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.)

LogPrint(BCLog::BENCH, "      - Connect %u transactions: %.2fms (%.3fms/tx, %.3fms/txin) [%.2fs (%.2fms/blk)]\n", (unsigned)block.vtx.size(), MILLI * (nTime3 - nTime2), MILLI * (nTime3 - nTime2) / block.vtx.size(), nInputs <= 1 ? 0 : MILLI * (nTime3 - nTime2) / (nInputs-1), nTimeConnect * MICRO, nTimeConnect * MILLI / nBlocksTotal);
    CAmount blockReward = nFees + GetBlockSubsidy(pindex->nHeight, chainparams.GetConsensus());
    if (block.vtx[0]->GetValueOut() > blockReward) {
        LogPrintf("ERROR: ConnectBlock(): coinbase pays too much (actual=%d vs limit=%d)\n", block.vtx[0]->GetValueOut(), blockReward);
        return state.Invalid(BlockValidationResult::BLOCK_CONSENSUS, "bad-cb-amount");
    }
    if (!control.Wait()) {
        LogPrintf("ERROR: %s: CheckQueue failed\n", __func__);
        return state.Invalid(BlockValidationResult::BLOCK_CONSENSUS, "block-validation-failed");
    }
    int64_t nTime4 = GetTimeMicros(); nTimeVerify += nTime4 - nTime2;
    LogPrint(BCLog::BENCH, "    - Verify %u txins: %.2fms (%.3fms/txin) [%.2fs (%.2fms/blk)]\n", nInputs - 1, MILLI * (nTime4 - nTime2), nInputs <= 1 ? 0 : MILLI * (nTime4 - nTime2) / (nInputs-1), nTimeVerify * MICRO, nTimeVerify * MILLI / nBlocksTotal);
    if (fJustCheck)
        return true;
    if (!WriteUndoDataForBlock(blockundo, state, pindex, chainparams))
        return false;
    if (!pindex->IsValid(BLOCK_VALID_SCRIPTS)) {
        pindex->RaiseValidity(BLOCK_VALID_SCRIPTS);
        setDirtyBlockIndex.insert(pindex);
    }
    assert(pindex->phashBlock);
    
    view.SetBestBlock(pindex->GetBlockHash());
    int64_t nTime5 = GetTimeMicros(); nTimeIndex += nTime5 - nTime4;
    LogPrint(BCLog::BENCH, "    - Index writing: %.2fms [%.2fs (%.2fms/blk)]\n", MILLI * (nTime5 - nTime4), nTimeIndex * MICRO, nTimeIndex * MILLI / nBlocksTotal);
    int64_t nTime6 = GetTimeMicros(); nTimeCallbacks += nTime6 - nTime5;
    LogPrint(BCLog::BENCH, "    - Callbacks: %.2fms [%.2fs (%.2fms/blk)]\n", MILLI * (nTime6 - nTime5), nTimeCallbacks * MICRO, nTimeCallbacks * MILLI / nBlocksTotal);
    return true;
}
CoinsCacheSizeState CChainState::GetCoinsCacheSizeState(const CTxMemPool& tx_pool)