Do you have a code example that doesn't work as expected? Thanks.

hi @cherrymui,
thanks for your fast response!

here is a code example: https://go.dev/play/p/BfjeTIOkq-M

From the definition of io.EOF:

Functions should return EOF only to signal a graceful end of input. If the EOF occurs unexpectedly in a structured data stream, the appropriate error is either ErrUnexpectedEOF or some other error giving more detail.

This is an unexpected end of input, so the error is correct.

hi @seankhliao,
indeed the error itself is correct - I never said something different!
Im pretty sure that the "wrapping" in L346 is wrong! By returning the io.EOF with "%v" it is not wrapped and thus not useable with errors.Is().
By replaceing the "%v" with a "%w" it works as expected...
So its not about the error itself!

but allowing you to test against EOF is wrong, since EOF represents a clean exit, and this is not

hi @seankhliao,
the following code is used in mime/multipart/multipart.go#L346:

...
		if err != nil {
			return nil, fmt.Errorf("multipart: NextPart: %v", err)
		}
...

by using "%v" the caller can not check the error and it does not have to be an io.EOF error - it could be any error...
so the caller just sees the error but is not able to check what kind of error it is!

further I do not understand what you mean by "but allowing you to test against EOF is wrong, since EOF represents a clean exit, and this is not" - you should be allowed to test against any error (error wrapping - https://pkg.go.dev/errors) but thats a different topic...