atexit does not work in python-jl

[tkf]

$ python -c 'from atexit import register; register(lambda: print("hello"))'
hello

$ python-jl -c 'from atexit import register; register(lambda: print("hello"))'

[tkf]

Just calling Py_Finalize forces Python to call atexit.register hook:

diff --git a/julia/python_jl.py b/julia/python_jl.py
index 5f4aad5..dc63a0a 100644
--- a/julia/python_jl.py
+++ b/julia/python_jl.py
@@ -40,6 +40,7 @@ import PyCall
 PyCall.pyimport("julia")[:Julia](init_julia=false)

 let code = PyCall.pyimport("julia.pseudo_python_cli")[:main](ARGS)
+    ccall(PyCall.@pysym(:Py_Finalize), Cvoid, ())
     if code isa Integer
         exit(code)
     end

But it produces a segfault because PyCall has a bunch of references to PyObjects which are now pointing nowhere. (Maybe the problem with multiprocessing #206 (comment) also is relevant to this?)

$ python-jl -c 'from atexit import register; register(lambda: print("hello"))'
hello

signal (11): Segmentation fault
in expression starting at no file:0
unknown function (ip: 0x7f48dcadd971)
pydecref_ at /home/takafumi/.julia/dev/PyCall/src/PyCall.jl:102 [inlined]
pydecref at /home/takafumi/.julia/dev/PyCall/src/PyCall.jl:107
jl_apply_generic at /buildworker/worker/package_linux64/build/src/gf.c:2184
jl_apply at /buildworker/worker/package_linux64/build/src/julia.h:1537 [inlined]
run_finalizer at /buildworker/worker/package_linux64/build/src/gc.c:115
jl_gc_run_finalizers_in_list at /buildworker/worker/package_linux64/build/src/gc.c:210
jl_gc_run_all_finalizers at /buildworker/worker/package_linux64/build/src/gc.c:245
jl_atexit_hook at /buildworker/worker/package_linux64/build/src/init.c:260
unknown function (ip: 0x40151c)
__libc_start_main at /usr/lib/libc.so.6 (unknown line)
unknown function (ip: 0x4015b4)
Allocations: 11921591 (Pool: 11918737; Big: 2854); GC: 25

[stevengj]

PyCall could probably check if Python is finalizing before calling decref?

[tkf]

Yeah, I was thinking something like

const _finalized = Ref(false)

function Py_Finalize()
    # maybe clear pycall_gc (how?)
    _finalized[] = true
    ccall(@pysym(:Py_Finalize), Cvoid, ())
end

function pydecref_(o::Union{PyPtr,PyObject})
    if _finalized[]
        return
    end
    ccall(@pysym(:Py_DecRef), Cvoid, (PyPtr,), o)
end

function __init__()
    ...
    atexit(Py_Finalize)  # or maybe not...
end

But I haven't checked if it works.