[horde] Cannot export IMAP folder to MBOX

[Jens Wahnes]

Louis-Philippe Allard wrote:
> What is causing this?

I think the root cause is that the "download folder" feature does not 
work very resource efficiently on the server running Imp. When operating 
on a larger mail folder, it does use proper IMAP commands to retrieve 
messages from the server in an efficient manner, but it seems to hold 
either all of the messages or at least a good part of them in RAM, which 
causes memory exhaustion within PHP after a while, even with very high 
"memory_limit"s. The process also consumes substantial amounts of CPU 
time while running. This might stem from the process trying to write the 
output file line by line, but that's merely an assumption -- I haven't 
found out exactly what the cause is.

Either way, downloading larger mail folders is likely to fail with the 
current implementation.

As creating (and potentially zipping) a large mbox file clearly will be 
quite resource intensive on the server in any implementation, I think it 
might not even be possible to provide this kind of feature interactively 
through the website. Preparing a large mbox file will almost always take 
much more time to complete than today's users are expecting to wait for 
anything they click on a website. So for large mail folders, this might 
have to be a non-interactive service (e.g. the mbox file would be 
prepared in the background and an email notification with a download 
link would have to be sent to the user that requested the mbox file once 
it's ready for download.)

Jens