LongListDisk.writeLongsData() may corrupt data in snapshots

[OlegMazurov]

LongListDisk.writeLongsData() reads all current chunks from its backing file and writes them to the target file in logical order (from min to max index value).
Chunk file offsets are allocate dynamically and the last logical chunk (with largest index values) may be allocated in the middle of the backing file. Similarly, the last physical chunk may be in the middle of the index value range.
The last physical chunk may be truncated because the backing file is enlarged according to the actual data written to it (see LongListDisk.putToChunk()) and if the largest index value in that chunk is less than memoryChunkSize the chunk is effectively truncated: when such chunk is read into transferBuffer the number of bytes actually read is less than the limit of the buffer and the remaining part of the buffer is left unchanged, possibly containing data from the previous logical chunk. The entire buffer with the wrong data is then written into the target file (that was actually observed in #18136).

[artemananiev]

I don't think two chunks mentioned in the description are affected, but only one of them, which is the last chunk in the LongListDisk backing file (it may or may not be the chunk with the highest index). All chunks in the middle of the backing file are written in full regardless of their indexes.

The fix can be to explicitly append a full chunk (filled with zeroes) to the end of the backing file, when a new chunk is created.

[OlegMazurov]

I don't think two chunks mentioned in the description are affected...

I agree. I have updated the description.