Enhancement for ListInDisk

[guo-shaoge]

Bug Report

ListInDisk is a temporary storage for operators which need spill data to disk when memory limit is reached.

But there is a limitation of ListIndisk. After the user reads data from ListInDisk, he can no longer write.

That's because for each chunk added into ListInDisk, we try to seperate it as many fixed size segments(1024B), when a segemnt is full, we will compute its checksum and write it to disk. So for the last segment, its length may be samller than 1024.

When we read data in ListInDisk, we will first compute checksum of the last segment and flush it to disk even its length is smaller than 1024. By doing this, the code of ReadAt method will be simple and clean. Because length of the last segment may be smaller than 1024, so we cannot append any data.

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