[Dovecot] Plugin development - ostream and index cache size

[Philipp Brüll]

Hi,

I'm new to this list, so first thanks for the dovecot tool! So far, it 
has been great to work with.

Currently, I'm developing a scrambler plugin for dovecot, that should 
stores the mails encrypted. To do that, I've created the 
scrambler_ostream and scrambler_istream that does the encryption and 
that I hook in the mail storage process.

The scrambler_ostream is stacked on top of the output stream of the 
mail_save_context (mail_save_context->data.output) and the 
scrambler_istream is attached to the input stream when the virtual 
function istream_opened in the mail_private struct is called.

Basically, the encrypt/decrypt works, but during the process the data 
size changes slightly. And when I'm doing a FETCH BINARY[] via imap, it 
results in the error...

Cached message size smaller than expected (204 < 208)
Error: Corrupted index cache file 
/home/.../mail/mailboxes/INBOX/dbox-Mails/dovecot.index.cache: Broken 
physical size for mail UID 1

My guess is, that the FETCH function tries to deliver the unencrypted 
cache version of the mail (204 bytes) and not the stored version (208 
bytes).

How can I make dovecot cache the output of my scrambler_ostream or at 
least disable this caching for testing?

I've read already a lot of code, but I've found a hint yet. How does - 
for example - the zlib plugin handle this issue? I have read the code a 
couple of times, but still have no clue.

Best regards,
Philipp