Corrupted sizes in cache once again

[Stuart Henderson]

On 2023-02-01, Tim Evers <te-ml-ext at artfiles.de> wrote:
> I run a fairly large Dovecot Installation (around 100k mailboxes) on 
> several servers.
>
> gzip compression is on.
>
> Every once in a while I get the dreaded "cache corruption" messages in 
> the log:
>
> Error: Corrupted record in index cache file 
> /[redacted]/Maildir/dovecot.index.cache: UID 3868: Broken physical size 
> in mailbox INBOX: 
> read(zlib(/[redacted]/Maildir/cur/1674129792.M797543P21755.node2,S=8099,W=8276:2,)) 
> failed: Cached message size smaller than expected (2877 < 8099, 
> box=INBOX, UID=3868)
>
> Error: Corrupted record in index cache file 
> /[redacted]/Maildir/dovecot.index.cache: UID 3875: Broken physical size 
> in mailbox INBOX: 
> read(zlib(/[redacted]/Maildir/cur/1674212201.M985809P29112.node2,S=13907,W=14121:2,)) 
> failed: Cached message size smaller than expected (5533 < 8192, 
> box=INBOX, UID=3875)
>
> The first entry shows 2877 (size on disk) vs. 8099 (real size unzipped, 
> also in the filename: S=8099).
>
> The second entry shows 5533 (size on disk) vs. 8192 - this is not 
> correct in any way. Size on disk is 13907 as noted in the filename.
>
> Both mails were delivered trough LMTP and retrieved by the POP3 service.
>
> Anyone with an idea what might be happening here? I've read all 
> available info in the doc and in the previous discussions / bug reports, 
> but nothing seems to match my case. And where does that 8192 come from - 
> it looks suspicious?
>
> Version is 2.3.7.2 (Ubuntu 20.04)

2.3.7.2 is rather old now. There were definitely fixes regarding compression
around the 2.3.10-2.3.12 timeframe or thereabouts (I forget all the details
but it took a release or two before some remaining issues were sorted out
after changes in the area). I'd be looking to get it updated to a current
version first.