[doc] MemPoolAccept coins views

Author: glozow

src/validation.cpp

@@ -725,8 +725,27 @@ class MemPoolAccept
 
 private:
     CTxMemPool& m_pool;
+
+    /** Holds a cached view of available coins from the UTXO set, mempool, and artificial temporary coins (to enable package validation).
+     * The view doesn't track whether a coin previously existed but has now been spent. We detect conflicts in other ways:
+     * - conflicts within a transaction are checked in CheckTransaction (bad-txns-inputs-duplicate)
+     * - conflicts within a package are checked in IsWellFormedPackage (conflict-in-package)
+     * - conflicts with an existing mempool transaction are found in CTxMemPool::GetConflictTx and replacements are allowed
+     * The temporary coins should persist between individual transaction checks so that package validation is possible,
+     * but must be cleaned up when we finish validating a subpackage, whether accepted or rejected. The cache must also
+     * be cleared when mempool contents change (when a changeset is applied or when the mempool trims itself) because it
+     * can return cached coins that no longer exist in the backend. Use CleanupTemporaryCoins() anytime you are finished
+     * with a SubPackageState or call LimitMempoolSize().
+     */
     CCoinsViewCache m_view;
+
+    // These are the two possible backends for m_view.
+    /** When m_view is connected to m_viewmempool as its backend, it can pull coins from the mempool and from the UTXO
+     * set. This is also where temporary coins are stored. */
     CCoinsViewMemPool m_viewmempool;
+    /** When m_view is connected to m_dummy, it can no longer look up coins from the mempool or UTXO set (meaning no disk
+     * operations happen), but can still return coins it accessed previously. Useful for keeping track of which coins
+     * were pulled from disk. */
     CCoinsView m_dummy;
 
     Chainstate& m_active_chainstate;
@@ -1651,7 +1670,8 @@ void MemPoolAccept::CleanupTemporaryCoins()
     // (3) Confirmed coins don't need to be removed. The chainstate has not changed (we are
     // holding cs_main and no blocks have been processed) so the confirmed tx cannot disappear like
     // a mempool tx can. The coin may now be spent after we submitted a tx to mempool, but
-    // we have already checked that the package does not have 2 transactions spending the same coin.
+    // we have already checked that the package does not have 2 transactions spending the same coin
+    // and we check whether a mempool transaction spends conflicting coins (CTxMemPool::GetConflictTx).
     // Keeping them in m_view is an optimization to not re-fetch confirmed coins if we later look up
     // inputs for this transaction again.
     for (const auto& outpoint : m_viewmempool.GetNonBaseCoins()) {