The code currently assumes that once a block file is 'left', it and its corresponding rev file can be flushed, and won't be written to any further.

https://github.com/bitcoin/bitcoin/blob/e8e79958a7b2a0bf1b02adcce9f4d811eac37dfc/src/validation.cpp#L1742-L1743

(Flushing here additionally means to get rid of pre-allocated unused space.)

This is the case for the bl(oc)k files, but it turns out to not be the case for the rev files. During an IBD up to ~200k blocks, the following excerpt (one of several) was encountered (grepping out irrelevancy):
```
[...]
2020-01-07T14:19:09Z Pre-allocating up to position 0x7000000 in blk00013.dat
2020-01-07T14:19:13Z Pre-allocating up to position 0x8000000 in blk00013.dat
2020-01-07T14:19:14Z Pre-allocating up to position 0xf00000 in rev00013.dat
2020-01-07T14:19:16Z Leaving block file 13: CBlockFileInfo(blocks=1099, size=134068483, heights=183531...185655, time=2012-06-08...2012-06-22)
2020-01-07T14:19:16Z Pre-allocating up to position 0x1000000 in blk00014.dat
2020-01-07T14:19:17Z Pre-allocating up to position 0x100000 in rev00014.dat
2020-01-07T14:19:20Z Pre-allocating up to position 0x1000000 in rev00013.dat
2020-01-07T14:19:20Z Pre-allocating up to position 0x2000000 in blk00014.dat
2020-01-07T14:19:24Z Pre-allocating up to position 0x1100000 in rev00013.dat
2020-01-07T14:19:26Z Pre-allocating up to position 0x3000000 in blk00014.dat
2020-01-07T14:19:27Z Pre-allocating up to position 0x1200000 in rev00013.dat
2020-01-07T14:19:30Z Pre-allocating up to position 0x200000 in rev00014.dat
2020-01-07T14:19:33Z Pre-allocating up to position 0x300000 in rev00014.dat
2020-01-07T14:19:35Z Pre-allocating up to position 0x400000 in rev00014.dat
2020-01-07T14:19:37Z Pre-allocating up to position 0x500000 in rev00014.dat
2020-01-07T14:19:39Z Pre-allocating up to position 0x600000 in rev00014.dat
2020-01-07T14:19:42Z Pre-allocating up to position 0x700000 in rev00014.dat
2020-01-07T14:19:43Z Pre-allocating up to position 0x4000000 in blk00014.dat
2020-01-07T14:19:45Z Pre-allocating up to position 0x800000 in rev00014.dat
2020-01-07T14:19:49Z Pre-allocating up to position 0x900000 in rev00014.dat
[...]
```

But the only time we flush (and truncate the files to their correct size) is when we leave a block, and we already left block 13 here, so the rev files above end up with pre-allocated space that is never used. This is related to #17827.

This seems to repeat itself for every file that ended up larger than expected in https://github.com/bitcoin/bitcoin/pull/17887#issuecomment-571591432. (Note: #17887 is not a fix for this issue. #17887 makes the impact much smaller by fixing a separate blow-up bug, but it does not address re-flushing re-opened rev files.)

}
    
    std::vector<CBlockIndex*> vpindexToConnect;
    bool fContinue = true;
    int nHeight = pindexFork ? pindexFork->nHeight : -1;
    while (fContinue && nHeight != pindexMostWork->nHeight) {
        
        
        int nTargetHeight = std::min(nHeight + 32, pindexMostWork->nHeight);
        vpindexToConnect.clear();
        vpindexToConnect.reserve(nTargetHeight - nHeight);
        CBlockIndex *pindexIter = pindexMostWork->GetAncestor(nTargetHeight);
        while (pindexIter && pindexIter->nHeight != nHeight) {
            vpindexToConnect.push_back(pindexIter);
            pindexIter = pindexIter->pprev;
        }
        nHeight = nTargetHeight;
        
        for (CBlockIndex *pindexConnect : reverse_iterate(vpindexToConnect)) {
            if (!ConnectTip(state, chainparams, pindexConnect, pindexConnect == pindexMostWork ? pblock : std::shared_ptr<const CBlock>(), connectTrace, disconnectpool)) {
                if (state.IsInvalid()) {
                    
                    if (state.GetResult() != BlockValidationResult::BLOCK_MUTATED) {
                        InvalidChainFound(vpindexToConnect.front());
                    }
                    state = BlockValidationState();
                    fInvalidFound = true;
                    fContinue = false;
                    break;
                } else {