Skip not requested parts

[kesselb]

⚠️ Forwarding emails is broken in Mail 1.12. Please review this patch with Mail 1.11.

How to reproduce / test

1. Sent a message to yourself with three attachments.
2. Forward the message to yourself
3. Note that in the forwarded message only the first attachment is correct, attachment two and three are 0 byte.

When forwarding a message

handleAttachments is calling getRawAttachments (via handleForwardedAttachment) for each attachment to forward. getRawAttachments get the attachment id (which references a mime part of the original message) as argument.

getRawAttachments fetch the structure for the message.
In a second query we request the actual body for the attachment (buildAttachmentsPartsQuery).

getRawAttachments for Attachment 1

getRawAttachments for Attachment 2

getRawAttachments for Attachment 3

We see in the $attachments array that attachment 1 is at position 0, attachment 2 at 1 and attachment 3 at 2. The reason is that $structure->partIterator() always returns all parts of a message regardless if mail request the data for it or not.

For example getRawAttachments for attachment 3 is also reading the data for attachment 1 and get an empty string because we do not request to fetch the data for it.

mail/lib/Service/MailTransmission.php

Lines 549 to 550 in 1752cbb

	// Attaches attachment to new message
	$message->addRawAttachment($attachment['fileName'], $attachments[0]);

In handleForwardedAttachment mail always uses the attachment at position 0 for the data. for attachment 3 we add the empty data for attachment 1.

This patch changes the behavior to skip parts we did not request.

[ChristophWurst]

Makes sense

[ChristophWurst]

/backport to stable1.12

[st3iny]

Tested and works.

Tested on stable1.11 via git apply <(curl -fL https://github.com/nextcloud/mail/pull/6543.patch).