selftest: Add more notes on using selftest with namespaces

In particular, document how to hook up a testenv to a Windows VM
(ideally there should be a helper script to do this, but in the
meantime some instructions are better than nothing).

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>

selftest/ns/README

@@ -63,3 +63,100 @@ The veth interfaces are named vethX and vethX-br, where X is the
 SOCKET_WRAPPER_DEFAULT_IFACE for the testenv. The vethX-br interface is always
 added to the selftest0 bridge interface. 
 
+How do I use it?
+================
+To use namespaces instead of socket-wrapper, just add 'USE_NAMESPACES=1' to the
+make command, e.g.
+
+To run the 'quick' test cases using namespaces:
+USE_NAMESPACES=1 make test TESTS=quick
+
+To setup an ad_dc testenv using namespaces:
+USE_NAMESPACES=1 SELFTEST_TESTENV=ad_dc make testenv
+
+You can connect secondary shells to the namespace your testenv is running in.
+The command to do this is a little complicated, so a helper 'nsenter.sh' script
+gets autogenerated when the testenv is created. E.g. to connect to the testenv
+that the ad_dc is running in, use:
+./st/ad_dc/nsenter.sh
+
+This script also sets up the shell with all the same $SERVER/$USERNAME/etc
+variables that you normally get in xterm.
+
+To run the ad-dc-backup autobuild job using namespaces:
+USE_NAMESPACES=1 script/autobuild.py samba-ad-dc-backup --verbose --nocleanup \
+ --keeplogs --tail --testbase /tmp/samba-testbase
+
+Using the customdc testenv, you can basically now essentially your own
+light-weight samba VM. E.g.
+MY_BACKUP=/home/$USER/samba-backup-prod-domain.tar.bz2
+USE_NAMESPACES=1 BACKUP_FILE=$MY_BACKUP SELFTEST_TESTENV=customdc make testenv
+
+You can then talk to that DC in any other shell by using
+./st/customdc/nsenter.sh which enters the DC's network namespace (with
+all the $SERVER/etc env variables defined).
+
+How to join VMs to the testenv
+----------------------------------------
+I haven't tried this (beyond basic IP connectivity), but using namespaces it
+should now be possible to connect a Windows VM to a Samba testenv.
+
+1. Work out the main selftest.pl namespace PID manually, e.g.
+SELFTEST_PID= ps waux | grep selftest.pl
+
+2. Create a new veth to bridge between the selftest namespace and your PC's
+default namespace:
+sudo ip link add dev testenv-veth0 type veth peer name testenv-veth1
+
+3. Move one end of the veth tunnel into the selftest namespace:
+sudo ip link set testenv-veth1 netns $SELFTEST_PID
+
+4. Configure the veth end in the default namespace to be in the same subnet
+as the selftest network:
+sudo ip link set dev testenv-veth0 up
+sudo ip addr add 10.0.0.63/24 dev testenv-veth0
+
+5. Enter the selftest namespace, bring that end of the pipe up, and add it to
+to the main selftest0 bridge (that connects all the DCs together). We also need
+to add a default route from selftest back to your PC's default namespace.
+nsenter -t $SELFTEST_PID --net --user --preserve-credentials
+ip link set dev testenv-veth1 up
+ip link set testenv-veth1 master selftest0
+ip route add default via 10.0.0.63
+logout
+
+Your Windows VM and samba testenv should now be able to talk to each
+other over IP!
+
+6. The other step is to get DNS working. You probably need to add dns_hub
+(10.0.0.64) as a nameserver (at least on your Windows VM).
+
+This should work for using RSAT tools on samba, or joining Windows to Samba
+(depending on the schema version). Joining samba to Windows is a bit more
+tricky, as the namespaces are tied to the *running* samba process.
+
+What you'd probably want to do is run the join command to the windows VM
+outside of testenv, create an offline backup-file of the resulting DB, and
+then plug that backup-file into the customdc testenv. (And then follow the
+above veth/bridge steps to join samba to the VM).
+
+Note that the namespace disappears once you stop the testenv, so you'd
+need to do the above steps with creating the veth interface every time
+you restarted the testenv.
+
+Known limitations
+=================
+- When running a testenv, sometimes xterm can fail to startup, due to a
+  permissions problem with /dev/pts. This seems to be a particular problem
+  with the 'none' testenv.
+  A short-term work-around is to use a terminal that doesn't try to access
+  /dev/pts, e.g. just use bash as the terminal:
+  TERMINAL=bash TERMINAL_ARGS='--norc' USE_NAMESPACES=1 \
+    SELFTEST_TESTENV=none make testenv
+- Some test cases rely on socket-wrapper, so will fail when run using
+  namespaces.
+- Currently USE_NAMESPACES maps you (i.e. $USER) to root in the new namespace.
+  This means any test cases that rely on being a non-root user will fail (i.e.
+  anything that fails under 'sudo make test' will also fail with namespaces).
+- Namespaces should work within docker, but currently the 'unshare' system
+  call is disallowed on the gitlab CI runners.