Opening a comm using a target that does not exist makes the kernel crash

[martinRenou]

It crashes upon trying to register the comm on the comm manager:

Error: signal 11:
/home/martin/micromamba/envs/jupytergis/bin/../lib/libxeus-python.so.0(_ZN4xpyt15sigsegv_handlerEi+0x2b) [0x7f3a5c352b8b]
/lib64/libc.so.6(+0x1a090) [0x7f3a5ba10090]
/home/martin/micromamba/envs/jupytergis/bin/../lib/libxeus.so.11(_ZN4xeus13xcomm_manager13register_commENS_19xbasic_fixed_stringIcLm55ELi5ENS_13string_policy12silent_errorESt11char_traitsIcEEEPNS_5xcommE+0x12) [0x7f3a5c1bcac2]
/home/martin/micromamba/envs/jupytergis/bin/../lib/libxeus-python.so.0(+0x4f419) [0x7f3a5c2fb419]
/home/martin/micromamba/envs/jupytergis/bin/../lib/libxeus-python.so.0(+0x50a05) [0x7f3a5c2fca05]
/home/martin/micromamba/envs/jupytergis/bin/../lib/libxeus-python.so.0(+0x45573) [0x7f3a5c2f1573]
/home/martin/micromamba/envs/jupytergis/bin/../lib/libpython3.10.so.1.0(+0x13f4cd) [0x7f3a5bf3f4cd]
/home/martin/micromamba/envs/jupytergis/bin/../lib/libpython3.10.so.1.0(_PyObject_MakeTpCall+0x2eb) [0x7f3a5bf3864b]
/home/martin/micromamba/envs/jupytergis/bin/../lib/libpython3.10.so.1.0(+0x14b163) [0x7f3a5bf4b163]
/home/martin/micromamba/envs/jupytergis/bin/../lib/libpython3.10.so.1.0(PyVectorcall_Call+0xce) [0x7f3a5bf4bbce

We should either:

• follow ipykernel's behavior to not complain at all (automatically create the target)
• raise a proper human readable error

[JohanMabille]

According to the protocol, if the target does not exist, the kernel should reply with a comm_close message. Therefore, the target must be created before opening a comm.

However, this code is supposed to send the comm_close message, so the crash must be investigated.

[JohanMabille]

Ok the bug is due to this function which instantiates a default xtarget if not found in the map. The default constructor of xtarget cannot initialize its comm manager and I think xtarget should not be default constructible at all.