The documentation indicates that false/or the value will be returnned.
Using both null and false to indicate failure is confusing. Use only
false to indicate failure. It might be better in the future to use
exceptions for trying to read/write on missing cache configs.
Set '0664' as default value for the 'mask' setting in FileEngine::. Adjusted corresponding test cases.
Added new warning if the file could not be opened for writing.
Locking files prevents many issues with unserialize() warnings
caused by partially written files.
Fix a locking issue where files would be left locked.
Fixes#1988