Botond Dénes 04643fb223 multishard_combining_reader: prepare for read-ahead otliving the reader ...

When the multishard reader is destroyed there might be severeal pending
read-aheads running in the background. These read-aheads need their
associated reader to stay alive until after the read-ahead completes.
To solve this move the flat_mutation_reader into a struct and manage
this struct's lifetime through a shared pointer. Fibers associated with
read-aheads that might outlive the multishard reader will hold on to a
copy of the shard pointer keeping the underlying reader alive until they
complete. To avoid doing any extra work a flag is added to this state
which is set when the multishard reader is destroyed. When this flag is
set, pending continuations will return early.  All this is encapsulated
in multishard_combining_reader::shard_reader the multishard reader code
itself need not be changed.

2018-04-30 17:16:21 +03:00

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