nixos/nscd: Disable caching of group and passwd

Systemd provides an option for allocating DynamicUsers
which we want to use in NixOS to harden service configuration.
However, we discovered that the user wasn't allocated properly
for services. After some digging this turned out to be, of course,
a cache inconsistency problem.

When a DynamicUser creation is performed, Systemd check beforehand
whether the requested user already exists statically. If it does,
it bails out. If it doesn't, systemd continues with allocating the
user.

However, by checking whether the user exists,  nscd will store
the fact that the user does not exist in it's negative cache.
When the service tries to lookup what user is associated to its
uid (By calling whoami, for example), it will try to consult
libnss_systemd.so However this will read from the cache and tell
report that the user doesn't exist, and thus will return that
there is no user associated with the uid. It will continue
to do so for the cache duration time.  If the service
doesn't immediately looks up its username, this bug is not
triggered, as the cache will be invalidated around this time.
However, if the service is quick enough, it might end up
in a situation where it's incorrectly reported that the
user doesn't exist.

Preferably, we would not be using nscd at all. But we need to
use it because glibc reads  nss modules from /etc/nsswitch.conf
by looking relative to the global LD_LIBRARY_PATH.  Because LD_LIBRARY_PATH
is not set globally (as that would lead to impurities and ABI issues),
glibc will fail to find any nss modules.
Instead, as a hack, we start up nscd with LD_LIBRARY_PATH set
for only that service. Glibc will forward all nss syscalls to
nscd, which will then respect the LD_LIBRARY_PATH and only
read from locations specified in the NixOS config.
we can load nss modules in a pure fashion.

However, I think by accident, we just copied over the default
settings of nscd, which actually caches user and group lookups.
We already disable this when sssd is enabled, as this interferes
with the correct working of libnss_sss.so as it already
does its own caching of LDAP requests.
(See https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/6/html/deployment_guide/usingnscd-sssd)

Because nscd caching is now also interferring with libnss_systemd.so
and probably also with other nsss modules, lets just pre-emptively
disable caching for now for all options related to users and groups,
but keep it for caching hosts ans services lookups.

Note that we can not just put in /etc/nscd.conf:
enable-cache passwd no

As this will actually cause glibc to _not_ forward the call to nscd
at all, and thus never reach the nss modules. Instead we set
the negative and positive cache ttls  to 0 seconds as a workaround.
This way, Glibc will always forward requests to nscd, but results
will never be cached.

Fixes #50273

nixos/modules/services/system/nscd.conf

@@ -4,16 +4,16 @@ paranoia                no
 debug-level             0
 
 enable-cache            passwd          yes
-positive-time-to-live   passwd          600
-negative-time-to-live   passwd          20
+positive-time-to-live   passwd          0
+negative-time-to-live   passwd          0
 suggested-size          passwd          211
 check-files             passwd          yes
 persistent              passwd          no
 shared                  passwd          yes
 
 enable-cache            group           yes
-positive-time-to-live   group           3600
-negative-time-to-live   group           60
+positive-time-to-live   group           0
+negative-time-to-live   group           0
 suggested-size          group           211
 check-files             group           yes
 persistent              group           no