SyncEngine::slotDiscoveryFinished() and SyncFileStatusTracker::slotAboutToPropagate block the main thread ("Not responding")

[guruz]

We kind of have this already as an issue, but now @michaelstingl reported it in his video (see rocket chat).

Big loop of:

08-30 15:52:18:917 [ debug sync.statustracker ]	[ OCC::SyncFileStatusTracker::slotAboutToPropagate ]:	Investigating "Shared/owncloud/xxxx.pdf" OCC::SyncFileItem::NoStatus 8
08-30 15:52:18:918 [ warning sync.vfs ]:	fileStatusChanged: could not make handle for "C:/Users/mail/ownCloud/Shared/owncloud/xxxx.pdf"

Created following @HanaGemela 's #7439

probably not fixable for 2.6.0

[guruz]

Actually I think this is #5356
Why @jnweiger only finds this now and not earlier I don't know..did 2.6 regress in that regard?

[guruz]

Those are the functions:
https://github.com/owncloud/client/blob/v2.6.0-rc1/src/libsync/syncengine.cpp#L618
calls both
https://github.com/owncloud/client/blob/v2.6.0-rc1/src/libsync/syncfilestatustracker.cpp#L220
and

client/src/libsync/owncloudpropagator.cpp

Line 368 in f7d4b29

void OwncloudPropagator::start(const SyncFileItemVector &items)

[guruz]

On my fast macbook with syncing down cloud.owncloud.com (in placeholder mode) I get:

09-06 13:03:36:108 [ info sync.engine ]:	#### Discovery end #################################################### Discovery Finished 425791 ms
09-06 13:03:36:917 [ info sync.engine ]:	#### Discovery end #################################################### Reconcile (sorted OK) 426600 ms
09-06 13:03:37:173 [ info sync.engine ]:	#### Discovery end #################################################### Reconcile (aboutToPropagate OK) 426856 ms
09-06 13:03:37:277 [ info sync.engine ]:	#### Post-Reconcile end #################################################### Post-Reconcile Finished  426959 ms
09-06 13:03:37:277 [ info gui.folder ]:	#### Propagation start ####################################################
09-06 13:03:37:277 [ info gui.application ]:	Sync state changed for folder  "https://cloud.owncloud.com/remote.php/dav/files/guruz/" :  "Sync Running"

So the main thread is blocked ca 1150msec for [forgot to note the number] files/directories.

[guruz]

Wow, can I move the sorting to a thread?!

09-06 13:20:59:200 [ info sync.engine ]:	#### Discovery end #################################################### Reconcile (start sorting) 410181 ms
09-06 13:20:59:922 [ info sync.engine ]:	#### Discovery end #################################################### Reconcile (sorted OK) 410902 ms

[guruz]

09-06 13:53:25:078 [ info sync.engine ]:	#### Discovery end #################################################### Discovery Finished 408808 ms QThread(0x7fa9697124a0)
09-06 13:53:25:078 [ debug default ]	[ OCC::SyncEngine::slotDiscoveryFinished()::(anonymous class)::operator() ]:	Start sorting 63121 items in thread QThread(0x7fa96f781320, name = "Thread (pooled)")
09-06 13:53:25:791 [ debug default ]	[ OCC::SyncEngine::slotDiscoveryFinished()::(anonymous class)::operator() ]:	Ended sorting QThread(0x7fa96f781320, name = "Thread (pooled)")
09-06 13:53:25:792 [ info sync.engine ]:	#### Discovery end #################################################### Discovery Finished SORTED 409522 ms QThread(0x7fa9697124a0)
09-06 13:53:26:064 [ info sync.engine ]:	#### Discovery end #################################################### Reconcile (aboutToPropagate OK) 409795 ms
09-06 13:53:26:164 [ info sync.engine ]:	#### Post-Reconcile end #################################################### Post-Reconcile Finished  409895 ms