abort: bit out of range 0 - FD_SETSIZE on fd_set

[Sigmanificient]

Describe the bug

I ran this expression

nix-build -E '{ remote }:
  let
    parts = builtins.split ":" remote;
    owner = builtins.elemAt parts 0;
    branch = builtins.elemAt parts 2;
    pkgs = import (fetchTarball {
      url = "https://github.com/${owner}/nixpkgs/archive/${branch}.tar.gz";
    }) {};
  in with pkgs; mtail' --argstr "remote" "just1602:push_lwvvpzolsksr" --cores 10 -v

and ran into this following abort:

downloading 'https://github.com/just1602/nixpkgs/archive/push_lwvvpzolsksr.tar.gz'...
*** bit out of range 0 - FD_SETSIZE on fd_set ***: terminated
[1]    1111770 abort (core dumped)  nix-build -E  --argstr "remote" "just1602:push_lwvvpzolsksr" --cores 10 -v

It doesn't seems reproducible, but @lisanna-dettwyler told me it was worth opening an issue for.

Metadata

I have a coredump of it!

           PID: 1111770 (nix-build)
           TID: 1111770 (nix-build)
           UID: 1000 (sigmanificient)
           GID: 100 (users)
        Signal: 6 (ABRT) si_code: SI_TKILL
     Timestamp: Mon 2026-08-03 21:42:56 CEST (8min ago)
  Command Line: nix-build -E $'{ remote }:\n  let\n    parts = builtins.split ":" remote;\n    owner = builtins.elemAt parts 0;\n    branch = builtins.elemAt parts 2;\n    pkgs = import (fetchTarball {\n      url = "https://github.com/${owner}/nixpkgs/archive/${branch}.tar.gz";\n    }) {};\n  in with pkgs; mtail' --argstr remote just1602:push_lwvvpzolsksr --cores 10 -v
    Executable: /nix/store/v0670lgzdj7pcfk044g3hxwyjnk9jvrk-nix-2.34.8/bin/nix
 Control Group: /user.slice/user-1000.slice/user@1000.service/kitty-892765-0.scope
          Unit: user@1000.service
     User Unit: kitty-892765-0.scope
         Slice: user-1000.slice
     Owner UID: 1000 (sigmanificient)
       Boot ID: be90214b65174bdd995f0075e9b4852f
    Machine ID: 60faca36450245afb82ba7c7d14cbe8d
      Hostname: Sigmachine
       Storage: /var/lib/systemd/coredump/core.nix-build.1000.be90214b65174bdd995f0075e9b4852f.1111770.1785786176000000.zst (present)
  Size on Disk: 24.8M
       Message: Process 1111770 (nix-build) of user 1000 dumped core.
                
                Module libtbbmalloc.so.2 without build-id.
                Module libkeyutils.so.1 without build-id.
                Module libkrb5support.so.0 without build-id.
                Module libcom_err.so.3 without build-id.
                Module libk5crypto.so.3 without build-id.
                Module libkrb5.so.3 without build-id.
                Module libunistring.so.5 without build-id.
                Module libattr.so.1 without build-id.
                Module libpcre2-8.so.0 without build-id.
                Module libllhttp.so.9.4 without build-id.
                Module libaws-checksums.so.1.0.0 without build-id.
                Module libaws-c-sdkutils.so.1.0.0 without build-id.
                Module libaws-c-cal.so.1.0.0 without build-id.
                Module libaws-c-compression.so.1.0.0 without build-id.
                Module libs2n.so.1 without build-id.
                Module libaws-c-io.so.1.0.0 without build-id.
                Module libaws-c-http.so.1.0.0 without build-id.
                Module libaws-c-s3.so.0unstable without build-id.
                Module libaws-c-event-stream.so.1.0.0 without build-id.
                Module libaws-c-mqtt.so.1.0.0 without build-id.
                Module libgssapi_krb5.so.2 without build-id.
                Module libpsl.so.5 without build-id.
                Module libssh2.so.1 without build-id.
                Module libidn2.so.0 without build-id.
                Module libnghttp2.so.14 without build-id.
                Module libngtcp2.so.16 without build-id.
                Module libngtcp2_crypto_ossl.so.0 without build-id.
                Module libnghttp3.so.9 without build-id.
                Module libicuuc.so.76 without build-id.
                Module libicui18n.so.76 without build-id.
                Module libicudata.so.76 without build-id.
                Module libboost_random.so.1.89.0 without build-id.
                Module libboost_regex.so.1.89.0 without build-id.
                Module libbrotlicommon.so.1 without build-id.
                Module libtbb.so.12 without build-id.
                Module libxml2.so.16 without build-id.
                Module liblzma.so.5 without build-id.
                Module libz.so.1 without build-id.
                Module libbz2.so.1 without build-id.
                Module libzstd.so.1 without build-id.
                Module libacl.so.1 without build-id.
                Module libeditline.so.1 without build-id.
                Module liblowdown.so.3 without build-id.
                Module libgit2.so.1.9 without build-id.
                Module libaws-c-common.so.1 without build-id.
                Module libaws-c-auth.so.1.0.0 without build-id.
                Module libaws-crt-cpp.so without build-id.
                Module libseccomp.so.2 without build-id.
                Module libboost_url.so.1.89.0 without build-id.
                Module libboost_iostreams.so.1.89.0 without build-id.
                Module libboost_context.so.1.89.0 without build-id.
                Module libcpuid.so.17 without build-id.
                Module libbrotlienc.so.1 without build-id.
                Module libbrotlidec.so.1 without build-id.
                Module libblake3.so.0 without build-id.
                Module libarchive.so.13 without build-id.
                Module libgcc_s.so.1 without build-id.
                Module libstdc++.so.6 without build-id.
                Stack trace of thread 1111770:
                #0  0x000077d96e49fdcc __pthread_kill_implementation (libc.so.6 + 0x9fdcc)
                #1  0x000077d96e44265e raise (libc.so.6 + 0x4265e)
                #2  0x000077d96e429350 abort (libc.so.6 + 0x29350)
                #3  0x000077d96e42a3c8 __libc_message_impl.cold (libc.so.6 + 0x2a3c8)
                #4  0x000077d96e531b79 __fortify_fail (libc.so.6 + 0x131b79)
                #5  0x000077d96e531597 __fdelt_warn (libc.so.6 + 0x131597)
                #6  0x000077d96f7f5071 _ZN3nix8FdSource7hasDataEv (libnixutil.so.2.34.8 + 0x132071)
                #7  0x000077d96f57783d _ZN3nix11WorkerProto21BasicClientConnection19processStderrReturnEPNS_4SinkEPNS_6SourceEbb (libnixstore.so.2.34.8 + 0x37783d)
                #8  0x000077d96f578106 _ZN3nix11WorkerProto21BasicClientConnection13processStderrEPbPNS_4SinkEPNS_6SourceEbb.constprop.0 (libnixstore.so.2.34.8 + 0x378106)
                #9  0x000077d96f518747 _ZN3nix10FramedSink15writeUnbufferedESt17basic_string_viewIcSt11char_traitsIcEE (libnixstore.so.2.34.8 + 0x318747)
                #10 0x000077d96f7f36f6 _ZN3nix12BufferedSinkclESt17basic_string_viewIcSt11char_traitsIcEE (libnixutil.so.2.34.8 + 0x1306f6)
                #11 0x000077d96f7f4ba2 _ZN3nix6Source9drainIntoERNS_4SinkE (libnixutil.so.2.34.8 + 0x131ba2)
                #12 0x000077d96f521963 _ZN3nix11RemoteStore16ConnectionHandle14withFramedSinkENS_3funIFvRNS_4SinkEEEE (libnixstore.so.2.34.8 + 0x321963)
                #13 0x000077d96f525555 _ZN3nix11RemoteStore12addCAToStoreERNS_6SourceESt17basic_string_viewIcSt11char_traitsIcEENS_20ContentAddressMethodENS_13HashAlgorithmERKSt3setINS_9StorePathESt4lessISA_ESaISA_EENS_10RepairFlagE (libnixstore.so.2.34.8 + 0x325555)
                #14 0x000077d96f525bc6 _ZN3nix11RemoteStore18addToStoreFromDumpERNS_6SourceESt17basic_string_viewIcSt11char_traitsIcEENS_23FileSerialisationMethodENS_20ContentAddressMethodENS_13HashAlgorithmERKSt3setINS_9StorePathESt4lessISB_ESaISB_EENS_10RepairFlagE (libnixstore.so.2.34.8 + 0x325bc6)
                #15 0x000077d96f566de1 _ZNSt17_Function_handlerIFvRN3nix6SourceEEZNS0_5Store10addToStoreESt17basic_string_viewIcSt11char_traitsIcEERKNS0_10SourcePathENS0_20ContentAddressMethodENS0_13HashAlgorithmERKSt3setINS0_9StorePathESt4lessISF_ESaISF_EERNS0_3funIFbRKNSt7__cxx1112basic_stringIcS7_SaIcEEEEEENS0_10RepairFlagEEUlS2_E_E9_M_invokeERKSt9_Any_dataS2_.lto_priv.0 (libnixstore.so.2.34.8 + 0x366de1)
                #16 0x000077d96f7fc132 _ZN5boost7context6detail11fiber_entryINS1_12fiber_recordINS0_5fiberENS0_31basic_protected_fixedsize_stackINS0_12stack_traitsEEEZNS_11coroutines26detail14push_coroutineIbE13control_blockC4IS7_ZZN3nix12sourceToSinkENSE_3funIFvRNSE_6SourceEEEEEN12SourceToSinkclESt17basic_string_viewIcSt11char_traitsIcEEEUlRNS9_14pull_coroutineIbEEE_EENS0_12preallocatedEOT_OT0_EUlOS4_E_EEEEvNS1_10transfer_tE (libnixutil.so.2.34.8 + 0x139132)
                #17 0x000077d96f0cd18f make_fcontext (libboost_context.so.1.89.0 + 0x118f)
                #18 0x000004c001000100 n/a (n/a + 0x0)
                ELF object binary architecture: AMD x86-64

Checklist

• checked latest Nix manual (source)
• checked open bug issues and pull requests for possible duplicates
• reviewed the contributing guide

---

Add 👍 to issues you find important.

[xokdvium]

I think this might have been fixed by #15757

[xokdvium]

It also might be that maybe libgit2 is eating up too many file descriptors for a bunch of packfiles. Might be worth running this snippet, that nix still inexplicably doesn't do itself:

cd ~/.cache/nix/tarball-cache-v2 && git multi-pack-index write && git multi-pack-index repack && git multi-pack-index expire