'A safe read lock is a lock that does nothing if the thread already
has a write lock, otherwise it acquires a read lock. This is necessary
to prevent DowngradeLockErrors, which can happen when updating the
search cache in the presence of composite columns. Updating the search
cache holds an exclusive lock, but searching a composite column
involves reading field values via ProxyMetadata which tries to get a
shared lock. There may be other scenarios that trigger this as well.
This property returns a new lock object on every access. This lock
object is not recursive (for performance) and must only be used in a
with statement as ``with cache.safe_read_lock:`` otherwise bad things
will happen.'

@property
def safe_read_lock(self):

    return SafeReadLock(self.read_lock)
