Commitlog files contain multiple chunks. Each chunk starts as a single
(possibly, fragmented buffer). The size of that buffer in memory may be
larger than the size in the file.
cycle() was incorrectly using the in-memory size to write the whole
buffer to the file. That sometimes caused data corruption, since a
smaller on-file size was used to compute the offset of the next chunk
and there could be multiple chunk writes happening at the same time.
This patch solves the issue by ensuring that only the actual on-file
size of the chunk is written.
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