forget: Add --unsafe-allow-remove-all option

To prevent accidentally wiping all snapshots from a repository, that
option can only be used if either a snapshot filter or a keep policy is
specified.

Essentially, the option allows `forget --tag something
--unsafe-allow-remove-all` calls to remove all snapshots with a specific
tag.

cmd/restic/cmd_forget.go

@@ -89,6 +89,8 @@ type ForgetOptions struct {
 	WithinYearly  restic.Duration
 	KeepTags      restic.TagLists
 
+	UnsafeAllowRemoveAll bool
+
 	restic.SnapshotFilter
 	Compact bool
 
@@ -118,6 +120,7 @@ func init() {
 	f.VarP(&forgetOptions.WithinMonthly, "keep-within-monthly", "", "keep monthly snapshots that are newer than `duration` (eg. 1y5m7d2h) relative to the latest snapshot")
 	f.VarP(&forgetOptions.WithinYearly, "keep-within-yearly", "", "keep yearly snapshots that are newer than `duration` (eg. 1y5m7d2h) relative to the latest snapshot")
 	f.Var(&forgetOptions.KeepTags, "keep-tag", "keep snapshots with this `taglist` (can be specified multiple times)")
+	f.BoolVar(&forgetOptions.UnsafeAllowRemoveAll, "unsafe-allow-remove-all", false, "allow deleting all snapshots of a snapshot group")
 
 	initMultiSnapshotFilter(f, &forgetOptions.SnapshotFilter, false)
 	f.StringArrayVar(&forgetOptions.Hosts, "hostname", nil, "only consider snapshots with the given `hostname` (can be specified multiple times)")
@@ -211,7 +214,14 @@ func runForget(ctx context.Context, opts ForgetOptions, pruneOptions PruneOption
 		}
 
 		if policy.Empty() {
-			return fmt.Errorf("no policy was specified, no snapshots will be removed")
+			if opts.UnsafeAllowRemoveAll {
+				if opts.SnapshotFilter.Empty() {
+					return errors.Fatal("--unsafe-allow-remove-all is not allowed unless a snapshot filter option is specified")
+				}
+				// UnsafeAllowRemoveAll together with snapshot filter is fine
+			} else {
+				return errors.Fatal("no policy was specified, no snapshots will be removed")
+			}
 		}
 
 		if !gopts.JSON {

internal/restic/snapshot_policy.go

@@ -94,7 +94,11 @@ func (e ExpirePolicy) String() (s string) {
 		s += fmt.Sprintf("all snapshots within %s of the newest", e.Within)
 	}
 
-	s = "keep " + s
+	if s == "" {
+		s = "remove"
+	} else {
+		s = "keep " + s
+	}
 
 	return s
 }
@@ -186,16 +190,6 @@ func ApplyPolicy(list Snapshots, p ExpirePolicy) (keep, remove Snapshots, reason
 	// sort newest snapshots first
 	sort.Stable(list)
 
-	if p.Empty() {
-		for _, sn := range list {
-			reasons = append(reasons, KeepReason{
-				Snapshot: sn,
-				Matches:  []string{"policy is empty"},
-			})
-		}
-		return list, remove, reasons
-	}
-
 	if len(list) == 0 {
 		return list, nil, nil
 	}