s3: enable content hash calculation for uploads

internal/backend/b2/b2.go

@@ -200,6 +200,7 @@ func (be *b2Backend) Save(ctx context.Context, h restic.Handle, rd restic.Rewind
 	debug.Log("Save %v, name %v", h, name)
 	obj := be.bucket.Object(name)
 
+	// b2 always requires sha1 checksums for uploaded file parts
 	w := obj.NewWriter(ctx)
 	n, err := io.Copy(w, rd)
 	debug.Log("  saved %d bytes, err %v", n, err)

internal/backend/s3/s3.go

@@ -270,6 +270,8 @@ func (be *Backend) Save(ctx context.Context, h restic.Handle, rd restic.RewindRe
 
 	opts := minio.PutObjectOptions{StorageClass: be.cfg.StorageClass}
 	opts.ContentType = "application/octet-stream"
+	// the only option with the high-level api is to let the library handle the checksum computation
+	opts.SendContentMd5 = true
 
 	debug.Log("PutObject(%v, %v, %v)", be.cfg.Bucket, objName, rd.Length())
 	info, err := be.client.PutObject(ctx, be.cfg.Bucket, objName, ioutil.NopCloser(rd), int64(rd.Length()), opts)