range::is_wrap_around() and range::contains() rely on total ordering
on values to work properly. Current ring_position_comparator was only
imposing a weak ordering (token positions equal to all key positions
with that token).
range::before() and range::after() can't work for weak ordering. If
the bound is exclusive, we don't know if user-provided token position
is inside or outside.
Also, is_wrap_around() can't properly detect wrap around in all
cases. Consider this case:
(1) ]A; B]
(2) [A; B]
For A = (tok1) and B = (tok1, key1), (1) is a wrap around and (2) is
not. Without total ordering between A and B, range::is_wrap_around() can't
tell that.
I think the simplest soution is to define a total ordering on
ring_position by making token positions positioned either before or
after all keys with that token.
Singular range is a range of the form [x; x]. Internally such ranges
are optimized to avoid storing the value twice, so _end is empty.
Client shouldn't have to special case for it if it's not interested,
so make end() and split() work for singular ranges too.
query::range::serialized_size() calls a template method (also
called serialized_size()) within a lambda, however it is called with
a const 'this' while the template is non-const-qualified. gcc 5
rightly rejects it (nicely annotating the template as a near miss).
Fix by making the template static; it doesn't need 'this' anyway.
This change abstracts reading from on-disk data sources behind a single
reader which is then composed with memtable readers. This change also
abstracts all data sources behind a single reader obtained via
column_family::make_reader(). That reader is then used by algorithms
like column_family::for_all_partitions() or
column_family::query(). Having those abstractions will make it easier
to add row cache, because it will be encapsulated in a single place.
Current model was not really correct because Origin doesn't support
querying of partition ranges by their value. We can query slices
according to dht::decorated_key ordering, which orders partitions
first by token then by key value.
ring_position encapsulates range constraint. Key value is optional, in
which case only token is constrained.
partitions_ranges will be manipulated upon to be split for different
destination, so provide it separately from read_command to not copy the
later for each destination.
