New helper function `FindFilteredSnapshots` to iterate over snapshots

This helper function takes a set of filters and/or a list of snapshots
from the commandline. It returns a channel of *Snapshot.

When snapshot ids are given, they are checked for validity and their
corresponding Snapshots returned. The snapshot id "latest" is handled
special to return either the last snapshot (no filters) or the last
snapshot matching the filters.

When no arguments are given, the filters are applied over all available
snapshots and these are returned.

src/cmds/restic/find.go

@@ -0,0 +1,78 @@
+package main
+
+import (
+	"context"
+
+	"restic"
+	"restic/repository"
+)
+
+// FindFilteredSnapshots yields Snapshots, either given explicitly by `snapshotIDs` or filtered from the list of all snapshots.
+func FindFilteredSnapshots(ctx context.Context, repo *repository.Repository, host string, tags []string, paths []string, snapshotIDs []string) <-chan *restic.Snapshot {
+	out := make(chan *restic.Snapshot)
+	go func() {
+		defer close(out)
+		if len(snapshotIDs) != 0 {
+			var (
+				id         restic.ID
+				usedFilter bool
+				err        error
+			)
+			ids := make(restic.IDs, 0, len(snapshotIDs))
+			// Process all snapshot IDs given as arguments.
+			for _, s := range snapshotIDs {
+				if s == "latest" {
+					id, err = restic.FindLatestSnapshot(repo, paths, tags, host)
+					if err != nil {
+						Warnf("Ignoring %q, no snapshot matched given filter (Paths:%v Tags:%v Host:%v)\n", s, paths, tags, host)
+						usedFilter = true
+						continue
+					}
+				} else {
+					id, err = restic.FindSnapshot(repo, s)
+					if err != nil {
+						Warnf("Ignoring %q, it is not a snapshot id\n", s)
+						continue
+					}
+				}
+				ids = append(ids, id)
+			}
+
+			// Give the user some indication their filters are not used.
+			if !usedFilter && (host != "" || len(tags) != 0 || len(paths) != 0) {
+				Warnf("Ignoring filters as there are explicit snapshot ids given\n")
+			}
+
+			for _, id := range ids.Uniq() {
+				sn, err := restic.LoadSnapshot(repo, id)
+				if err != nil {
+					Warnf("Ignoring %q, could not load snapshot: %v\n", id, err)
+					continue
+				}
+				select {
+				case <-ctx.Done():
+					return
+				case out <- sn:
+				}
+			}
+			return
+		}
+
+		for id := range repo.List(restic.SnapshotFile, ctx.Done()) {
+			sn, err := restic.LoadSnapshot(repo, id)
+			if err != nil {
+				Warnf("Ignoring %q, could not load snapshot: %v\n", id, err)
+				continue
+			}
+			if (host != "" && host != sn.Hostname) || !sn.HasTags(tags) || !sn.HasPaths(paths) {
+				continue
+			}
+			select {
+			case <-ctx.Done():
+				return
+			case out <- sn:
+			}
+		}
+	}()
+	return out
+}