Mount result read-only in Query-to-File

[vogti]

When executing arbitrary queries via Query-to-File, the result is materialized in the mounted folder. Since updating is anyway not support for arbitrary queries (this is only supported in the table mode), it would be beneficial to mount the folder read-only.

Since FUSE supports mounting file systems read-only, fixing this issue breaks down into finding the required options and setting them when executing arbitrary queries.