feat: Do not change permissions on existing data directory

[Gu1nness]

• When we attach an existing volume with charmed mongodb $DATA directory already existing, the former version would break when trying to change the permissions of $DATA.
• This version, similar as [DPE-4133] Do not change permission on existing data directory charmed-kafka-snap#40 fixes this issue

[Mehdi-Bendriss]

Thanks Neha! I have a couple of questions

[Mehdi-Bendriss]

1. how can ! -d "$DATA" ever evaluate to True given $DATA was potentially created right before?
2. we could perhaps defer the creation of $DATA and simplify the condition with:

if [ ! -d "${f}" ]; then
    mkdir -p "${DATA}"
    chmod -R 770 "${SNAP_COMMON}"
    chmod 750 "${DATA}"
fi

[Gu1nness]

Indeed I don't think -d "$DATA" can be false here indeed.
The important check here is stat -c '%u' "$DATA" == 0 to check the owner.
If root is the owner, we should update the permissions, otherwise it means we are installing on existing data and we should not change this.

[Mehdi-Bendriss]

Should this be recursive?

[Gu1nness]

No, as there will be no data in here in case we just created the directory, and we enter this block only if that's the case.
For a bit more exaplanation, there will be a complementary code on the charm side when attaching a storage to grant the right permissions to the attached storage, like they did in kafka operator.

[Mehdi-Bendriss]

same comment as above

[Gu1nness]

Same answer as above :)