Use a pager to iterate through a GCS bucket

Iterating through a GCS bucket can be resource intensive, especially
when doing it recursively for all objects in the bucket.

This commit tries to resolve that by using a pager with a fixed page
size and reusing a single slice to process results.

go.mod

@@ -27,6 +27,7 @@ require (
 	golang.org/x/oauth2 v0.4.0
 	golang.org/x/sync v0.1.0
 	google.golang.org/api v0.103.0
+	google.golang.org/grpc v1.53.0
 	gopkg.in/alecthomas/kingpin.v2 v2.2.6
 	gopkg.in/yaml.v2 v2.4.0
 )
@@ -92,7 +93,6 @@ require (
 	golang.org/x/xerrors v0.0.0-20220907171357-04be3eba64a2 // indirect
 	google.golang.org/appengine v1.6.7 // indirect
 	google.golang.org/genproto v0.0.0-20230110181048-76db0878b65f // indirect
-	google.golang.org/grpc v1.53.0 // indirect
 	google.golang.org/protobuf v1.28.1 // indirect
 	gopkg.in/check.v1 v1.0.0-20201130134442-10cb98267c6c // indirect
 	gopkg.in/ini.v1 v1.67.0 // indirect

go.sum

@@ -491,7 +491,6 @@ golang.org/x/sys v0.0.0-20210615035016-665e8c7367d1/go.mod h1:oPkhp1MJrh7nUepCBc
 golang.org/x/sys v0.0.0-20210616045830-e2b7044e8c71/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
 golang.org/x/sys v0.0.0-20211216021012-1d35b9e2eb4e/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
 golang.org/x/sys v0.0.0-20220114195835-da31bd327af9/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
-golang.org/x/sys v0.0.0-20220209214540-3681064d5158/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
 golang.org/x/sys v0.0.0-20220715151400-c0bba94af5f8/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
 golang.org/x/sys v0.5.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
 golang.org/x/sys v0.6.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=

providers/gcs/gcs.go

@@ -108,25 +108,30 @@ func (b *Bucket) Iter(ctx context.Context, dir string, f func(string) error, opt
 		delimiter = ""
 	}
 
-	it := b.bkt.Objects(ctx, &storage.Query{
+	const pageSize = 1000
+	items := make([]*storage.ObjectAttrs, 0, pageSize)
+	it := iterator.NewPager(b.bkt.Objects(ctx, &storage.Query{
 		Prefix:    dir,
 		Delimiter: delimiter,
-	})
+	}), pageSize, "")
 	for {
 		select {
 		case <-ctx.Done():
 			return ctx.Err()
 		default:
 		}
-		attrs, err := it.Next()
-		if err == iterator.Done {
-			return nil
-		}
+		token, err := it.NextPage(&items)
 		if err != nil {
 			return err
 		}
-		if err := f(attrs.Prefix + attrs.Name); err != nil {
-			return err
+		for _, attrs := range items {
+			if err := f(attrs.Prefix + attrs.Name); err != nil {
+				return err
+			}
+		}
+		items = items[:0]
+		if token == "" {
+			return nil
 		}
 	}
 }