policy: reduce MAX_PEER_TX_ANNOUNCEMENTS

Maintaining up to 100000 INVs per peer is excessive. A Dogecoin
Core node will never send more than 7 invs per second.

Backported from: de11b0a
Original Author: Pieter Wuille <[email protected]>

qa/rpc-tests/p2p-tx-download.py

@@ -18,6 +18,7 @@
 INBOUND_PEER_TX_DELAY = 2  # seconds
 TXID_RELAY_DELAY = 2 # seconds
 MAX_GETDATA_IN_FLIGHT = 100
+MAX_PEER_TX_ANNOUNCEMENTS = 5000
 
 class TxDownloadTestNode(SingleNodeConnCB):
     def __init__(self):
@@ -51,6 +52,11 @@ def is_closed():
             return self.connection.state == "closed"
         return wait_until(is_closed, timeout=30)
 
+    def wait_until_numgetdata(self, num):
+        def has_num():
+            return len(self.tx_getdata_received) == num
+        return wait_until(has_num, timeout=60)
+
     def disconnect(self):
         self.connection.disconnect_node()
         return self.wait_for_disconnect()
@@ -80,6 +86,7 @@ def run_test(self):
         self.test_invblock_resolution()
         self.test_disconnect_fallback()
         self.test_notfound_fallback()
+        self.test_max_announcements()
 
     def setup_network(self):
         # set up full nodes
@@ -261,5 +268,23 @@ def test_disconnect_fallback(self):
         # the losing peer is now the fallback and received a getdata message
         assert self.wait_for_getdata([txid], [loser])
 
+    def test_max_announcements(self):
+        # create a test node
+        peer = self.create_testnode()
+        peer.wait_for_verack()
+
+        hashes = []
+        for _ in range(MAX_PEER_TX_ANNOUNCEMENTS):
+            hashes.append(self.next_fake_txid())
+
+        peer.send_tx_inv(hashes)
+        peer.wait_until_numgetdata(MAX_PEER_TX_ANNOUNCEMENTS)
+        peer.sync_with_ping()
+
+        # send one more - this should never come back.
+        extratx = self.next_fake_txid()
+        peer.send_tx_inv([extratx])
+        assert not self.any_received_getdata(extratx, [peer])
+
 if __name__ == '__main__':
     TxDownloadTest().main()

src/net_processing.cpp

@@ -52,8 +52,15 @@ struct IteratorComparator
 
 /** Maximum number of in-flight transactions from a peer */
 static constexpr int32_t MAX_PEER_TX_IN_FLIGHT = 100;
-/** Maximum number of announced transactions from a peer */
-static constexpr int32_t MAX_PEER_TX_ANNOUNCEMENTS = 2 * MAX_INV_SZ;
+/**
+ * Maximum number of transactions to consider for requesting, per peer.
+ *
+ * It provides a reasonable DoS limit to per-peer memory usage spent on
+ * announcements, while covering peers continuously sending INVs at the maximum
+ * rate for several minutes (see INVENTORY_BROADCAST_MAX), while not receiving
+ * the actual transaction (from any peer) in response to requests for them.
+ */
+static constexpr int32_t MAX_PEER_TX_ANNOUNCEMENTS = 5000;
 /** How many microseconds to delay requesting transactions from inbound peers */
 static constexpr int64_t NONPREF_PEER_TX_DELAY = 2 * 1000000; // 2 seconds
 /** How many microseconds to delay requesting transactions from overloaded peers */