D25185: Icon: solve threading issue on when the source is http

[Aleix Pol Gonzalez]

apol created this revision.
apol added reviewers: Kirigami, mart.
Herald added a project: Kirigami.
Herald added a subscriber: plasma-devel.
apol requested review of this revision.

REVISION SUMMARY
  For starters, QQmlEngine::networkAccessManager is not reentrant and would give
  us a warning along the lines of:
  
    discover(68504)/ QObject::QObject|QNetworkAccessManager::QNetworkAccessManager QObject: Cannot create children for a parent that is in a different thread.
    (Parent is QQmlApplicationEngine(0x5563bf8dcad0), parent's thread is QThread(0x5563bf825f70), current thread is QSGRenderThread(0x5563bfef7830)
  
  That happens because Icon::findIcon is called from Icon::updatePaintNode which
  is called from the render thread.
  
  This patch changes it so the QNAM bits happen upon setSource, so downloading
  the image will be handled by the main thread and we will only be grabbing it
  from an already prepared QImage.
  This patch also includes a QMutex, to keep both threads split.

TEST PLAN
  I don't get the warning anymore and if I agressively scroll I don't
  get crashes. This will potentially fix a bunch of bugs in Discover since we use
  extensively Kirigami.Icon + http.

REPOSITORY
  R169 Kirigami

BRANCH
  master

REVISION DETAIL
  https://phabricator.kde.org/D25185

AFFECTED FILES
  src/icon.cpp
  src/icon.h

To: apol, #kirigami, mart
Cc: plasma-devel, fbampaloukas, GB_2, domson, dkardarakos, ngraham, apol, ahiemstra, davidedmundson, mart, hein
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