s3ng: Provide objectSize when uploading

According to the documentation for `PutObject`,
setting objectSize to -1 causes a multipart Put
operation until the input stream reaches EOF,
which can result in high memory usage.
Something similar has already been reported in
minio/minio-go#1496

This PR changes the behavior so it tries to determine
the file size before uploading it. Should the given
io.Reader not be a file, it falls back to using `-1`.

In my testing this change has reduced the memory
usage when uploading files to OICS quite a bit:
From over 1GB for a
single user, down to <300MB.

changelog/unreleased/provide-size-for-s3-upload.md

@@ -0,0 +1,5 @@
+Bugfix: Provide file size when uploading with S3ng storage
+
+The memory usage could be high when uploading bigger files using the S3ng storage. 
+By providing the actual file size, the upload partSize can now be properly determined, reducing
+the overall memory usage.

pkg/storage/fs/s3ng/blobstore/blobstore.go

@@ -22,6 +22,7 @@ import (
 	"context"
 	"io"
 	"net/url"
+	"os"
 
 	"github.com/minio/minio-go/v7"
 	"github.com/minio/minio-go/v7/pkg/credentials"
@@ -60,7 +61,16 @@ func New(endpoint, region, bucket, accessKey, secretKey string) (*Blobstore, err
 
 // Upload stores some data in the blobstore under the given key
 func (bs *Blobstore) Upload(key string, reader io.Reader) error {
-	_, err := bs.client.PutObject(context.Background(), bs.bucket, key, reader, -1, minio.PutObjectOptions{ContentType: "application/octet-stream"})
+	size := int64(-1)
+	if file, ok := reader.(*os.File); ok {
+		info, err := file.Stat()
+		if err != nil {
+			return errors.Wrapf(err, "could not determine file size for object '%s'", key)
+		}
+		size = info.Size()
+	}
+
+	_, err := bs.client.PutObject(context.Background(), bs.bucket, key, reader, size, minio.PutObjectOptions{ContentType: "application/octet-stream"})
 
 	if err != nil {
 		return errors.Wrapf(err, "could not store object '%s' into bucket '%s'", key, bs.bucket)