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fix crashes using multithreaded julia from python #758

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fix crashes using multithreaded julia from python #758

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7d7bf7e
fix: rare crash due to memoty race condition
Apr 12, 2026
104103c
fix: crash in atexit
Apr 16, 2026
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24 changes: 23 additions & 1 deletion src/JlWrap/C.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -25,6 +25,11 @@ const PyJuliaBase_Type = Ref(C.PyNULL)
const PYJLVALUES = []
# unused indices in PYJLVALUES
const PYJLFREEVALUES = Int[]
# lock protecting PYJLVALUES and PYJLFREEVALUES from concurrent modification
const PYJL_LOCK = Threads.SpinLock()
# set to true by an atexit hook so _pyjl_dealloc can skip Julia runtime calls
# (Base.GC.enable, lock) after jl_atexit_hook has torn down the runtime
const JL_EXITING = Ref(false)

function _pyjl_new(t::C.PyPtr, ::C.PyPtr, ::C.PyPtr)
alloc = C.PyType_GetSlot(t, C.Py_tp_alloc)
Expand All @@ -38,9 +43,17 @@ end

function _pyjl_dealloc(o::C.PyPtr)
idx = C.@ft UnsafePtr{PyJuliaValueObject}(o).value[]
if idx != 0
if idx != 0 && !JL_EXITING[]
# Disable GC to prevent push! from triggering a GC that runs finalizers
# re-entrantly (the finalizer chain: push! → alloc → GC → py_finalizer →
# enqueue → Py_DecRef → _pyjl_dealloc → push! on same vector →
# ConcurrencyViolationError). The lock guards against true multi-thread races.
prev = Base.GC.enable(false)
lock(PYJL_LOCK)
PYJLVALUES[idx] = nothing
push!(PYJLFREEVALUES, idx)
unlock(PYJL_LOCK)
Base.GC.enable(prev)
end
(C.@ft UnsafePtr{PyJuliaValueObject}(o).weaklist[!]) == C.PyNULL || C.PyObject_ClearWeakRefs(o)
freeptr = C.PyType_GetSlot(C.Py_Type(o), C.Py_tp_free)
Expand Down Expand Up @@ -365,6 +378,9 @@ end

function __init__()
init_c()
atexit() do
JL_EXITING[] = true
end
end

PyJuliaValue_IsNull(o) = Base.GC.@preserve o (C.@ft UnsafePtr{PyJuliaValueObject}(C.asptr(o)).value[]) == 0
Expand All @@ -375,13 +391,19 @@ PyJuliaValue_SetValue(_o, @nospecialize(v)) = Base.GC.@preserve _o begin
o = C.asptr(_o)
idx = C.@ft UnsafePtr{PyJuliaValueObject}(o).value[]
if idx == 0
# Disable GC to prevent push!/pop! from triggering a GC whose finalizers
# re-entrantly resize the same vectors (see _pyjl_dealloc comment).
prev = Base.GC.enable(false)
lock(PYJL_LOCK)
if isempty(PYJLFREEVALUES)
push!(PYJLVALUES, v)
idx = length(PYJLVALUES)
else
idx = pop!(PYJLFREEVALUES)
PYJLVALUES[idx] = v
end
unlock(PYJL_LOCK)
Base.GC.enable(prev)
C.@ft UnsafePtr{PyJuliaValueObject}(o).value[] = idx
else
PYJLVALUES[idx] = v
Expand Down
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