cmd/serve/s3: return object listings in key order

The S3 ListObjects response from `rclone serve s3` was sorting object
contents by modification time instead of object key. This made the
listing order incompatible with S3 clients which expect lexicographic
key ordering.

In particular, `aws s3 sync` assumes both source and destination
iterators are ordered by key. With the old modtime ordering it could
misidentify files as missing or outdated and re-download objects that
were already up to date.

Change the pager to sort returned objects by key and add a regression
test which uses keys and modtimes arranged so the old behaviour would
fail.

Fixes #9002

cmd/serve/s3/pager.go

@@ -13,9 +13,9 @@ func (db *s3Backend) pager(list *gofakes3.ObjectList, page gofakes3.ListBucketPa
 	sort.Slice(list.CommonPrefixes, func(i, j int) bool {
 		return list.CommonPrefixes[i].Prefix < list.CommonPrefixes[j].Prefix
 	})
-	// sort by modtime
+	// sort by key name
 	sort.Slice(list.Contents, func(i, j int) bool {
-		return list.Contents[i].LastModified.Before(list.Contents[j].LastModified.Time)
+		return list.Contents[i].Key < list.Contents[j].Key
 	})
 	tokens := page.MaxKeys
 	if tokens == 0 {

cmd/serve/s3/pager_test.go

@@ -0,0 +1,32 @@
+package s3
+
+import (
+	"testing"
+	"time"
+
+	"github.com/rclone/gofakes3"
+)
+
+func TestPagerSortsContentsByKey(t *testing.T) {
+	list := gofakes3.NewObjectList()
+	list.Add(&gofakes3.Content{
+		Key:          "b.txt",
+		LastModified: gofakes3.NewContentTime(time.Unix(100, 0)),
+	})
+	list.Add(&gofakes3.Content{
+		Key:          "a.txt",
+		LastModified: gofakes3.NewContentTime(time.Unix(200, 0)),
+	})
+
+	got, err := (&s3Backend{}).pager(list, gofakes3.ListBucketPage{MaxKeys: 2})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(got.Contents) != 2 {
+		t.Fatalf("expected 2 contents, got %d", len(got.Contents))
+	}
+
+	if got.Contents[0].Key != "a.txt" || got.Contents[1].Key != "b.txt" {
+		t.Fatalf("expected lexicographic key order [a.txt b.txt], got [%s %s]", got.Contents[0].Key, got.Contents[1].Key)
+	}
+}