[hostname-config] improve hostname-config process (#3676)

We noticed in tests/production that there is a low probability failure
where /etc/hosts could have some garbage characters before the entry for
local host name. The consequence is that all sudo command would be very
slow. In extreme cases it would prevent some services from starting
properly.

I suspect that the /etc/hosts file might be opened by some process causing
the issue. Editing contents with new file level and replace the whole file
should be safer.

Signed-off-by: Ying Xie <ying.xie@microsoft.com>

files/image_config/hostname/hostname-config.sh

@@ -8,9 +8,15 @@ hostname -F /etc/hostname
 
 # Remove the old hostname entry from hosts file.
 # But, 'localhost' entry is used by multiple applications. Don't remove it altogether.
+# Edit contents of /etc/hosts and put in /etc/hosts.new
 if [ $CURRENT_HOSTNAME  != "localhost" ] || [ $CURRENT_HOSTNAME == $HOSTNAME ] ;  then
- sed -i "/\s$CURRENT_HOSTNAME$/d" /etc/hosts
+    sed "/\s$CURRENT_HOSTNAME$/d" /etc/hosts > /etc/hosts.new
+else
+    cp -f /etc/hosts /etc/hosts.new
 fi
 
-echo "127.0.0.1 $HOSTNAME" >> /etc/hosts
+echo "127.0.0.1 $HOSTNAME" >> /etc/hosts.new
 
+# Swap file: hosts.new and hosts
+mv -f /etc/hosts     /etc/hosts.old
+mv -f /etc/hosts.new /etc/hosts