partial bitcoin#22778: Reduce resource usage for inbound block-relay-…

…only connections

includes:
- 290a8da

src/net_processing.cpp

@@ -272,25 +272,34 @@ struct Peer {
 
     struct TxRelay {
         mutable RecursiveMutex m_bloom_filter_mutex;
-        // We use m_relay_txs for two purposes -
-        // a) it allows us to not relay tx invs before receiving the peer's version message
-        // b) the peer may tell us in its version message that we should not relay tx invs
-        //    unless it loads a bloom filter.
+        /** Whether the peer wishes to receive transaction announcements.
+         *
+         * This is initially set based on the fRelay flag in the received
+         * `version` message. If initially set to false, it can only be flipped
+         * to true if we have offered the peer NODE_BLOOM services and it sends
+         * us a `filterload` or `filterclear` message. See BIP37. */
         bool m_relay_txs GUARDED_BY(m_bloom_filter_mutex){false};
+        /** A bloom filter for which transactions to announce to the peer. See BIP37. */
         std::unique_ptr<CBloomFilter> m_bloom_filter PT_GUARDED_BY(m_bloom_filter_mutex) GUARDED_BY(m_bloom_filter_mutex){nullptr};
 
         mutable RecursiveMutex m_tx_inventory_mutex;
-        // inventory based relay
+        /** A filter of all the txids that the peer has announced to
+         *  us or we have announced to the peer. We use this to avoid announcing
+         *  the same txid to a peer that already has the transaction. */
         CRollingBloomFilter m_tx_inventory_known_filter GUARDED_BY(m_tx_inventory_mutex){50000, 0.000001};
-        // Set of transaction ids we still have to announce.
-        // They are sorted by the mempool before relay, so the order is not important.
+        /** Set of transaction ids we still have to announce. We use the
+         *  mempool to sort transactions in dependency order before relay, so
+         *  this does not have to be sorted. */
         std::set<uint256> m_tx_inventory_to_send GUARDED_BY(m_tx_inventory_mutex);
-        // List of non-tx/non-block inventory items
+        /** List of non-tx/non-block inventory items */
         std::vector<CInv> vInventoryOtherToSend GUARDED_BY(m_tx_inventory_mutex);
-        // Used for BIP35 mempool sending, also protected by m_tx_inventory_mutex
+        /** Whether the peer has requested us to send our complete mempool. Only
+         *  permitted if the peer has NetPermissionFlags::Mempool. See BIP35. */
         bool m_send_mempool GUARDED_BY(m_tx_inventory_mutex){false};
-        // Last time a "MEMPOOL" request was serviced.
+        /** The last time a BIP35 `mempool` request was serviced. */
         std::atomic<std::chrono::seconds> m_last_mempool_req{0s};
+        /** The next time after which we will send an `inv` message containing
+         *  transaction announcements to this peer. */
         std::chrono::microseconds m_next_inv_send_time GUARDED_BY(NetEventsInterface::g_msgproc_mutex){0};
     };