Fix off-by-one bug in find_outdated_snapshot().

The man page sayeth:

	"dss removes any snapshots older than n times u",

where n is the number of unit intervals and u is the duration of
a unit interval. As intervals count from zero, this means that a
snapshot should be considered outdated if its interval number
is greater _or equal_ than n.

However, the current code only removes snapshots in intervals
strictly greater than n. Fix this bug and clarify the documentation.

2 files changed, 13 insertions, 7 deletions

diff --git a/dss.c b/dss.c
index 6101f17..f04be8f 100644
--- a/dss.c
+++ b/dss.c
@@ -385,14 +385,14 @@ static struct snapshot *find_outdated_snapshot(struct snapshot_list *sl)
 	int i;
 	struct snapshot *s;
 
-	DSS_DEBUG_LOG("looking for snapshots belonging to intervals greater than %d\n",
+	DSS_DEBUG_LOG("looking for snapshots belonging to intervals >= %d\n",
 		conf.num_intervals_arg);
 	FOR_EACH_SNAPSHOT(s, i, sl) {
 		if (snapshot_is_being_created(s))
 			continue;
 		if (is_reference_snapshot(s))
 			continue;
-		if (s->interval <= conf.num_intervals_arg)
+		if (s->interval < conf.num_intervals_arg)
 			continue;
 		return s;
 	}
diff --git a/dss.ggo b/dss.ggo
index 23f2685..506fbf1 100644
--- a/dss.ggo
+++ b/dss.ggo
@@ -110,11 +110,17 @@ groupoption "prune" P
 "Remove redundant and outdated snapshots"
 group="command"
 details="
-	A snapshot is considered outdated if it belongs to an interval
-	greater than the maximum number of intervals. It is said to be
-	redundant if it belongs to an interval that already contains
-	more than the desired number of snapshots. This command gets
-	rid of such snapshots.
+	A snapshot is considered outdated if its interval number
+	is greater or equal than the specified number of unit
+	intervals. See the \"Intervals\" section below for the precise
+	definition of these terms.
+
+	A snapshot is said to be redundant if it belongs to an
+	interval that already contains more than the desired number
+	of snapshots.
+
+	The prune command gets rid of both outdated and redundant
+	snapshots.
 "
 
 groupoption "ls" L