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dix: create empty selection objects as-needed in dixLookupSelection()

Enrico Weigelt, metux IT consult . requested to merge metux/xserver:submit/fix-SelectSelectionInput into master

For now, new selection objects are only created in ProcSetSelectionOwner() when dixLookupSelection() can't find the requested one (returns BadMatch).

When somebody's trying to listen on a not-yet existing selection, via XFixesSelectSelectionInput() -- XFIXES:SelectSelectionInput message -- he's also getting BadMatch. This isn't neccessarily completely wrong, the spec doesn't really tell anything about those situations (it doens't tell anything about selection's lifetimes, just their ownerships). But there are real- world clients not expecting an error here and crashing - the problem popped up just recently, due to a necessary security fix (remote memory corruption plus XACE missing to catch SelectSelectionInput) that alread went unnoticed for far too long (*1).

So, it's better being polite and interpret the spec in the way that any potential selection exists as soon as it's used by someone. (in fact, they never get deleted anyways, just cleared).

*1) 601fd0fd

Signed-off-by: Enrico Weigelt, metux IT consult info@metux.net

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