Merge #19848: Remove mempool global from interfaces

fa9ee52556f493e4a896e2570ca1a3102d777d9a doc: Add doxygen comment to IsRBFOptIn (MarcoFalke)
faef4fc9b4990e563022b6ab595cb02c4060c216 Remove mempool global from interfaces (MarcoFalke)
fa831684e54783f6b40533ca218eb7636bdae667 refactor: Add IsRBFOptInEmptyMempool (MarcoFalke)

Pull request description:

  The chain interface has an `m_node` member, which has a pointer to the mempool global. Use the pointer instead of the global to prepare the removal of the mempool global. See #19556

ACKs for top commit:
  jnewbery:
    utACK fa9ee52556
  darosior:
    ACK fa9ee52
  hebasto:
    re-ACK fa9ee52556f493e4a896e2570ca1a3102d777d9a, since my [previous](https://github.com/bitcoin/bitcoin/pull/19848#pullrequestreview-482403942) review:

Tree-SHA512: 11b4c1446f0860a743fdaa67f95c52bf0262d0a4f888be0eaf07ee497448965d32be414111bf016bd568f2989cde923430e3a3889e224057b73c499f06de7199

3 files changed, 41 insertions, 15 deletions

diff --git a/src/interfaces/chain.cpp b/src/interfaces/chain.cpp
index 313c1265de..13d885a20d 100644
--- a/src/interfaces/chain.cpp
+++ b/src/interfaces/chain.cpp
@@ -276,13 +276,15 @@ public:
     }
     RBFTransactionState isRBFOptIn(const CTransaction& tx) override
     {
-        LOCK(::mempool.cs);
-        return IsRBFOptIn(tx, ::mempool);
+        if (!m_node.mempool) return IsRBFOptInEmptyMempool(tx);
+        LOCK(m_node.mempool->cs);
+        return IsRBFOptIn(tx, *m_node.mempool);
     }
     bool hasDescendantsInMempool(const uint256& txid) override
     {
-        LOCK(::mempool.cs);
-        auto it = ::mempool.GetIter(txid);
+        if (!m_node.mempool) return false;
+        LOCK(m_node.mempool->cs);
+        auto it = m_node.mempool->GetIter(txid);
         return it && (*it)->GetCountWithDescendants() > 1;
     }
     bool broadcastTransaction(const CTransactionRef& tx,
@@ -298,7 +300,9 @@ public:
     }
     void getTransactionAncestry(const uint256& txid, size_t& ancestors, size_t& descendants) override
     {
-        ::mempool.GetTransactionAncestry(txid, ancestors, descendants);
+        ancestors = descendants = 0;
+        if (!m_node.mempool) return;
+        m_node.mempool->GetTransactionAncestry(txid, ancestors, descendants);
     }
     void getPackageLimits(unsigned int& limit_ancestor_count, unsigned int& limit_descendant_count) override
     {
@@ -307,6 +311,7 @@ public:
     }
     bool checkChainLimits(const CTransactionRef& tx) override
     {
+        if (!m_node.mempool) return true;
         LockPoints lp;
         CTxMemPoolEntry entry(tx, 0, 0, 0, false, 0, lp);
         CTxMemPool::setEntries ancestors;
@@ -315,8 +320,9 @@ public:
         auto limit_descendant_count = gArgs.GetArg("-limitdescendantcount", DEFAULT_DESCENDANT_LIMIT);
         auto limit_descendant_size = gArgs.GetArg("-limitdescendantsize", DEFAULT_DESCENDANT_SIZE_LIMIT) * 1000;
         std::string unused_error_string;
-        LOCK(::mempool.cs);
-        return ::mempool.CalculateMemPoolAncestors(entry, ancestors, limit_ancestor_count, limit_ancestor_size,
+        LOCK(m_node.mempool->cs);
+        return m_node.mempool->CalculateMemPoolAncestors(
+            entry, ancestors, limit_ancestor_count, limit_ancestor_size,
             limit_descendant_count, limit_descendant_size, unused_error_string);
     }
     CFeeRate estimateSmartFee(int num_blocks, bool conservative, FeeCalculation* calc) override
@@ -329,7 +335,8 @@ public:
     }
     CFeeRate mempoolMinFee() override
     {
-        return ::mempool.GetMinFee(gArgs.GetArg("-maxmempool", DEFAULT_MAX_MEMPOOL_SIZE) * 1000000);
+        if (!m_node.mempool) return {};
+        return m_node.mempool->GetMinFee(gArgs.GetArg("-maxmempool", DEFAULT_MAX_MEMPOOL_SIZE) * 1000000);
     }
     CFeeRate relayMinFee() override { return ::minRelayTxFee; }
     CFeeRate relayIncrementalFee() override { return ::incrementalRelayFee; }
@@ -395,8 +402,9 @@ public:
     }
     void requestMempoolTransactions(Notifications& notifications) override
     {
-        LOCK2(::cs_main, ::mempool.cs);
-        for (const CTxMemPoolEntry& entry : ::mempool.mapTx) {
+        if (!m_node.mempool) return;
+        LOCK2(::cs_main, m_node.mempool->cs);
+        for (const CTxMemPoolEntry& entry : m_node.mempool->mapTx) {
             notifications.transactionAddedToMempool(entry.GetSharedTx());
         }
     }
diff --git a/src/policy/rbf.cpp b/src/policy/rbf.cpp
index f8b17d18d5..4b55934891 100644
--- a/src/policy/rbf.cpp
+++ b/src/policy/rbf.cpp
@@ -36,3 +36,9 @@ RBFTransactionState IsRBFOptIn(const CTransaction& tx, const CTxMemPool& pool)
     }
     return RBFTransactionState::FINAL;
 }
+
+RBFTransactionState IsRBFOptInEmptyMempool(const CTransaction& tx)
+{
+    // If we don't have a local mempool we can only check the transaction itself.
+    return SignalsOptInRBF(tx) ? RBFTransactionState::REPLACEABLE_BIP125 : RBFTransactionState::UNKNOWN;
+}
diff --git a/src/policy/rbf.h b/src/policy/rbf.h
index d335fbbb36..f84e6e5286 100644
--- a/src/policy/rbf.h
+++ b/src/policy/rbf.h
@@ -7,16 +7,28 @@
 
 #include <txmempool.h>
 
+/** The rbf state of unconfirmed transactions */
 enum class RBFTransactionState {
+    /** Unconfirmed tx that does not signal rbf and is not in the mempool */
     UNKNOWN,
+    /** Either this tx or a mempool ancestor signals rbf */
     REPLACEABLE_BIP125,
-    FINAL
+    /** Neither this tx nor a mempool ancestor signals rbf */
+    FINAL,
 };
 
-// Determine whether an in-mempool transaction is signaling opt-in to RBF
-// according to BIP 125
-// This involves checking sequence numbers of the transaction, as well
-// as the sequence numbers of all in-mempool ancestors.
+/**
+ * Determine whether an unconfirmed transaction is signaling opt-in to RBF
+ * according to BIP 125
+ * This involves checking sequence numbers of the transaction, as well
+ * as the sequence numbers of all in-mempool ancestors.
+ *
+ * @param tx   The unconfirmed transaction
+ * @param pool The mempool, which may contain the tx
+ *
+ * @return     The rbf state
+ */
 RBFTransactionState IsRBFOptIn(const CTransaction& tx, const CTxMemPool& pool) EXCLUSIVE_LOCKS_REQUIRED(pool.cs);
+RBFTransactionState IsRBFOptInEmptyMempool(const CTransaction& tx);
 
 #endif // BITCOIN_POLICY_RBF_H