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When bitcoind is running in pruned mode, producing a hashlist with ./linearize-hashes.py linearize.cfg > hashlist.txt and then executing linearize-data.py linearize.cfg will produce:
Read 313001 hashes
Input file /home/dan/.bitcoin/blocks/blk00000.dat
Premature end of block data
This happens because linearize-data starts by attempting to process blk00000.dat regardless of whether or not blk00000.dat actually exists - this may not be the case if working with a pruned node.
This PR adds a function which finds the first block file that does exist, and calls that function when the BlockDataCopier is initialized.
This is a refactor of #16431.
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