cmd/snapshots: Add option to limit snapshots list

This patch adds a `--latest` option to limit snapshots list to the n
last snapshots. It is very similar to the `--last` one but does not
limit to one entry. It also deprecates the `--last` flag usage in
favor of `--latest 1`

Output example:

    $ restic snapshots --latest 2
    repository 0d3eb989 opened successfully, password is correct
    ID        Time                 Host        Tags        Paths
    ------------------------------------------------------------
    5a33bdcc  2020-12-14 12:30:00  local                   /home
    73887d8e  2020-12-15 12:30:00  local                   /home
    ------------------------------------------------------------
    2 snapshots

Signed-off-by: Sébastien Gross <seb•ɑƬ•chezwam•ɖɵʈ•org>

changelog/unreleased/pull-3167

@@ -0,0 +1,9 @@
+Enhancement: Allow limiting the snapshots list
+
+The `--last` option allowed limiting the output of the `snapshots`
+command to the latest snapshot for each host. The new `--latest n`
+option allows limiting the output to the latest `n` snapshots.
+
+This change deprecate `--last` in favour of `--latest 1`
+
+https://github.com/restic/restic/pull/3167

cmd/restic/cmd_snapshots.go

@@ -36,7 +36,8 @@ type SnapshotOptions struct {
 	Tags    restic.TagLists
 	Paths   []string
 	Compact bool
-	Last    bool
+	Last    bool // This option should be removed in favour of Latest.
+	Latest  int
 	GroupBy string
 }
 
@@ -51,6 +52,12 @@ func init() {
 	f.StringArrayVar(&snapshotOptions.Paths, "path", nil, "only consider snapshots for this `path` (can be specified multiple times)")
 	f.BoolVarP(&snapshotOptions.Compact, "compact", "c", false, "use compact output format")
 	f.BoolVar(&snapshotOptions.Last, "last", false, "only show the last snapshot for each host and path")
+	err := f.MarkDeprecated("last", "use --latest 1")
+	if err != nil {
+		// MarkDeprecated only returns an error when the flag is not found
+		panic(err)
+	}
+	f.IntVar(&snapshotOptions.Latest, "latest", 0, "only show the last `n` snapshots for each host and path")
 	f.StringVarP(&snapshotOptions.GroupBy, "group-by", "g", "", "string for grouping snapshots by host,paths,tags")
 }
 
@@ -82,7 +89,11 @@ func runSnapshots(opts SnapshotOptions, gopts GlobalOptions, args []string) erro
 
 	for k, list := range snapshotGroups {
 		if opts.Last {
-			list = FilterLastSnapshots(list)
+			// This branch should be removed in the same time
+			// that --last.
+			list = FilterLastestSnapshots(list, 1)
+		} else if opts.Latest > 0 {
+			list = FilterLastestSnapshots(list, opts.Latest)
 		}
 		sort.Sort(sort.Reverse(list))
 		snapshotGroups[k] = list
@@ -125,21 +136,22 @@ func newFilterLastSnapshotsKey(sn *restic.Snapshot) filterLastSnapshotsKey {
 	return filterLastSnapshotsKey{sn.Hostname, strings.Join(paths, "|")}
 }
 
-// FilterLastSnapshots filters a list of snapshots to only return the last
-// entry for each hostname and path. If the snapshot contains multiple paths,
-// they will be joined and treated as one item.
-func FilterLastSnapshots(list restic.Snapshots) restic.Snapshots {
+// FilterLastestSnapshots filters a list of snapshots to only return
+// the limit last entries for each hostname and path. If the snapshot
+// contains multiple paths, they will be joined and treated as one
+// item.
+func FilterLastestSnapshots(list restic.Snapshots, limit int) restic.Snapshots {
 	// Sort the snapshots so that the newer ones are listed first
 	sort.SliceStable(list, func(i, j int) bool {
 		return list[i].Time.After(list[j].Time)
 	})
 
 	var results restic.Snapshots
-	seen := make(map[filterLastSnapshotsKey]bool)
+	seen := make(map[filterLastSnapshotsKey]int)
 	for _, sn := range list {
 		key := newFilterLastSnapshotsKey(sn)
-		if !seen[key] {
-			seen[key] = true
+		if seen[key] < limit {
+			seen[key]++
 			results = append(results, sn)
 		}
 	}