng" __futex_abstimed_wait_common64 (cancel=true, private=0, abstime=0x0, clockid=0, expected=0, futex_word=0x7f8c8802ba98) at ../sysdeps/nptl/futex-internal.c:74
  5 LWP 18010 "apt-cacher-ng" __futex_abstimed_wait_common64 (cancel=true, private=0, abstime=0x0, clockid=0, expected=0, futex_word=0x7f8c8802ba98) at ../sysdeps/nptl/futex-internal.c:74
  6 LWP 18014 "apt-cacher-ng" __futex_abstimed_wait_common64 (cancel=true, private=0, abstime=0x7f8c7bffeca0, clockid=1, expected=0, futex_word=0x7f8c94dacd38) at ../sysdeps/nptl/futex-internal.c:74
  7 LWP 19462 "apt-cacher-ng" 0x00007f8c94949b31 in __GI___select (nfds=15, readfds=0x7f8c915daa00, writefds=0x7f8c915daa80, exceptfds=0x0, timeout=<optimised out>) at ../sysdeps/unix/sysv/linux/select.c:49
  8 LWP 21198 "apt-cacher-ng" __futex_abstimed_wait_common64 (cancel=true, private=0, abstime=0x0, clockid=0, expected=0, futex_word=0x7f8c94dae090) at ../sysdeps/nptl/futex-internal.c:74
  9 LWP 22284 "apt-cacher-ng" 0x00007f8c94949b31 in __GI___select (nfds=19, readfds=0x7f8c7b7fda00, writefds=0x7f8c7b7fda80, exceptfds=0x0, timeout=<optimised out>) at ../sysdeps/unix/sysv/linux/select.c:49
(gdb) thread 7
[Switching to thread 7 (LWP 19462)]
#0 0x00007f8c94949b31 in __GI___select (nfds=15, readfds=0x7f8c915daa00, writefds=0x7f8c915daa80, exceptfds=0x0, timeout=<optimised out>) at ../sysdeps/unix/sysv/linux/select.c:49
Download failed: Function not implemented. Continuing without source file ./misc/../sysdeps/unix/sysv/linux/select.c.
49 ../sysdeps/unix/sysv/linux/select.c: No such file or directory.
(gdb) bt
#0 0x00007f8c94949b31 in __GI___select (nfds=15, readfds=0x7f8c915daa00, writefds=0x7f8c915daa80, exceptfds=0x0, timeout=<optimised out>) at ../sysdeps/unix/sysv/linux/select.c:49
#1 0x00007f8c94d135f8 in acng::dlcon::Impl::ExchangeData(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&, std::shared_ptr<acng::tcpconnect>&, std::__cxx11::list<acng::dlcon::Impl::tDlJob, std::allocator<acng::dlcon::Impl::tDlJob> >&) () from /lib/x86_64-linux-gnu/libsupacng.so
#2 0x00007f8c94d15636 in acng::dlcon::Impl::WorkLoop() () from /lib/x86_64-linux-gnu/libsupacng.so
#3 0x00007f8c94b1c694 in ?? () from /lib/x86_64-linux-gnu/libstdc++.so.6
#4 0x00007f8c94c64450 in start_thread (arg=0x7f8c915db640) at pthread_create.c:473
#5 0x00007f8c94952d53 in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

[Ponnuvel Palaniyappan (pponnuvel)]

Attached apt-cacher-ng logs.

[Jean-Marie Mineau (histausse)]

I seem to have the same issue:

```
histausse@hindley:~$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 20.04.3 LTS
Release: 20.04
Codename: focal

histausse@hindley:~$ sudo dpkg -l | grep apt-cacher
ii apt-cacher-ng 3.3.1-2build1 amd64 caching proxy server for software repositories

histausse@hindley:~$ sudo ps -eo pid,ppid,cmd,%mem,%cpu --sort=-%cpu | head
    PID PPID CMD %MEM %CPU
    765 1 /usr/sbin/apt-cacher-ng -c 0.6 71.9
```

[Eduard Bloch (edi-gmx)]

Please upgrade to Debian Stable version, 3.6.4-1 .

[Rolf Leggewie (r0lf)]

Edward, as you know, that is not an option for LTS users.

[Rolf Leggewie (r0lf)]

And besides, there have been reports similar to this for years. Best to try and figure out what is going on.