Empty attachment with wrong size

[rqueizan]

Describe the bug
In previous versions of the library, when an empty attachment (without content) was read, it came with zero size.
Previous Version: MailKit: 2.8.0, MimeKit: 2.9.1
In the new versions it returns with the size between 1 and 2 bytes (octets).
New Version: MailKit: 2.10.0-2.10.1, MimeKit: 2.10.0-2.10.1

I detect the bug when run the test that cover that scenary.

Platform (please complete the following information):

• OS: Windows 10 x64
• .NET Runtime: net core app
• .NET Framework: Net Core 2.0 (Target Framework)
• MailKit Version: 2.10.0-2.10.1, MimeKit Version: 2.10.0-2.10.1

To Reproduce
Steps to reproduce the behavior:

1. I sent an email with empty attachment.
2. When read the email, the attachment report size greather than zero.
3. When download de attachment, the size indicate that is greather than zero.

Expected behavior
The size of an empty attachment must be zero.

[jstedfast]

Can you provide a sample app? I'm not sure how you are determining size.

[rqueizan]

Obtener Outlook para Android

[jstedfast]

That just gave me a file listing in a comment, but no actual source code that I can look at to see what you are doing so I can better understand the issue.

[jstedfast]

@rqueizan I don't see it. You can't reply to GitHub notifications with attachments, it will only post the message text as a comment.

[rqueizan]

For some reason looks like the mail fail when sent. In the web I can't see the email, but in the client looks like was sent. When I send email with old version, and read with old/new version work fine. The problem is when with new version I send the email. The server report at empty attachment with 2 bytes of size. See the document. If you prefer code in place of picture, let me know. In picture are more easy to read by the color assigned by the IDE.

[jstedfast]

It's best not to reply to these comments using your email client because GitHub doesn't handle emails very well.

It's best to go to https://github.com/jstedfast/MailKit/issues/1136 and write your comment manually.

[rqueizan]

Mial Kit Bug.docx

[rqueizan]

Review the word document. I explain my way for fetch mails and how to build my own representations of the mail.

[rqueizan]

I ran my tests with the old version, I send an empty attachment (an image, 0 bytes).
When I read the mail in the folder, it reports that it has a zero-byte attachment, the same thing happens when downloading.
If I read that same email from the folder with the new version of the library, it reports zero bytes, as with the old version.
However, when sending the mail with the new version of the library, the mail reports the empty attachment with 2 bytes of content (I suppose it is a lf, cr). When downloaded it comes blank.
The problem is that when estimating the size it fails, because in the previous versions it gave 0 bytes, in the new one it comes with 2 bytes.

[jstedfast]

This is what the latest version of MimeKit generates with an empty attachment:

From: Sender Name <[email protected]>
Date: Tue, 19 Jan 2021 17:13:23 -0500
Subject: Test message with an empty attachment
Message-Id: <ZTBROVH3RCU4.6L5JRL70PLKC1@DESKTOP-FD9TH05>
To: Recipient Name <[email protected]>
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="=-D2my5Z+Mr009Y3Ov908Jcw=="

--=-D2my5Z+Mr009Y3Ov908Jcw==
Content-Type: text/plain; charset=utf-8

This is the message text.
--=-D2my5Z+Mr009Y3Ov908Jcw==
Content-Type: image/png; name=empty.png
Content-Disposition: attachment; filename=empty.png

--=-D2my5Z+Mr009Y3Ov908Jcw==--

There's no way to remove the blank line w/o causing invalid MIME.

I also could not find any code changes to the way MIME is written out (lots of changes to the parser, though).

That said, I don't think it's related to the parser changes either because you seem to be using MailKit's BodyPart API which does not make use of MimeKit's parser (it uses the IMAP server's MIME parser instead).

So I'm kind of confused...

[jstedfast]

Can you verify that MimeKit 2.8.0 produces different output from 2.9.1? (or 2.10.x if you'd rather use 2.10 is fine).

[rqueizan]

[rqueizan]

The four picture represent de comparative use of new library and old library.
1st Sending mail
2nd Creating a copy in Sent folder.
3rd Reading a mail received.
4th Reading the copy in Sent folder.

[rqueizan]

This are the complete logs:
MKBnew.txt (2.10.X)
MKBold.txt (2.8.0)
With MailKit 2.9.0 I have the same problem.

[jstedfast]

in the 2.10.x version (I assume that's what the "MKBnew.txt" file represents), I can definitely see that there are too many blank lines for an empty MIME part:

C: --=-McaGg1eFrZRnn5DQ89W9+A==
C: Content-Type: text/plain; charset=utf-8
C:
C:
C: --=-McaGg1eFrZRnn5DQ89W9+A==
C: Content-Type: text/html; charset=utf-8
C:
C:
C: --=-McaGg1eFrZRnn5DQ89W9+A==--
C:
C: --=-0MJMUrUdjJcFsuJ+pHOTpQ==
C: Content-Type: IMAGE/png; name=emptyImage.png
C: Content-Transfer-Encoding: base64
C: Content-Disposition: attachment; filename=emptyImage.png
C:
C:
C:
C: --=-0MJMUrUdjJcFsuJ+pHOTpQ==--

The image/png, for example, has 3 blank lines. It should only have 1 like my example a few comments above has.

I'm not sure why you are getting 3. I can't seem to reproduce that here. How are you creating the image/png attachment?

[jstedfast]

Okay, so my local test case was broken and now that I fixed it, I'm getting 2 blank lines. But I think that's still ok. The issue you are having is that you are getting 3 blank lines and I still can't figure out how that is happening for you.

[jstedfast]

Does your empty.png have a newline in it? Or is it a 0-length file?

[jstedfast]

Ok, so when using the BodyBuilder, I can reproduce this...

[jstedfast]

Ah, ok, so in 2.9.0, I made the BodyBuilder.Attachments.Add() method always set the Content-Transfer-ENcoding to base64 for non-text content (based on mime-type), so the empty.png attachment went from not having an encoding to having an encoding.

The base64 encoder would always flush the content with a new-line sequence, even if the content that was base64 encoded was 0-length.

[rqueizan]

See my code, how I build and send the email.
I send an empty Stream, 0-length.
I use the body builder.

[jstedfast]

Yep, my patch fixes that issue.