document and work around issue w/ files disappearing from mirrors..

The local mirror had all the files disappear last September causing
lots of releases to be missing, and subsequently deleted from the
available list for fetching...

addinfo.sh

@@ -29,6 +29,12 @@
 
 set -e
 
+#
+# Note: It appears that sometimes files can "disappear" from the server, so,
+# uncompress snapshot.complete.idx.xz to both snapshot.complete.idx and
+# snapshot.idx, and then run "sh addinfo.sh -c YYYYMMDD" with the oldest
+# known snapshot.
+
 usage() {
 	echo "Usage: $0 [ -m ] <file>"
 	echo "Usage: $0 -c <date>"
@@ -82,6 +88,18 @@ while ! mkdir "$0.running"; do
 done
 
 if [ x"$complete" = x"1" ]; then
+	if [ ! -f snapshot.complete.idx ]; then
+		echo 'snapshot.complete.idx does not exist, aborting...'
+		rmdir "$0.running"
+		exit 5
+	fi
+
+	if [ ! -f snapshot.idx ]; then
+		echo 'snapshot.idx does not exist, aborting...'
+		rmdir "$0.running"
+		exit 5
+	fi
+
 	sort -u snapshot.complete.idx | xz > snapshot.complete.idx.xz
 	awk '$5 == "xxx" || $5 >= "'"$1"'" {
 			if (!system("wget --method=HEAD " $9))