shared: add helper for flushing nscd caches

Apparently, people do use nscd, hence play somewhat nice with it, and
let's explicitly flush nscd caches whenever we register a new
user/group.

This patch only adds the actual refresh request invocation. Later
commits then issue this call at appropriate moments.

Note that the nscd protocol is not officially documented though very
simple. This code is written very defensively so that incompatibilities
don't affect us much.

Given that glibc really has a duty to maintain compat between
differently compiled programs and their system nscd they can't break API
and thus it should be safe for us to implement an alternative,
minimalistic client.

Ideally this kind of explicit, global cache flushing would not be necessary.
However nscd currently has no cache coherency protocol, hence we can't
really implement this better. The only concept it knows is a TTL for
positive hosts lookups. Hoewver for negative lookups or any of the other
tables nothing is available.

1 files changed, 11 insertions, 0 deletions

diff --git a/src/test/meson.build b/src/test/meson.build
index 9863ecc8f0..ea049a6fba 100644
--- a/src/test/meson.build
+++ b/src/test/meson.build
@@ -94,6 +94,17 @@ tests += [
           libblkid],
          '', 'manual'],
 
+        [['src/test/test-nscd-flush.c'],
+         [libcore,
+          libshared],
+         [threads,
+          librt,
+          libseccomp,
+          libselinux,
+          libmount,
+          libblkid],
+         '', 'manual'],
+
         [['src/test/test-loopback.c'],
          [libcore,
           libshared],