Simplify coding around path_contains_parent_reference().
Given the existing stipulation that path_contains_parent_reference()
must only be invoked on canonicalized paths, we can simplify things
in the wake of commit
c10f830c5. It is now only possible to see
".." at the start of a relative path. That means we can simplify
path_contains_parent_reference() itself quite a bit, and it makes
the two existing outside call sites dead code, since they'd already
checked that the path is absolute.
We could now fold path_contains_parent_reference() into its only
remaining caller path_is_relative_and_below_cwd(). But it seems
better to leave it as a separately callable function, in case any
extensions are using it.
Also document the pre-existing requirement for
path_is_relative_and_below_cwd's input to be likewise canonicalized.
Shenhao Wang and Tom Lane
Discussion: https://postgr.es/m/OSBPR01MB4214FA221FFE046F11F2AD74F2D49@OSBPR01MB4214.jpnprd01.prod.outlook.com