internal/restic: Use IDSet.Clone + use maps package

One place where IDSet.Clone is useful was reinventing it, using a
conversion to list, a sort, and a conversion back to map.

Also, use the stdlib "maps" package to implement as much of IDSet as
possible. This requires changing one caller, which assumed that cloning
nil would return a non-nil IDSet.

internal/repository/index/master_index.go

@@ -347,6 +347,9 @@ func (mi *MasterIndex) Rewrite(ctx context.Context, repo restic.Unpacked, exclud
 
 	// copy excludePacks to prevent unintended sideeffects
 	excludePacks = excludePacks.Clone()
+	if excludePacks == nil {
+		excludePacks = restic.NewIDSet()
+	}
 	debug.Log("start rebuilding index of %d indexes, excludePacks: %v", len(indexes), excludePacks)
 	wg, wgCtx := errgroup.WithContext(ctx)
 

internal/restic/idset.go

@@ -1,6 +1,9 @@
 package restic
 
-import "sort"
+import (
+	"maps"
+	"sort"
+)
 
 // IDSet is a set of IDs.
 type IDSet map[ID]struct{}
@@ -44,28 +47,10 @@ func (s IDSet) List() IDs {
 }
 
 // Equals returns true iff s equals other.
-func (s IDSet) Equals(other IDSet) bool {
-	if len(s) != len(other) {
-		return false
-	}
-
-	for id := range s {
-		if _, ok := other[id]; !ok {
-			return false
-		}
-	}
-
-	// length + one-way comparison is sufficient implication of equality
-
-	return true
-}
+func (s IDSet) Equals(other IDSet) bool { return maps.Equal(s, other) }
 
 // Merge adds the blobs in other to the current set.
-func (s IDSet) Merge(other IDSet) {
-	for id := range other {
-		s.Insert(id)
-	}
-}
+func (s IDSet) Merge(other IDSet) { maps.Copy(s, other) }
 
 // Intersect returns a new set containing the IDs that are present in both sets.
 func (s IDSet) Intersect(other IDSet) (result IDSet) {
@@ -106,8 +91,4 @@ func (s IDSet) String() string {
 	return "{" + str[1:len(str)-1] + "}"
 }
 
-func (s IDSet) Clone() IDSet {
-	c := NewIDSet()
-	c.Merge(s)
-	return c
-}
+func (s IDSet) Clone() IDSet { return maps.Clone(s) }

internal/restic/lock.go

@@ -187,7 +187,7 @@ func (l *Lock) checkForOtherLocks(ctx context.Context) error {
 
 		// Store updates in new IDSet to prevent data races
 		var m sync.Mutex
-		newCheckedIDs := NewIDSet(checkedIDs.List()...)
+		newCheckedIDs := checkedIDs.Clone()
 		err = ForAllLocks(ctx, l.repo, checkedIDs, func(id ID, lock *Lock, err error) error {
 			if err != nil {
 				// if we cannot load a lock then it is unclear whether it can be ignored