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Don't detach the auto-GC thread#15614

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Don't detach the auto-GC thread#15614
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join-auto-gc-thread

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@edolstra edolstra commented Apr 1, 2026

Motivation

It could still be running after this is destroyed. So we need to join it in the LocalStore destructor.

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@edolstra
Don't detach the auto-GC thread
4a09c1f 
It could still be running after `this` is destroyed. So we need to
join it in the LocalStore destructor.
@edolstra edolstra requested a review from Ericson2314 as a code owner April 1, 2026 20:03
@github-actions github-actions Bot added the store Issues and pull requests concerning the Nix store label Apr 1, 2026
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xokdvium commented Apr 1, 2026

cc @dramforever

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xokdvium reviewed Apr 1, 2026
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Comment thread src/libstore/local-store.cc
Comment on lines +435 to +439
{
auto state(_state->lock());
if (state->gcThread.joinable())
state->gcThread.join();
}
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@xokdvium xokdvium Apr 1, 2026

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This can't throw, right?

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@dramforever dramforever Apr 2, 2026

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Should we also try to interrupt the GC thread here? Not sure how though.

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xokdvium commented Apr 2, 2026

@dramforever says it doesn't help with the use-after-free issue of static globals. Unclear what's going on there. We don't have any global static store instances AFAIK.

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dramforever commented Apr 2, 2026

To be clear this is the crash I still got (the second trace being the crashing thread):

Details 
Thread 2 (LWP 27962):
#0  0x00007f27934a7f18 in unlink_chunk.isra () from /nix/store/vr7ds8vwbl2fz7pr221d5y0f8n9a5wda-glibc-2.40-218/lib/libc.so.6
#1  0x00007f27934a8117 in malloc_consolidate () from /nix/store/vr7ds8vwbl2fz7pr221d5y0f8n9a5wda-glibc-2.40-218/lib/libc.so.6
#2  0x00007f27934a9460 in _int_free_maybe_consolidate.part.0 () from /nix/store/vr7ds8vwbl2fz7pr221d5y0f8n9a5wda-glibc-2.40-218/lib/libc.so.6
#3  0x00007f27934a9b12 in _int_free () from /nix/store/vr7ds8vwbl2fz7pr221d5y0f8n9a5wda-glibc-2.40-218/lib/libc.so.6
#4  0x00007f27934ac3ee in free () from /nix/store/vr7ds8vwbl2fz7pr221d5y0f8n9a5wda-glibc-2.40-218/lib/libc.so.6
#5  0x00005555d1d86d08 in std::__new_allocator<std::__detail::_State<char> >::deallocate (this=<optimized out>, __p=<optimized out>, __n=<optimized out>) at /nix/store/yps9cjmi8g58mh0f7pw902br7gxwg2dz-gcc-14.3.0/include/c++/14.3.0/bits/new_allocator.h:172
#6  std::allocator<std::__detail::_State<char> >::deallocate (this=<optimized out>, __p=<optimized out>, __n=<optimized out>) at /nix/store/yps9cjmi8g58mh0f7pw902br7gxwg2dz-gcc-14.3.0/include/c++/14.3.0/bits/allocator.h:208
#7  std::allocator_traits<std::allocator<std::__detail::_State<char> > >::deallocate (__a=..., __p=<optimized out>, __n=<optimized out>) at /nix/store/yps9cjmi8g58mh0f7pw902br7gxwg2dz-gcc-14.3.0/include/c++/14.3.0/bits/alloc_traits.h:550
#8  std::_Vector_base<std::__detail::_State<char>, std::allocator<std::__detail::_State<char> > >::_M_deallocate (this=0x5555de945a58, __p=<optimized out>, __n=<optimized out>) at /nix/store/yps9cjmi8g58mh0f7pw902br7gxwg2dz-gcc-14.3.0/include/c++/14.3.0/bits/stl_vector.h:392
#9  std::_Vector_base<std::__detail::_State<char>, std::allocator<std::__detail::_State<char> > >::_M_deallocate (this=0x5555de945a58, __p=<optimized out>, __n=<optimized out>) at /nix/store/yps9cjmi8g58mh0f7pw902br7gxwg2dz-gcc-14.3.0/include/c++/14.3.0/bits/stl_vector.h:388
#10 std::_Vector_base<std::__detail::_State<char>, std::allocator<std::__detail::_State<char> > >::~_Vector_base (this=0x5555de945a58, __in_chrg=<optimized out>) at /nix/store/yps9cjmi8g58mh0f7pw902br7gxwg2dz-gcc-14.3.0/include/c++/14.3.0/bits/stl_vector.h:371
#11 std::vector<std::__detail::_State<char>, std::allocator<std::__detail::_State<char> > >::~vector (this=0x5555de945a58, __in_chrg=<optimized out>) at /nix/store/yps9cjmi8g58mh0f7pw902br7gxwg2dz-gcc-14.3.0/include/c++/14.3.0/bits/stl_vector.h:751
#12 std::__detail::_NFA<std::__cxx11::regex_traits<char> >::~_NFA (this=0x5555de945a20, __in_chrg=<optimized out>) at /nix/store/yps9cjmi8g58mh0f7pw902br7gxwg2dz-gcc-14.3.0/include/c++/14.3.0/bits/regex_automaton.h:220
#13 std::destroy_at<std::__detail::_NFA<std::__cxx11::regex_traits<char> > > (__location=0x5555de945a20) at /nix/store/yps9cjmi8g58mh0f7pw902br7gxwg2dz-gcc-14.3.0/include/c++/14.3.0/bits/stl_construct.h:88
#14 std::_Destroy<std::__detail::_NFA<std::__cxx11::regex_traits<char> > > (__pointer=0x5555de945a20) at /nix/store/yps9cjmi8g58mh0f7pw902br7gxwg2dz-gcc-14.3.0/include/c++/14.3.0/bits/stl_construct.h:149
#15 std::allocator_traits<std::allocator<void> >::destroy<std::__detail::_NFA<std::__cxx11::regex_traits<char> > > (__p=0x5555de945a20) at /nix/store/yps9cjmi8g58mh0f7pw902br7gxwg2dz-gcc-14.3.0/include/c++/14.3.0/bits/alloc_traits.h:720
#16 std::_Sp_counted_ptr_inplace<std::__detail::_NFA<std::__cxx11::regex_traits<char> >, std::allocator<void>, (__gnu_cxx::_Lock_policy)2>::_M_dispose (this=0x5555de945a10) at /nix/store/yps9cjmi8g58mh0f7pw902br7gxwg2dz-gcc-14.3.0/include/c++/14.3.0/bits/shared_ptr_base.h:616
#17 0x00007f27949783ea in std::_Sp_counted_base<(__gnu_cxx::_Lock_policy)2>::_M_release (this=0x5555de945a10) at /nix/store/yps9cjmi8g58mh0f7pw902br7gxwg2dz-gcc-14.3.0/include/c++/14.3.0/bits/shared_ptr_base.h:346
#18 std::_Sp_counted_base<(__gnu_cxx::_Lock_policy)2>::_M_release (this=0x5555de945a10) at /nix/store/yps9cjmi8g58mh0f7pw902br7gxwg2dz-gcc-14.3.0/include/c++/14.3.0/bits/shared_ptr_base.h:317
#19 0x00007f27949c101a in std::__shared_count<(__gnu_cxx::_Lock_policy)2>::~__shared_count (this=0x7f27949fef58 <nix::parseFlakeIdRef(nix::fetchers::Settings const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, bool)::flakeRegex+24>, __in_chrg=<optimized out>) at /nix/store/yps9cjmi8g58mh0f7pw902br7gxwg2dz-gcc-14.3.0/include/c++/14.3.0/bits/shared_ptr_base.h:1069
#20 std::__shared_ptr<std::__detail::_NFA<std::__cxx11::regex_traits<char> > const, (__gnu_cxx::_Lock_policy)2>::~__shared_ptr (this=0x7f27949fef50 <nix::parseFlakeIdRef(nix::fetchers::Settings const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, bool)::flakeRegex+16>, __in_chrg=<optimized out>) at /nix/store/yps9cjmi8g58mh0f7pw902br7gxwg2dz-gcc-14.3.0/include/c++/14.3.0/bits/shared_ptr_base.h:1525
#21 std::shared_ptr<std::__detail::_NFA<std::__cxx11::regex_traits<char> > const>::~shared_ptr (this=0x7f27949fef50 <nix::parseFlakeIdRef(nix::fetchers::Settings const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, bool)::flakeRegex+16>, __in_chrg=<optimized out>) at /nix/store/yps9cjmi8g58mh0f7pw902br7gxwg2dz-gcc-14.3.0/include/c++/14.3.0/bits/shared_ptr.h:175
#22 std::__cxx11::basic_regex<char, std::__cxx11::regex_traits<char> >::~basic_regex (this=0x7f27949fef40 <nix::parseFlakeIdRef(nix::fetchers::Settings const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, bool)::flakeRegex>, __in_chrg=<optimized out>) at /nix/store/yps9cjmi8g58mh0f7pw902br7gxwg2dz-gcc-14.3.0/include/c++/14.3.0/bits/regex.h:557
#23 0x00007f2793444271 in __run_exit_handlers () from /nix/store/vr7ds8vwbl2fz7pr221d5y0f8n9a5wda-glibc-2.40-218/lib/libc.so.6
#24 0x00007f279344433e in exit () from /nix/store/vr7ds8vwbl2fz7pr221d5y0f8n9a5wda-glibc-2.40-218/lib/libc.so.6
#25 0x00007f279342a4df in __libc_start_call_main () from /nix/store/vr7ds8vwbl2fz7pr221d5y0f8n9a5wda-glibc-2.40-218/lib/libc.so.6
#26 0x00007f279342a59b in __libc_start_main_impl () from /nix/store/vr7ds8vwbl2fz7pr221d5y0f8n9a5wda-glibc-2.40-218/lib/libc.so.6
#27 0x00005555d1cfebe5 in _start ()

Thread 1 (LWP 27978):
#0  0x00007f2794658aab in boost::re_detail_500::perl_matcher<char const*, std::allocator<boost::sub_match<char const*> >, boost::regex_traits<char, boost::cpp_regex_traits<char> > >::match_all_states (this=this@entry=0x7f2771b24bd0) at /nix/store/zw6w6chqvqsh2c6jfsb4pw9bpgdkmjnh-boost-1.87.0-dev/include/boost/regex/v5/perl_matcher_non_recursive.hpp:199
#1  0x00007f2794673eea in boost::re_detail_500::perl_matcher<char const*, std::allocator<boost::sub_match<char const*> >, boost::regex_traits<char, boost::cpp_regex_traits<char> > >::match_prefix (this=0x7f2771b24bd0) at /nix/store/zw6w6chqvqsh2c6jfsb4pw9bpgdkmjnh-boost-1.87.0-dev/include/boost/regex/v5/perl_matcher_common.hpp:306
#2  boost::re_detail_500::perl_matcher<char const*, std::allocator<boost::sub_match<char const*> >, boost::regex_traits<char, boost::cpp_regex_traits<char> > >::match_imp (this=this@entry=0x7f2771b24bd0) at /nix/store/zw6w6chqvqsh2c6jfsb4pw9bpgdkmjnh-boost-1.87.0-dev/include/boost/regex/v5/perl_matcher_common.hpp:196
#3  0x00007f279464c7f9 in boost::re_detail_500::perl_matcher<char const*, std::allocator<boost::sub_match<char const*> >, boost::regex_traits<char, boost::cpp_regex_traits<char> > >::match (this=0x7f2771b24bd0) at /nix/store/zw6w6chqvqsh2c6jfsb4pw9bpgdkmjnh-boost-1.87.0-dev/include/boost/regex/v5/perl_matcher_common.hpp:172
#4  boost::regex_match<char const*, std::allocator<boost::sub_match<char const*> >, char, boost::regex_traits<char, boost::cpp_regex_traits<char> > > (first=0x7f23640025a3 "42", last=<optimized out>, m=..., e=..., flags=boost::regex_constants::match_any) at /nix/store/zw6w6chqvqsh2c6jfsb4pw9bpgdkmjnh-boost-1.87.0-dev/include/boost/regex/v5/regex_match.hpp:39
#5  boost::regex_match<char, boost::regex_traits<char, boost::cpp_regex_traits<char> > > (str=0x7f23640025a3 "42", e=..., flags=boost::regex_constants::match_default) at /nix/store/zw6w6chqvqsh2c6jfsb4pw9bpgdkmjnh-boost-1.87.0-dev/include/boost/regex/v5/regex_match.hpp:75
#6  nix::findRuntimeRootsUnchecked[abi:cxx11](nix::StoreDirConfig const&) (config=...) at ../src/libstore/local-gc.cc:74
#7  0x00007f27945ca722 in nix::LocalStore::findRuntimeRoots (this=0x5555de838870, roots=..., censor=true) at ../src/libstore/gc.cc:334
#8  0x00007f27945cc4f5 in nix::LocalStore::collectGarbage (this=0x5555de838870, options=..., results=...) at ../src/libstore/gc.cc:519
#9  0x00007f27945c6aff in operator() (__closure=0x5555deb8a028) at ../src/libstore/gc.cc:907
#10 0x00007f27938ed064 in execute_native_thread_routine () from /nix/store/0p8b2lqk47fvxm9hc6c8mnln5l8x51q1-gcc-14.3.0-lib/lib/libstdc++.so.6
#11 0x00007f279349a97a in start_thread () from /nix/store/vr7ds8vwbl2fz7pr221d5y0f8n9a5wda-glibc-2.40-218/lib/libc.so.6
#12 0x00007f2793522d2c in __clone3 () from /nix/store/vr7ds8vwbl2fz7pr221d5y0f8n9a5wda-glibc-2.40-218/lib/libc.so.6

And the crash is here in the boost::regex_match accessing a now-gone digitsRegex:

nix/src/libstore/local-gc.cc

Lines 69 to 74 in 0187919

static const auto digitsRegex = boost::regex(R"(^\d+$)");
static const auto mapRegex = boost::regex(R"(^\s*\S+\s+\S+\s+\S+\s+\S+\s+\S+\s+(/\S+)\s*$)");
auto storePathRegex = boost::regex(quoteRegexChars(config.storeDir) + R"(/[0-9a-z]+[0-9a-zA-Z\+\-\._\?=]*)");
while (errno = 0, ent = readdir(procDir.get())) {
checkInterrupt();
if (boost::regex_match(ent->d_name, digitsRegex)) {

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@xokdvium xokdvium xokdvium approved these changes

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