[docs] Improve commenting in coins.cpp|h

Remove references to 'pruned' coins, which don't exist since the move
to per-txout coins db.

Author: jnewbery

src/coins.cpp

@@ -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.
             }
         }
     }

src/coins.h

@@ -109,19 +109,45 @@ class SaltedOutpointHasher
     }
 };
 
+/**
+ * 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 @@ class CCoinsViewCache : public CCoinsViewBacked
     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 @@ class CCoinsViewCache : public CCoinsViewBacked
     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);