[Users] [Bug 3460] CM gets stuck in a loop while scanning folder tree

[noreply at thewildbeast.co.uk]

http://www.thewildbeast.co.uk/claws-mail/bugzilla/show_bug.cgi?id=3460

--- Comment #5 from Tore Anderson <tore at fud.no> ---
After digging a bit more into this I realise that it is probably the server
that's misbehaving, with LSUB reporting the same folder multiple times if the
it contains sub-folders. From the log:

[12:23:26] IMAP4> 908 LSUB "" "%" 
[...]
[12:23:35] IMAP4< * LSUB (\NoSelect) "/" "MS" 
[...]
[12:23:35] IMAP4< * LSUB (\NoSelect) "/" "MS" 
[...]
[12:23:35] IMAP4< * LSUB () "/" "MS" 
[...]
[12:23:35] IMAP4< * LSUB (\NoSelect) "/" "MS" 
[...]
[12:23:35] IMAP4< * LSUB (\NoSelect) "/" "MS" 
[...]
[12:23:35] IMAP4< 908 OK LSUB completed 

Predictably enough, this makes CM descend into the "MS" folder five times:

[13:17:31] IMAP4> 2097 LSUB "" "MS/%" 
[13:17:34] IMAP4< * LSUB () "/" "MS/MS0" 
[13:17:34] IMAP4< * LSUB () "/" "MS/QA" 
[13:17:34] IMAP4< * LSUB () "/" "MS/Rootmail" 
[13:17:34] IMAP4< * LSUB () "/" "MS/Vaktlogg" 
[13:17:34] IMAP4< 2097 OK LSUB completed 
[...]
[13:19:13] IMAP4> 2137 LSUB "" "MS/%" 
[13:19:15] IMAP4< * LSUB () "/" "MS/MS0" 
[13:19:15] IMAP4< * LSUB () "/" "MS/QA" 
[13:19:15] IMAP4< * LSUB () "/" "MS/Vaktlogg" 
[13:19:15] IMAP4< * LSUB () "/" "MS/Rootmail" 
[13:19:15] IMAP4< 2137 OK LSUB completed 
[...]
[13:35:44] IMAP4> 2495 LSUB "" "MS/%" 
[13:35:46] IMAP4< * LSUB () "/" "MS/MS0" 
[13:35:46] IMAP4< * LSUB () "/" "MS/QA" 
[13:35:46] IMAP4< * LSUB () "/" "MS/Rootmail" 
[13:35:46] IMAP4< * LSUB () "/" "MS/Vaktlogg" 
[13:35:46] IMAP4< 2495 OK LSUB completed 
[...]
[14:27:55] IMAP4> 3632 LSUB "" "MS/%" 
[14:27:57] IMAP4< * LSUB () "/" "MS/MS0" 
[14:27:57] IMAP4< * LSUB () "/" "MS/QA" 
[14:27:57] IMAP4< * LSUB () "/" "MS/Rootmail" 
[14:27:57] IMAP4< * LSUB () "/" "MS/Vaktlogg" 
[14:27:57] IMAP4< 3632 OK LSUB completed 
[...]
[14:28:08] IMAP4> 3637 LSUB "" "MS/%" 
[14:28:11] IMAP4< * LSUB () "/" "MS/Vaktlogg" 
[14:28:11] IMAP4< * LSUB () "/" "MS/MS0" 
[14:28:11] IMAP4< * LSUB () "/" "MS/Rootmail" 
[14:28:11] IMAP4< * LSUB () "/" "MS/QA" 
[14:28:11] IMAP4< 3637 OK LSUB completed 

With deep folder hierarchies this causes an exponential increase in the amount
of scanning to be done, and couple that with the rather slow (multi-second)
LSUB performance of the mail server, it's only to be expected that the total
time it takes to complete the scan gets very long.

I've opened a bug report with Zibmra about this:
https://bugzilla.zimbra.com/show_bug.cgi?id=100321

That said, CM's handling of this situation could possibly be improved here - if
it simply ignored any duplicate folders returned, the exponential amount of
scanning would have been avoided.

Another thing worth considering is to use the "*" wild-card in order to get the
full list of subscribed folders from a single LSUB command:

1 LSUB "" "*"
[...]
* LSUB () "/" "MS/Vaktlogg"
* LSUB () "/" "MS/Rootmail"
* LSUB () "/" "MS/QA"
* LSUB () "/" "MS"
* LSUB () "/" "MS/MS0"
[...]
1 OK LSUB completed

This would cause a considerably faster folder scanning (even disregarding the
exponential scanning issue) than currently, because only a single LSUB command
is used, rather than one for each folder in the hierarchy. With a big hierarchy
and a server like Zimbra which spends a few seconds on each LSUB command, this
could cause enormous savings (in my case, it's the difference between "might as
well leave for lunch now" and "in the blink of an eye").

Another thing worth mentioning here, is that CM appears trust the entry with
the \NoSelect flag, given the conflicting entries above. So even though the
"MS" folder is actually a completely regular folder containing messages, I
cannot enter it in the CM UI, it is "grayed out". I would suggest to reverse
this logic, i.e., only trust that \NoSelect is actually the case IFF all the
returned LSUB entries have the flag set.

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