Bug 3462

Summary: Speed up folder scanning with «LSUB "" "*"»
Product: Claws Mail (GTK 2) Reporter: Tore Anderson <tore>
Component: Folders/IMAPAssignee: users
Status: NEW ---    
Severity: enhancement    
Priority: P3    
Version: 3.11.1   
Hardware: PC   
OS: Linux   

Description Tore Anderson 2015-07-05 21:57:50 UTC 
When scanning the folder tree, CM will recursively search for subscribed folders using multiple LSUB commands using the "%" wildcard, like so:

1 LSUB "" "%"
* LSUB () "/" "Foo"
* LSUB () "/" "Bar"
1 OK LSUB completed
2 LSUB "" "Foo/%"
* [...]
3 LSUB "" "Bar/%"
* [...]

...and so on for every level of every branch of the hierarchy. With large hierarchies, this means a lot of LSUB commands will be issued, which makes the folder tree scanning slow - especially if the IMAP server takes a few seconds to respond to each LSUB command. (My employer's Zimbra server uses 2-3 seconds.)

I suggest that this multi-command recursive method be replaced by a single command:

LSUB "" "*"

This makes the server return the entire list of subscribed folders in one go. If CM had used this approach, the entire folder scanning procedure would have taken 2-3 seconds, rather than the current 2-3 seconds multiplied by the number of nodes in the folder hierarchy. With a large folder hierarchy, that's a huge speedup.

Note: It might well be that the folder scanning procedure when in the show all folders mode (i.e., including non-subscribed ones) would also benefit from a similar change, only that it would apply to the LIST command rather than LSUB. I haven't checked that, though.
Comment 1 Tore Anderson 2015-07-12 13:39:35 UTC 
Actually, I think the whole folder scanning and subscription logic could be greatly improved. I suggest you consider an approach that does something like the following:

1) Get the entire list of folders, using 'LIST "" "*"'
2) Get the entire list of subscribed folders, using 'LSUB "" "*"'
3) Walk through every folder in set #1, checking if each folder exists in set #2

At this point you know the full folder list, as well as the subscribed/unsubscribed state of each folder. These steps could be performed at startup, and whenever the user requests to look for new folders.

Second, the interface for managing subscriptions could show a pop-up with the merged set from #3 in a standard tree-like structure, and with a checkbox beside each folder indicating its subscribed/unsubscrubed state as well as allowing the user to easily toggle it.

This seems to be how most other MUAs does it, for what it's worth.