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This patch makes sure that every time we need to create a new generation number - the very first step in the creation of a new SSTable, the respective CF is already initialized and populated. Failure to do so can lead to data being overwritten. Extensive details about why this is important can be found in Scylla's Github Issue #1014 Nothing should be writing to SSTables before we have the chance to populate the existing SSTables and calculate what should the next generation number be. However, if that happens, we want to protect against it in a way that does not involve overwriting existing tables. This is one of the ways to do it: every column family starts in an unwriteable state, and when it can finally be written to, we mark it as writeable. Note that this *cannot* be a part of add_column_family. That adds a column family to a db in memory only, and if anybody is about to write to a CF, that was most likely already called. We need to call this explicitly when we are sure we're ready to issue disk operations safely. Signed-off-by: Glauber Costa <glauber@scylladb.com>