D7595: Skip reload when another reload is already in progress

[Henrik Fehlauer]

rkflx created this revision.
Restricted Application added a project: Okular.

REVISION SUMMARY
  When pressing and holding [F5], Okular would reload the current
  document multiple times and eventually get stuck with no document
  showing and the "Reloading the document..." message reappearing every
  second in an infinite loop. On exit, Okular then would sometimes
  segfault, although often in different lines of code.
  
  These issues can be avoided by only allowing one reload progressing at
  any time. Any further reload requests are discarded.
  
  Implementation:
  
  - use std::unique_lock, as QMutexLocker lacks support for try_lock
  - use std::mutex, as QMutex supports try_lock only since Qt 5.8
  
  Note: The "Reload" menu entry and shortcut are already disabled while
  fetching the document (most noticable when using a slow network link).
  The new lock protects against *overlapping* reload requests coming in
  after fetching completes but before rendering finishes.
  
  Thanks to @aacid for finding the fix, this merely improves the
  implementation by using a mutex with RAII instead of a non-atomic and
  non-dry boolean.

TEST PLAN
  - Builds with both GCC 4.8 / Qt 5.6 and GCC 7.1 / Qt 5.9.
  - No hiccups and crashes anymore when holding [F5].
  - Behaviour for slow downloading remote documents unchanged.

REPOSITORY
  R223 Okular

BRANCH
  reload-mutex (branched from Applications/17.08)

REVISION DETAIL
  https://phabricator.kde.org/D7595

AFFECTED FILES
  part.cpp
  part.h

To: rkflx, #okular
Cc: aacid
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