Returning string from Python callback to Go - and PyBytes_AsString() SIGSEGV

[nickeygit]

Hi,

Thanks for the great tool!

Description

Trying to return a string from Python callback to Go - and getting SIGSEGV:

> python test.py
fs.CallBack(22, cbfun)...
in python cbfun: FieldI:  42  FieldS:  str field  ival:  22  sval:  str field
fs.CallBackIf(22, cbfunif)...
in python cbfunif: FieldI:  42  FieldS:  str field  ival:  22  ifval:  str field
fs.CallBackRval(22, cbfunrval)...
in python cbfunrval: FieldI:  42  FieldS:  str field  ival:  22  ifval:  str field

(nothing more in output, literally)

Gdb shows more:

(gdb) back
#0  0x000055b7519d2627 in PyErr_Format ()
#1  0x000055b7519003a6 in ?? ()
#2  0x00007f330e0d0dab in _cgo_86d28a1f6e85_Cfunc_PyBytes_AsString () from /app/out/_funcs.cpython-310-x86_64-linux-gnu.so
#3  0x00007f330e098c41 in runtime.asmcgocall () at /usr/local/go/src/runtime/asm_amd64.s:923
#4  0x000000c000006700 in ?? ()
#5  0x00007ffee5d3e9a0 in ?? ()
#6  0x000055b763a8eee0 in ?? ()
#7  0x000000c000044008 in ?? ()
#8  0x00007ffee5d3ea58 in ?? ()
#9  0x00007f330e0a0c21 in crosscall2 () at /usr/local/go/src/runtime/cgo/asm_amd64.s:43
#10 0x00007f330e0d0c07 in funcs_FunStruct_CallBackRval () from /app/out/_funcs.cpython-310-x86_64-linux-gnu.so
#11 0x00007f330e0d5176 in _wrap__funcs_funcs_FunStruct_CallBackRval () from /app/out/_funcs.cpython-310-x86_64-linux-gnu.so
#12 0x000055b7519cbdae in ?? ()
#13 0x000055b7519c252b in _PyObject_MakeTpCall ()
#14 0x000055b7519bb16b in _PyEval_EvalFrameDefault ()
#15 0x000055b7519cc6ac in _PyFunction_Vectorcall ()
#16 0x000055b7519b4b2b in _PyEval_EvalFrameDefault ()
#17 0x000055b7519b1096 in ?? ()
#18 0x000055b751aa6f66 in PyEval_EvalCode ()
#19 0x000055b751ad1e98 in ?? ()
#20 0x000055b751acb79b in ?? ()
#21 0x000055b751ad1be5 in ?? ()
#22 0x000055b751ad10c8 in _PyRun_SimpleFileObject ()
#23 0x000055b751ad0d13 in _PyRun_AnyFileObject ()
#24 0x000055b751ac370e in Py_RunMain ()
#25 0x000055b751a99dfd in Py_BytesMain ()
#26 0x00007f330e733d90 in ?? () from /lib/x86_64-linux-gnu/libc.so.6
#27 0x00007f330e733e40 in __libc_start_main () from /lib/x86_64-linux-gnu/libc.so.6
#28 0x000055b751a99cf5 in _start ()

Trying _examples/funcs/
Just 2 changes to ret type string:

funcs.go:

-func (fs *FunStruct) CallBackRval(i int, fun func(fs *FunStruct, i int, v interface{}) bool) {
+func (fs *FunStruct) CallBackRval(i int, fun func(fs *FunStruct, i int, v interface{}) string) {

test.py:

in def cbfunrval(afs, ival, ifval):
-    return True
+    return "some string"

Versions

Gopy:
commit 1d7f3a2 (HEAD -> master, tag: v0.4.10, origin/master, origin/HEAD)

Python 3.10.12 (main, Jul 29 2024, 16:56:48) [GCC 11.4.0] on linux
go version go1.23.2 linux/amd64

The Reason

Generated out/funcs.go:

funcs_FunStruct_CallBackRval is generated to pass _fcret without type checks:

...
            _fcret := C.PyObject_CallObject(_fun_arg, _fcargs)
            C.gopy_decref(_fcargs)
            C.gopy_err_handle()
            C.PyGILState_Release(_gstate)
            return C.GoString(C.PyBytes_AsString(_fcret))
...

And PyBytes_AsString - panics as _fcret is not bytes.

Workaround

But, changing ret type to bytes - and things work fine:
test.py:

in def cbfunrval(afs, ival, ifval):
-    return "some string"
+    return b"some string"

> python test.py
fs.CallBack(22, cbfun)...
in python cbfun: FieldI:  42  FieldS:  str field  ival:  22  sval:  str field
fs.CallBackIf(22, cbfunif)...
in python cbfunif: FieldI:  42  FieldS:  str field  ival:  22  ifval:  str field
fs.CallBackRval(22, cbfunrval)...
in python cbfunrval: FieldI:  42  FieldS:  str field  ival:  22  ifval:  str field
got return value: some string
fs.CallBack(32, cls.ClassFun)...
in python class fun: FieldI:  42  FieldS:  str field  ival:  32  sval:  str field
cls.CallSelf...
in python class fun: FieldI:  42  FieldS:  str field  ival:  77  sval:  str field
fs.ObjArg with nil
got nil
fs.ObjArg with fs
ofs FieldI: 42 FieldS: str field
OK

Fix

So, need to test-cover PyBytes_AsString calls with more types.

Thank you!