[Development] Backporting the "stop unloading plugins" change to 5.6

[André Pönitz]

On Thu, Oct 13, 2016 at 03:30:22PM +0100, Sergio Martins wrote:
> On 2016-10-12 20:59, Thiago Macieira wrote:
> >Hello
> >
> >We've got a number of issues that got fixed in 5.7 by the change that made
> >QFactoryLoader stop unloading plugins (notably, the Network Manager bearer
> >plugin). Given that QtDBus in 5.6 is now heavily threaded, a number of new
> >issues have cropped up. I've managed to fix some, but reports from Ubuntu
> >developers indicate the latest fix only uncovered the next issue.
> 
> Hi,
> 
> 
> More importantly, this also solves all the QStringLiteral related crashes
> we've been seeing.

These would be fixable by going back to QLatin1String.

> We need this in 5.6, but it's too big a behaviour change for an LTS release.
> We need at least a variable to turn it on/off.
> 
> 
> Suggestion which me and peppe discussed:
> 
> Make it opt-in in 5.6.3 and write in the ChangeLog that there will be a
> behaviour change in 5.6.4.
> Make it opt-out in 5.6.4 (bringing it on-par with 5.8).
> 
> This gives the user a pre-warning of 3 months, it can still be turned off
> and fixes many crashes.

I still consider the approach of not unloading plugins fundamentally
wrong. It only deepens the trench between Qt and valid approaches
at software architectures.

Andre'