Merge #15267: doc: explain AcceptToMemoryPoolWorker's coins_to_uncache

5d26205272 doc: explain AcceptToMemoryPoolWorker's coins_to_uncache (James O'Beirne)

Pull request description:

  I found ATMPW's `coins_to_uncache` a little hard to understand (see #15264). This adds some doc for posterity.

ACKs for commit 5d2620:
  jnewbery:
    ACK 5d262052728acdaa2d108a35ba9921a23b3d761a

Tree-SHA512: 088508fa78012fab8680663c4e30f5cee29768416c2ca8b8b2abc29b6ac7067c5a589674f0254474a7ccc95477889d41719760f5796792bf492f51b3dd499c6c

2 files changed, 20 insertions, 0 deletions

diff --git a/src/coins.h b/src/coins.h
index d39ebf9062..482e233e8c 100644
--- a/src/coins.h
+++ b/src/coins.h
@@ -294,6 +294,10 @@ public:
     bool HaveInputs(const CTransaction& tx) const;
 
 private:
+    /**
+     * @note this is marked const, but may actually append to `cacheCoins`, increasing
+     * memory usage.
+     */
     CCoinsMap::iterator FetchCoin(const COutPoint &outpoint) const;
 };
 
diff --git a/src/validation.cpp b/src/validation.cpp
index 599d6027c8..6bbce9d99a 100644
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -562,6 +562,13 @@ static bool CheckInputsFromMempoolAndCache(const CTransaction& tx, CValidationSt
     return CheckInputs(tx, state, view, true, flags, cacheSigStore, true, txdata);
 }
 
+/**
+ * @param[out] coins_to_uncache   Return any outpoints which were not previously present in the
+ *                                coins cache, but were added as a result of validating the tx
+ *                                for mempool acceptance. This allows the caller to optionally
+ *                                remove the cache additions if the associated transaction ends
+ *                                up being rejected by the mempool.
+ */
 static bool AcceptToMemoryPoolWorker(const CChainParams& chainparams, CTxMemPool& pool, CValidationState& state, const CTransactionRef& ptx,
                               bool* pfMissingInputs, int64_t nAcceptTime, std::list<CTransactionRef>* plTxnReplaced,
                               bool bypass_limits, const CAmount& nAbsurdFee, std::vector<COutPoint>& coins_to_uncache, bool test_accept) EXCLUSIVE_LOCKS_REQUIRED(cs_main)
@@ -657,6 +664,10 @@ static bool AcceptToMemoryPoolWorker(const CChainParams& chainparams, CTxMemPool
             if (!pcoinsTip->HaveCoinInCache(txin.prevout)) {
                 coins_to_uncache.push_back(txin.prevout);
             }
+
+            // Note: this call may add txin.prevout to the coins cache
+            // (pcoinsTip.cacheCoins) by way of FetchCoin(). It should be removed
+            // later (via coins_to_uncache) if this tx turns out to be invalid.
             if (!view.HaveCoin(txin.prevout)) {
                 // Are inputs missing because we already have the tx?
                 for (size_t out = 0; out < tx.vout.size(); out++) {
@@ -978,6 +989,11 @@ static bool AcceptToMemoryPoolWithTime(const CChainParams& chainparams, CTxMemPo
     std::vector<COutPoint> coins_to_uncache;
     bool res = AcceptToMemoryPoolWorker(chainparams, pool, state, tx, pfMissingInputs, nAcceptTime, plTxnReplaced, bypass_limits, nAbsurdFee, coins_to_uncache, test_accept);
     if (!res) {
+        // Remove coins that were not present in the coins cache before calling ATMPW;
+        // this is to prevent memory DoS in case we receive a large number of
+        // invalid transactions that attempt to overrun the in-memory coins cache
+        // (`CCoinsViewCache::cacheCoins`).
+
         for (const COutPoint& hashTx : coins_to_uncache)
             pcoinsTip->Uncache(hashTx);
     }