D7139: Fix QSortFilterProxyModelPrivate::updateChildrenMapping crash in libtaskmanager

[Eike Hein]

hein created this revision.
Restricted Application added a project: Plasma.

REVISION SUMMARY
  TaskGroupingProxyModel uses a simple QVector<QVector<int>> populated
  with source model row indices to represent the task group tree. To
  implement QAbstractItemModel::parent(), its implementation of index()
  encodes row indices of the top-level vector into the internal ids of
  child item model indices. This allows parent() to produce the parent
  model index by simply decoding the parent row from the passed-in child
  index and call index() with that row.
  
  Top-level row indices shift up and down as the list of top-level items
  changes, invalidating those internal ids. QModelIndex is not meant to
  be stored, and the proxy model does take care of updating any persis-
  tent model indexes with new ids, so this should be fine.
  
  However, where it falls apart is that as internal ids are invalidated,
  a QSortFilterProxyModel on top of this proxy (i.e. TasksModel) may end
  up with multiple indexes with identical internal ids in its mappings,
  causing it to mess up its mappings as it uses them (e.g. taking things
  from them). This causes the often-reported crash/assert there.
  
  The fix is to refactor index()/parent() not to rely on row indices as
  internal ids, but instead use pointers to internal data structures
  instead.
  
  This patch achieves this by changing the map to QVector<QVector<int> *>.
  This screams fugly, but the alternative would basically just be to
  create some wrapper struct to hide the fugly appeareance a little,
  which I don't think is worth it.
  
  On the flip side, it saves a QVector::replace() call as a multable
  vector iterator can work directly on a vector without making a copy,
  and it's now no longer necessary to update the persistent model
  indices.
  
  BUG:381006

REPOSITORY
  R120 Plasma Workspace

BRANCH
  master

REVISION DETAIL
  https://phabricator.kde.org/D7139

AFFECTED FILES
  libtaskmanager/taskgroupingproxymodel.cpp

To: hein, #plasma, davidedmundson
Cc: plasma-devel, ZrenBot, progwolff, lesliezhai, ali-mohamed, jensreuterberg, abetts, sebas, apol, mart, lukas
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