Re: [PATCH v2 4/5] reftable/writer: fix writing multi-level indices

> When finishing a section we will potentially write an index that makes
> it more efficient to look up relevant blocks. The index records written
> will encode, for each block of the indexed section, what the offset of
> that block is as well as the last key of that block. Thus, the reader
> would iterate through the index records to find the first key larger or
> equal to the wanted key and then use the encoded offset to look up the
> desired block.
> 
> When there are a lot of blocks to index though we may end up writing
> multiple index blocks, too. To not require a linear search across all
> index blocks we instead end up writing a multi-level index. Instead of
> referring to the block we are after, an index record may point to
> another index block. The reader will then access the highest-level index
> and follow down the chain of index blocks until it hits the sought-after
> block.
> 
> It has been observed though that it is impossible to seek ref records of
> the last ref block when using a multi-level index. While the multi-level
> index exists and looks fine for most of the part, the highest-level
> index was missing an index record pointing to the last block of the next
> index. Thus, every additional level made more refs become unseekable at
> the end of the ref section.

> 
> The root cause is that we are not flushing the last block of the current
> level once done writing the level. Consequently, it wasn't recorded in
> the blocks that need to be indexed by the next-higher level and thus we
> forgot about it.
> 
> Fix this bug by flushing blocks after we have written all index records.