fix: detect imip messages from outlook.com

[kesselb]

Fix #10660

Adjust the parsing logic in MessageMapper to correctly detect iMIP messages in flat email structures where the text/calendar part is not wrapped in multipart/alternative.

The implementation loops over the message parts and then tries to fetch another part. This works for Gmail because the iMIP message is wrapped in multipart/alternative, but fails for Outlook.com, where the text/calendar part is not nested.

partIterator returns all parts, including nested ones.

Gmail:

body  
└─ multipart/alternative  
   ├─ text/plain  
   ├─ text/html  
   └─ text/calendar  
└─ application/ics  

Outlook.com:

body  
├─ text/plain  
├─ text/html  
└─ text/calendar  

[ChristophWurst]

I'm assuming writing a unit test for this is not trivial, is it?

[kesselb]

I'm assuming writing a unit test for this is not trivial, is it?

We should have tests for it 🙏

Richard did some tests for smime, I will have a look if we can start from there.

[ChristophWurst]

😻

[kesselb]

/backport to stable4.2

[kesselb]

/backport to stable3.7

[backportbot]

The backport to stable3.7 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable3.7
git pull origin stable3.7

# Create the new backport branch
git checkout -b backport/10661/stable3.7

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts, resolve them
git cherry-pick b9653556

# Push the cherry pick commit to the remote repository and open a pull request
git push origin backport/10661/stable3.7

Error: No changes found in backport branch

---

Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.

[ChristophWurst]

/backport to stable4.1

[SebastianKrupinski]

RFC examples of possible iMip Messages

https://datatracker.ietf.org/doc/html/rfc2447#section-4