[Development] QtNetwork changes from QtDD SF 2011

[shane.kearns at accenture.com]

> Disk cache:
> * Get implementation from somewhere which is good enough for most
> applications
> * Set caching on by default: no

Hi,

Although the discussion seems to have moved on to an inter-process safe disk cache, there is a thread safe wrapper for QNetworkDiskCache attached to https://bugreports.qt.nokia.com/browse/QTBUG-20500 which can be used as a basis if QMutex locking should be used before IPC locking.

We have QSystemSemaphore for IPC locking, which may be more portable than file locking.
Both are vulnerable to malicious locking, although passing a randomised key to child processes could help the webkit case.

Unix has advisory file locks, and Windows has mandatory file locks, which could be used to prevent opening of a cache file that is locked by another process. QFile doesn't support them in Qt4.

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