MESSAGE quota resource implemention

[Bron Gondwana]

On Mon, Sep 05, 2011 at 02:32:40PM +1000, Greg Banks wrote:
> Ok, I'm now convinced my first attempt at annotation quotas sucked too
> hard, here's how I want to re-implement it.  Let me know what you think.

>  - add a 32b mailbox index header entry to track the storage in bytes
> used by all annotations for the mailbox itself or for messages in the
> mailbox

Why not 64b?  Admittedly hanging gigabytes of annotations off a single
folders is probably evil, but this is the mailbox header - 4 bytes per
mailbox to not even have to think about it is super-cheap.  We keep a
handful of "blanks" around in the header already.

>  - at header upgrade time, write the special value ~0 into this field,
> meaning "unknown"

Funky.  If you didn't want to get all special-value about it, you could
use a flag over in the flags field as well, there are only about 5 of
the 32 in use so far.

>  - in mailbox_commit_quota(), if the field is not "unknown", then
> calculate the delta in usage and apply to the quota db.

I assume this is done the same way it's done now?  By taking a "snapshot"
of the value at the start, and calculating a diff to apply at the end.
That was actually a filthy workaround (one of many, dammit) for using
unsigned datatypes.  If we made this a signed 64 bit datatype as well,
with a flag off to the side for "unknown", then we could actually store
a diff in the mailbox object directly - and update both this field and
the quota's value during the commit.

>  - make reconstruct scan the annotations db to figure out the correct
> value for this new field.

Yep - that makes sense.  Reconstruct already does this for the
quota_mailbox_used field, doesn't it.  And any other derived
"aggregate" values in the header.

> This means that the annotation STORE and SETMETADATA paths will be
> updating the quota db in the same place that APPEND et al do,
> mailbox_commit_quota().  This should work around Bug #3529 and also make
> the code neater.

Nice.

Bron.