banman: save the banlist in a JSON format on disk

Save the banlist in `banlist.json` instead of `banlist.dat`.

This makes it possible to store Tor v3 entries in the banlist on disk
(and any other addresses that cannot be serialized in addrv1 format).

Only read `banlist.dat` if it exists and `banlist.json` does not
exist (first start after an upgrade).

Supersedes https://github.com/bitcoin/bitcoin/pull/20904
Resolves https://github.com/bitcoin/bitcoin/issues/19748

src/addrdb.cpp

@@ -11,13 +11,72 @@
 #include <cstdint>
 #include <hash.h>
 #include <logging/timer.h>
+#include <netbase.h>
 #include <random.h>
 #include <streams.h>
 #include <tinyformat.h>
+#include <univalue.h>
+#include <util/settings.h>
 #include <util/system.h>
 
+CBanEntry::CBanEntry(const UniValue& json)
+    : nVersion(json["version"].get_int()), nCreateTime(json["ban_created"].get_int64()),
+      nBanUntil(json["banned_until"].get_int64())
+{
+}
+
+UniValue CBanEntry::ToJson() const
+{
+    UniValue json(UniValue::VOBJ);
+    json.pushKV("version", nVersion);
+    json.pushKV("ban_created", nCreateTime);
+    json.pushKV("banned_until", nBanUntil);
+    return json;
+}
+
 namespace {
 
+static const char* BANMAN_JSON_ADDR_KEY = "address";
+
+/**
+ * Convert a `banmap_t` object to a JSON array.
+ * @param[in] bans Bans list to convert.
+ * @return a JSON array, similar to the one returned by the `listbanned` RPC. Suitable for
+ * passing to `BanMapFromJson()`.
+ */
+UniValue BanMapToJson(const banmap_t& bans)
+{
+    UniValue bans_json(UniValue::VARR);
+    for (const auto& it : bans) {
+        const auto& address = it.first;
+        const auto& ban_entry = it.second;
+        UniValue j = ban_entry.ToJson();
+        j.pushKV(BANMAN_JSON_ADDR_KEY, address.ToString());
+        bans_json.push_back(j);
+    }
+    return bans_json;
+}
+
+/**
+ * Convert a JSON array to a `banmap_t` object.
+ * @param[in] bans_json JSON to convert, must be as returned by `BanMapToJson()`.
+ * @param[out] bans Bans list to create from the JSON.
+ * @throws std::runtime_error if the JSON does not have the expected fields or they contain
+ * unparsable values.
+ */
+void BanMapFromJson(const UniValue& bans_json, banmap_t& bans)
+{
+    for (const auto& ban_entry_json : bans_json.getValues()) {
+        CSubNet subnet;
+        const auto& subnet_str = ban_entry_json[BANMAN_JSON_ADDR_KEY].get_str();
+        if (!LookupSubNet(subnet_str, subnet)) {
+            throw std::runtime_error(
+                strprintf("Cannot parse banned address or subnet: %s", subnet_str));
+        }
+        bans.insert_or_assign(subnet, CBanEntry{ban_entry_json});
+    }
+}
+
 template <typename Stream, typename Data>
 bool SerializeDB(Stream& stream, const Data& data)
 {
@@ -119,18 +178,54 @@ bool DeserializeFileDB(const fs::path& path, Data& data, int version)
 }
 } // namespace
 
-CBanDB::CBanDB(fs::path ban_list_path) : m_ban_list_path(std::move(ban_list_path))
+CBanDB::CBanDB(fs::path ban_list_path)
+    : m_banlist_dat(ban_list_path.string() + ".dat"),
+      m_banlist_json(ban_list_path.string() + ".json")
 {
 }
 
 bool CBanDB::Write(const banmap_t& banSet)
 {
-    return SerializeFileDB("banlist", m_ban_list_path, banSet, CLIENT_VERSION);
+    std::vector<std::string> errors;
+    if (util::WriteSettings(m_banlist_json, {{JSON_KEY, BanMapToJson(banSet)}}, errors)) {
+        return true;
+    }
+
+    for (const auto& err : errors) {
+        error("%s", err);
+    }
+    return false;
 }
 
-bool CBanDB::Read(banmap_t& banSet)
+bool CBanDB::Read(banmap_t& banSet, bool& dirty)
 {
-    return DeserializeFileDB(m_ban_list_path, banSet, CLIENT_VERSION);
+    // If the JSON banlist does not exist, then try to read the non-upgraded banlist.dat.
+    if (!fs::exists(m_banlist_json)) {
+        // If this succeeds then we need to flush to disk in order to create the JSON banlist.
+        dirty = true;
+        return DeserializeFileDB(m_banlist_dat, banSet, CLIENT_VERSION);
+    }
+
+    dirty = false;
+
+    std::map<std::string, util::SettingsValue> settings;
+    std::vector<std::string> errors;
+
+    if (!util::ReadSettings(m_banlist_json, settings, errors)) {
+        for (const auto& err : errors) {
+            LogPrintf("Cannot load banlist %s: %s\n", m_banlist_json.string(), err);
+        }
+        return false;
+    }
+
+    try {
+        BanMapFromJson(settings[JSON_KEY], banSet);
+    } catch (const std::runtime_error& e) {
+        LogPrintf("Cannot parse banlist %s: %s\n", m_banlist_json.string(), e.what());
+        return false;
+    }
+
+    return true;
 }
 
 CAddrDB::CAddrDB()