rootless: improve automatic range split

sort.Search returns the smallest index, so provide the available IDs
in decreasing order.

It fixes an issue when splitting the current mappings over multiple
available IDs.

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

pkg/rootless/rootless.go

@@ -137,7 +137,7 @@ func GetAvailableGids() (int64, error) {
 // It assumes availableMappings is sorted by ID.
 func findIDInMappings(id int64, availableMappings []user.IDMap) *user.IDMap {
 	i := sort.Search(len(availableMappings), func(i int) bool {
-		return availableMappings[i].ID >= id
+		return availableMappings[i].ID <= id
 	})
 	if i < 0 || i >= len(availableMappings) {
 		return nil
@@ -157,7 +157,7 @@ func MaybeSplitMappings(mappings []spec.LinuxIDMapping, availableMappings []user
 	overflow.Size = 0
 	consumed := 0
 	sort.Slice(availableMappings, func(i, j int) bool {
-		return availableMappings[i].ID < availableMappings[j].ID
+		return availableMappings[i].ID > availableMappings[j].ID
 	})
 	for {
 		cur := overflow

pkg/rootless/rootless_test.go

@@ -98,4 +98,61 @@ func TestMaybeSplitMappings(t *testing.T) {
 	if !reflect.DeepEqual(newMappings, desiredMappings) {
 		t.Fatal("wrong mappings generated")
 	}
+
+	mappings = []spec.LinuxIDMapping{
+		{
+			ContainerID: 0,
+			HostID:      0,
+			Size:        4,
+		},
+	}
+	desiredMappings = []spec.LinuxIDMapping{
+		{
+			ContainerID: 0,
+			HostID:      0,
+			Size:        1,
+		},
+		{
+			ContainerID: 1,
+			HostID:      1,
+			Size:        1,
+		},
+		{
+			ContainerID: 2,
+			HostID:      2,
+			Size:        1,
+		},
+		{
+			ContainerID: 3,
+			HostID:      3,
+			Size:        1,
+		},
+	}
+	availableMappings = []user.IDMap{
+		{
+			ID:       0,
+			ParentID: 0,
+			Count:    1,
+		},
+		{
+			ID:       1,
+			ParentID: 1,
+			Count:    1,
+		},
+		{
+			ID:       2,
+			ParentID: 2,
+			Count:    1,
+		},
+		{
+			ID:       3,
+			ParentID: 3,
+			Count:    1,
+		},
+	}
+
+	newMappings = MaybeSplitMappings(mappings, availableMappings)
+	if !reflect.DeepEqual(newMappings, desiredMappings) {
+		t.Fatal("wrong mappings generated")
+	}
 }