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.)

{
    LOCK(cs_main);
    CCoinsView& db = this->CoinsDB();
    CCoinsViewCache cache(&db);
    std::vector<uint256> hashHeads = db.GetHeadBlocks();
    if (hashHeads.empty()) return true; 
    if (hashHeads.size() != 2) return error("ReplayBlocks(): unknown inconsistent state");
    uiInterface.ShowProgress(_("Replaying blocks...").translated, 0, false);
    LogPrintf("Replaying blocks\n");
    const CBlockIndex* pindexOld = nullptr;  
    const CBlockIndex* pindexNew;            
    const CBlockIndex* pindexFork = nullptr; 
    if (m_blockman.m_block_index.count(hashHeads[0]) == 0) {
        return error("ReplayBlocks(): reorganization to unknown block requested");
    }
    pindexNew = m_blockman.m_block_index[hashHeads[0]];
    if (!hashHeads[1].IsNull()) { 
        if (m_blockman.m_block_index.count(hashHeads[1]) == 0) {
            return error("ReplayBlocks(): reorganization from unknown block requested");
        }
        pindexOld = m_blockman.m_block_index[hashHeads[1]];
        pindexFork = LastCommonAncestor(pindexOld, pindexNew);
        assert(pindexFork != nullptr);
    }
    
    while (pindexOld != pindexFork) {
        if (pindexOld->nHeight > 0) { 
            CBlock block;
            if (!ReadBlockFromDisk(block, pindexOld, params.GetConsensus())) {
                return error("RollbackBlock(): ReadBlockFromDisk() failed at %d, hash=%s", pindexOld->nHeight, pindexOld->GetBlockHash().ToString());