fuse: Better check for whether snapshots changed

We previously checked whether the set of snapshots might have changed
based only on their number, which fails when as many snapshots are
forgotten as are added. Check for the SHA-256 of their id's instead.

internal/fuse/snapshots_dirstruct.go

@@ -4,6 +4,7 @@
 package fuse
 
 import (
+	"bytes"
 	"context"
 	"fmt"
 	"path"
@@ -14,6 +15,8 @@ import (
 
 	"github.com/restic/restic/internal/debug"
 	"github.com/restic/restic/internal/restic"
+
+	"github.com/minio/sha256-simd"
 )
 
 type MetaDirData struct {
@@ -39,7 +42,7 @@ type SnapshotsDirStructure struct {
 	// that way we don't need path processing special cases when using the entries tree
 	entries map[string]*MetaDirData
 
-	snCount   int
+	hash      [sha256.Size]byte // Hash at last check.
 	lastCheck time.Time
 }
 
@@ -49,7 +52,6 @@ func NewSnapshotsDirStructure(root *Root, pathTemplates []string, timeTemplate s
 		root:          root,
 		pathTemplates: pathTemplates,
 		timeTemplate:  timeTemplate,
-		snCount:       -1,
 	}
 }
 
@@ -302,10 +304,26 @@ func (d *SnapshotsDirStructure) updateSnapshots(ctx context.Context) error {
 	if err != nil {
 		return err
 	}
-	// sort snapshots ascending by time (default order is descending)
-	sort.Sort(sort.Reverse(snapshots))
 
-	if d.snCount == len(snapshots) {
+	// Sort snapshots ascending by time, using the id to break ties.
+	// This needs to be done before hashing.
+	sort.Slice(snapshots, func(i, j int) bool {
+		si, sj := snapshots[i], snapshots[j]
+		if si.Time.Equal(sj.Time) {
+			return bytes.Compare(si.ID()[:], sj.ID()[:]) < 0
+		}
+		return si.Time.Before(sj.Time)
+	})
+
+	// We update the snapshots when the hash of their id's changes.
+	h := sha256.New()
+	for _, sn := range snapshots {
+		h.Write(sn.ID()[:])
+	}
+	var hash [sha256.Size]byte
+	h.Sum(hash[:0])
+
+	if d.hash == hash {
 		d.lastCheck = time.Now()
 		return nil
 	}
@@ -316,7 +334,7 @@ func (d *SnapshotsDirStructure) updateSnapshots(ctx context.Context) error {
 	}
 
 	d.lastCheck = time.Now()
-	d.snCount = len(snapshots)
+	d.hash = hash
 	d.makeDirs(snapshots)
 	return nil
 }