fix alternc-slaves detection and warn the user if there's no key to authorize

debian/alternc.postinst

@@ -158,19 +158,26 @@ EOF
     # multi-server configuration: we create an alternc account with
     # authorized keys. since this is the master, we do not give him a
     # valid shell, but we still need the user for proper perms
-    if [ ! -z "$ALTERNC_SLAVES" && "$ALTERNC_SLAVES" != "localhost" ] ; then
+    if [ ! -z "$ALTERNC_SLAVES" ] && [ "$ALTERNC_SLAVES" != "localhost" ] ; then
         if ! grep -q alternc /etc/passwd ; then
             echo "Creating alternc account"
             adduser --quiet --system --uid 342 --home $ALTERNC_LOC --shell /bin/false --ingroup adm alternc
         fi
-        key=`cat ~root/.ssh/id_dsa.pub`
-        if ! grep -q "$key" $ALTERNC_LOC/.ssh/authorized_keys ; then
-            echo "Authorizing root ssh key to access the common alternc account"
-            mkdir -p $ALTERNC_LOC/.ssh
-            echo "$key" >> $ALTERNC_LOC/.ssh/authorized_keys
-            chown -R alternc:adm $ALTERNC_LOC/.ssh
-            chmod -R og-rwx $ALTERNC_LOC/.ssh
+        if [ -r ~root/.ssh/id_dsa.pub ]; then
+            key=`cat ~root/.ssh/id_dsa.pub`
+            if ! grep -q "$key" $ALTERNC_LOC/.ssh/authorized_keys ; then
+                echo "Authorizing root ssh key to access the common alternc account"
+                mkdir -p $ALTERNC_LOC/.ssh
+                echo "$key" >> $ALTERNC_LOC/.ssh/authorized_keys
+                chown -R alternc:adm $ALTERNC_LOC/.ssh
+                chmod -R og-rwx $ALTERNC_LOC/.ssh
+            fi
+        else
+            echo "No SSH key in "~root/.ssh/id_dsa.pub
+            echo "create one and reconfigure alternc to propagate SSH keys"
         fi
+    else
+        echo "AlternC slaves not configured ($ALTERNC_SLAVES)"
     fi
 
     # /var/alternc/dns/d/www.example.com