multipart: fix chunked uploads storing truncated objects when the source ends early

If the source supplied fewer bytes than its declared size, the
multipart upload was completed anyway, storing a truncated object and
reporting a successful upload.

Check the number of bytes read from the source against the declared
size before finalising and abort the upload with an error if they do
not match.

This affects all backends which use lib/multipart for chunked uploads:
azureblob, b2, drime, internxt, oracleobjectstorage, s3 and shade.

This was found by the new FsPutShortEOF and TestRcatSizeShortEOF
integration tests.

lib/multipart/multipart.go

@@ -121,6 +121,13 @@ func UploadMultipart(ctx context.Context, src fs.ObjectInfo, in io.Reader, opt U
 		return nil, err
 	}
 
+	// Check the source supplied the number of bytes it declared before
+	// finalising, otherwise a truncated object would be created on the
+	// backend. Returning an error here aborts the upload.
+	if size >= 0 && off != size {
+		return nil, fmt.Errorf("multipart upload: expected %d bytes in input, but got %d: %w", size, off, io.ErrUnexpectedEOF)
+	}
+
 	err = chunkWriter.Close(ctx)
 	if err != nil {
 		return nil, fmt.Errorf("multipart upload: failed to finalise: %w", err)