Segfault in multi-threaded GC when running under gvisor/Claude Code Web

[Keno]

We've been receiving several reports that Julia crashes in the Claude Code web sandbox. The sandbox appears to be using gvisor, which was previously reported to have this issue in #50181. I'm opening this issue to track getting this fixed in both upstreams. I can reproduce the following with a recent gvisor release and julia nightly:

keno@demeter6:~$ /home/keno/.juliaup/bin/julia +nightly --threads=8
        Info A new `nightly` version is available. Install with `juliaup update`.
               _
   _       _ _(_)_     |  Documentation: https://docs.julialang.org
  (_)     | (_) (_)    |
   _ _   _| |_  __ _   |  Type "?" for help, "]?" for Pkg help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 1.14.0-DEV.1492 (2026-01-06)
 _/ |\__'_|_|_|\__'_|  |  Commit f4911b6694d (14 days old master)
|__/                   |

julia> for _ in 1:5
           tasks = [Threads.@spawn begin
               data = [rand(10000) for _ in 1:10]
               sum(sum.(data))
           end for _ in 1:50]
           [fetch(t) for t in tasks]
       end

julia> 
keno@demeter6:~$ runsc --rootless --platform=kvm --network=none do /home/keno/.juliaup/bin/julia +nightly --threads=8
        Info A new `nightly` version is available. Install with `juliaup update`.
               _
   _       _ _(_)_     |  Documentation: https://docs.julialang.org
  (_)     | (_) (_)    |
   _ _   _| |_  __ _   |  Type "?" for help, "]?" for Pkg help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 1.14.0-DEV.1492 (2026-01-06)
 _/ |\__'_|_|_|\__'_|  |  Commit f4911b6694d (14 days old master)
|__/                   |

julia> for _ in 1:5
           tasks = [Threads.@spawn begin
               data = [rand(10000) for _ in 1:10]
               sum(sum.(data))
           end for _ in 1:50]
           [fetch(t) for t in tasks]
       end

[1] signal 11 (1): Segmentation fault
in expression starting at REPL[1]:1
ijl_gc_safepoint at /cache/build/tester-amdci4-14/julialang/julia-master/src/jlapi.c:786
ijl_task_get_next at /cache/build/tester-amdci4-14/julialang/julia-master/src/scheduler.c:459
wait at ./task.jl:1246
wait_forever at ./task.jl:1168
jfptr_wait_forever_64366.1 at /home/keno/.julia/juliaup/julia-nightly/lib/julia/sys.so (unknown line)
jl_apply at /cache/build/tester-amdci4-14/julialang/julia-master/src/julia.h:2285 [inlined]
start_task at /cache/build/tester-amdci4-14/julialang/julia-master/src/task.c:1272
Allocations: 1 (Pool: 1; Big: 0); GC: 0

[Keno]

This also fails with --platform=systrap, so the root cause identification in the original issue is likely incomplete.

[Keno]

Filed upstream.