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[docs] Improve commenting in coins.cpp|h

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Remove references to 'pruned' coins, which don't exist since the move
to per-txout coins db.
This commit is contained in:
John Newbery 2020-01-30 15:52:36 -05:00
parent 646f0ada02
commit c205979031
2 changed files with 65 additions and 28 deletions
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49
src/coins.cpp
View file

@ -77,8 +77,21 @@ void CCoinsViewCache::AddCoin(const COutPoint &outpoint, Coin&& coin, bool possi
}
if (!possible_overwrite) {
if (!it->second.coin.IsSpent()) {
throw std::logic_error("Adding new coin that replaces non-pruned entry");
throw std::logic_error("Attempted to overwrite an unspent coin (when possible_overwrite is false)");
}
// If the coin exists in this cache as a spent coin and is DIRTY, then
// its spentness hasn't been flushed to the parent cache. We're
// re-adding the coin to this cache now but we can't mark it as FRESH.
// If we mark it FRESH and then spend it before the cache is flushed
// we would remove it from this cache and would never flush spentness
// to the parent cache.
//
// Re-adding a spent coin can happen in the case of a re-org (the coin
// is 'spent' when the block adding it is disconnected and then
// re-added when it is also added in a newly connected block).
//
// If the coin doesn't exist in the current cache, or is spent but not
// DIRTY, then it can be marked FRESH.
fresh = !(it->second.flags & CCoinsCacheEntry::DIRTY);
}
it->second.coin = std::move(coin);
@ -152,11 +165,11 @@ bool CCoinsViewCache::BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlockIn
}
CCoinsMap::iterator itUs = cacheCoins.find(it->first);
if (itUs == cacheCoins.end()) {
// The parent cache does not have an entry, while the child does
// We can ignore it if it's both FRESH and pruned in the child
// The parent cache does not have an entry, while the child cache does.
// We can ignore it if it's both spent and FRESH in the child
if (!(it->second.flags & CCoinsCacheEntry::FRESH && it->second.coin.IsSpent())) {
// Otherwise we will need to create it in the parent
// and move the data up and mark it as dirty
// Create the coin in the parent cache, move the data up
// and mark it as dirty.
CCoinsCacheEntry& entry = cacheCoins[it->first];
entry.coin = std::move(it->second.coin);
cachedCoinsUsage += entry.coin.DynamicMemoryUsage();
@ -169,19 +182,18 @@ bool CCoinsViewCache::BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlockIn
}
}
} else {
// Assert that the child cache entry was not marked FRESH if the
// parent cache entry has unspent outputs. If this ever happens,
// it means the FRESH flag was misapplied and there is a logic
// error in the calling code.
// Found the entry in the parent cache
if ((it->second.flags & CCoinsCacheEntry::FRESH) && !itUs->second.coin.IsSpent()) {
throw std::logic_error("FRESH flag misapplied to cache entry for base transaction with spendable outputs");
// The coin was marked FRESH in the child cache, but the coin
// exists in the parent cache. If this ever happens, it means
// the FRESH flag was misapplied and there is a logic error in
// the calling code.
throw std::logic_error("FRESH flag misapplied to coin that exists in parent cache");
}
// Found the entry in the parent cache
if ((itUs->second.flags & CCoinsCacheEntry::FRESH) && it->second.coin.IsSpent()) {
// The grandparent does not have an entry, and the child is
// modified and being pruned. This means we can just delete
// it from the parent.
// The grandparent cache does not have an entry, and the coin
// has been spent. We can just delete it from the parent cache.
cachedCoinsUsage -= itUs->second.coin.DynamicMemoryUsage();
cacheCoins.erase(itUs);
} else {
@ -190,11 +202,10 @@ bool CCoinsViewCache::BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlockIn
itUs->second.coin = std::move(it->second.coin);
cachedCoinsUsage += itUs->second.coin.DynamicMemoryUsage();
itUs->second.flags |= CCoinsCacheEntry::DIRTY;
// NOTE: It is possible the child has a FRESH flag here in
// the event the entry we found in the parent is pruned. But
// we must not copy that FRESH flag to the parent as that
// pruned state likely still needs to be communicated to the
// grandparent.
// NOTE: It isn't safe to mark the coin as FRESH in the parent
// cache. If it already existed and was spent in the parent
// cache then marking it FRESH would prevent that spentness
// from being flushed to the grandparent.
}
}
}

44
src/coins.h
View file

@ -109,19 +109,45 @@ public:
}
};
/**
* A Coin in one level of the coins database caching hierarchy.
*
* A coin can either be:
* - unspent or spent (in which case the Coin object will be nulled out - see Coin.Clear())
* - DIRTY or not DIRTY
* - FRESH or not FRESH
*
* Out of these 2^3 = 8 states, only some combinations are valid:
* - unspent, FRESH, DIRTY (e.g. a new coin created in the cache)
* - unspent, not FRESH, DIRTY (e.g. a coin changed in the cache during a reorg)
* - unspent, not FRESH, not DIRTY (e.g. an unspent coin fetched from the parent cache)
* - spent, FRESH, not DIRTY (e.g. a spent coin fetched from the parent cache)
* - spent, not FRESH, DIRTY (e.g. a coin is spent and spentness needs to be flushed to the parent)
*/
struct CCoinsCacheEntry
{
Coin coin; // The actual cached data.
unsigned char flags;
enum Flags {
DIRTY = (1 << 0), // This cache entry is potentially different from the version in the parent view.
FRESH = (1 << 1), // The parent view does not have this entry (or it is pruned).
/* Note that FRESH is a performance optimization with which we can
* erase coins that are fully spent if we know we do not need to
* flush the changes to the parent cache. It is always safe to
* not mark FRESH if that condition is not guaranteed.
/**
* DIRTY means the CCoinsCacheEntry is potentially different from the
* version in the parent cache. Failure to mark a coin as DIRTY when
* it is potentially different from the parent cache will cause a
* consensus failure, since the coin's state won't get written to the
* parent when the cache is flushed.
*/
DIRTY = (1 << 0),
/**
* FRESH means the parent cache does not have this coin or that it is a
* spent coin in the parent cache. If a FRESH coin in the cache is
* later spent, it can be deleted entirely and doesn't ever need to be
* flushed to the parent. This is a performance optimization. Marking a
* coin as FRESH when it exists unspent in the parent cache will cause a
* consensus failure, since it might not be deleted from the parent
* when this cache is flushed.
*/
FRESH = (1 << 1),
};
CCoinsCacheEntry() : flags(0) {}
@ -246,7 +272,7 @@ public:
bool HaveCoinInCache(const COutPoint &outpoint) const;
/**
* Return a reference to Coin in the cache, or a pruned one if not found. This is
* Return a reference to Coin in the cache, or coinEmpty if not found. This is
* more efficient than GetCoin.
*
* Generally, do not hold the reference returned for more than a short scope.
@ -258,8 +284,8 @@ public:
const Coin& AccessCoin(const COutPoint &output) const;
/**
* Add a coin. Set potential_overwrite to true if a non-pruned version may
* already exist.
* Add a coin. Set potential_overwrite to true if an unspent version may
* already exist in the cache.
*/
void AddCoin(const COutPoint& outpoint, Coin&& coin, bool potential_overwrite);