partial bitcoin#17428: Try to preserve outbound block-relay-only connections during restart

excludes:
- 0a85e5a7bc8dc6587963e2e37ac1b087a1fc97fe

doc/files.md

@@ -53,6 +53,7 @@ Subdirectory       | File(s)               | Description
 `indexes/blockfilter/basic/db/` | LevelDB database      | Blockfilter index LevelDB database for the basic filtertype; *optional*, used if `-blockfilterindex=basic`
 `indexes/blockfilter/basic/`    | `fltrNNNNN.dat`<sup>[\[2\]](#note2)</sup> | Blockfilter index filters for the basic filtertype; *optional*, used if `-blockfilterindex=basic`
 `wallets/`         |                       | [Contains wallets](#multi-wallet-environment); can be specified by `-walletdir` option; if `wallets/` subdirectory does not exist, a wallet resides in the data directory
+`./`               | `anchors.dat`         | Anchor IP address database, created on shutdown and deleted at startup. Anchors are last known outgoing block-relay-only peers that are tried to re-connect to on startup
 `evodb/`         |                       |special txes and quorums database
 `llmq/`          |                       |quorum signatures database
 `./`               | `banlist.dat`         | Stores the IPs/subnets of banned nodes

src/addrdb.cpp

@@ -9,6 +9,7 @@
 #include <chainparams.h>
 #include <clientversion.h>
 #include <hash.h>
+#include <logging/timer.h>
 #include <random.h>
 #include <streams.h>
 #include <tinyformat.h>
@@ -157,3 +158,22 @@ bool CAddrDB::Read(CAddrMan& addr, CDataStream& ssPeers)
     }
     return ret;
 }
+
+void DumpAnchors(const fs::path& anchors_db_path, const std::vector<CAddress>& anchors)
+{
+    LOG_TIME_SECONDS(strprintf("Flush %d outbound block-relay-only peer addresses to anchors.dat", anchors.size()));
+    SerializeFileDB("anchors", anchors_db_path, anchors);
+}
+
+std::vector<CAddress> ReadAnchors(const fs::path& anchors_db_path)
+{
+    std::vector<CAddress> anchors;
+    if (DeserializeFileDB(anchors_db_path, anchors)) {
+        LogPrintf("Loaded %i addresses from %s\n", anchors.size(), anchors_db_path.filename());
+    } else {
+        anchors.clear();
+    }
+
+    fs::remove(anchors_db_path);
+    return anchors;
+}

src/addrdb.h

@@ -11,9 +11,9 @@
 #include <serialize.h>
 
 #include <string>
-#include <map>
+#include <vector>
 
-class CSubNet;
+class CAddress;
 class CAddrMan;
 class CDataStream;
 
@@ -73,4 +73,20 @@ public:
     bool Read(banmap_t& banSet);
 };
 
+/**
+ * Dump the anchor IP address database (anchors.dat)
+ *
+ * Anchors are last known outgoing block-relay-only peers that are
+ * tried to re-connect to on startup.
+ */
+void DumpAnchors(const fs::path& anchors_db_path, const std::vector<CAddress>& anchors);
+
+/**
+ * Read the anchor IP address database (anchors.dat)
+ *
+ * Deleting anchors.dat is intentional as it avoids renewed peering to anchors after
+ * an unclean shutdown and thus potential exploitation of the anchor peer policy.
+ */
+std::vector<CAddress> ReadAnchors(const fs::path& anchors_db_path);
+
 #endif // BITCOIN_ADDRDB_H

src/net.cpp

@@ -64,6 +64,12 @@
 
 #include <math.h>
 
+/** Maximum number of block-relay-only anchor connections */
+static constexpr size_t MAX_BLOCK_RELAY_ONLY_ANCHORS = 2;
+static_assert (MAX_BLOCK_RELAY_ONLY_ANCHORS <= static_cast<size_t>(MAX_BLOCK_RELAY_ONLY_CONNECTIONS), "MAX_BLOCK_RELAY_ONLY_ANCHORS must not exceed MAX_BLOCK_RELAY_ONLY_CONNECTIONS.");
+/** Anchor IP address database file name */
+const char* const ANCHORS_DATABASE_FILENAME = "anchors.dat";
+
 // How often to dump addresses to peers.dat
 static constexpr std::chrono::minutes DUMP_PEERS_INTERVAL{15};
 
@@ -2336,6 +2342,13 @@ void CConnman::ThreadOpenConnections(const std::vector<std::string> connect)
         int nTries = 0;
         while (!interruptNet)
         {
+            // If we didn't find an appropriate destination after trying 100 addresses fetched from addrman,
+            // stop this loop, and let the outer loop run again (which sleeps, adds seed nodes, recalculates
+            // already-connected network ranges, ...) before trying new addrman addresses.
+            nTries++;
+            if (nTries > 100)
+                break;
+
             CAddrInfo addr = addrman.SelectTriedCollision();
 
             // SelectTriedCollision returns an invalid address if it is empty.
@@ -2365,13 +2378,6 @@ void CConnman::ThreadOpenConnections(const std::vector<std::string> connect)
                 break;
             }
 
-            // If we didn't find an appropriate destination after trying 100 addresses fetched from addrman,
-            // stop this loop, and let the outer loop run again (which sleeps, adds seed nodes, recalculates
-            // already-connected network ranges, ...) before trying new addrman addresses.
-            nTries++;
-            if (nTries > 100)
-                break;
-
             if (!IsReachable(addr))
                 continue;
 
@@ -2427,6 +2433,19 @@ void CConnman::ThreadOpenConnections(const std::vector<std::string> connect)
     }
 }
 
+std::vector<CAddress> CConnman::GetCurrentBlockRelayOnlyConns() const
+{
+    std::vector<CAddress> ret;
+    LOCK(cs_vNodes);
+    for (const CNode* pnode : vNodes) {
+        if (pnode->IsBlockRelayOnly()) {
+            ret.push_back(pnode->addr);
+        }
+    }
+
+    return ret;
+}
+
 std::vector<AddedNodeInfo> CConnman::GetAddedNodeInfo()
 {
     std::vector<AddedNodeInfo> ret;
@@ -3131,6 +3150,15 @@ bool CConnman::Start(CScheduler& scheduler, const Options& connOptions)
         }
     }
 
+    if (m_use_addrman_outgoing) {
+        // Load addresses from anchors.dat
+        m_anchors = ReadAnchors(GetDataDir() / ANCHORS_DATABASE_FILENAME);
+        if (m_anchors.size() > MAX_BLOCK_RELAY_ONLY_ANCHORS) {
+            m_anchors.resize(MAX_BLOCK_RELAY_ONLY_ANCHORS);
+        }
+        LogPrintf("%i block-relay-only anchors will be tried for connections.\n", m_anchors.size());
+    }
+
     uiInterface.InitMessage(_("Starting network threads...").translated);
 
     fAddressesInitialized = true;
@@ -3311,6 +3339,15 @@ void CConnman::StopNodes()
     if (fAddressesInitialized) {
         DumpAddresses();
         fAddressesInitialized = false;
+
+        if (m_use_addrman_outgoing) {
+            // Anchor connections are only dumped during clean shutdown.
+            std::vector<CAddress> anchors_to_dump = GetCurrentBlockRelayOnlyConns();
+            if (anchors_to_dump.size() > MAX_BLOCK_RELAY_ONLY_ANCHORS) {
+                anchors_to_dump.resize(MAX_BLOCK_RELAY_ONLY_ANCHORS);
+            }
+            DumpAnchors(GetDataDir() / ANCHORS_DATABASE_FILENAME, anchors_to_dump);
+        }
     }
 
     {

src/net.h

@@ -572,6 +572,11 @@ private:
     void RecordBytesRecv(uint64_t bytes);
     void RecordBytesSent(uint64_t bytes);
 
+    /**
+     * Return vector of current BLOCK_RELAY peers.
+     */
+    std::vector<CAddress> GetCurrentBlockRelayOnlyConns() const;
+
     // Whether the node should be passed out in ForEach* callbacks
     static bool NodeFullyConnected(const CNode* pnode);
 
@@ -679,6 +684,12 @@ private:
     NetEventsInterface* m_msgproc;
     BanMan* m_banman;
 
+    /**
+     * Addresses that were saved during the previous clean shutdown. We'll
+     * attempt to make block-relay-only connections to them.
+     */
+    std::vector<CAddress> m_anchors;
+
     /** SipHasher seeds for deterministic randomness */
     const uint64_t nSeed0, nSeed1;