Add <datadir>/settings.json persistent settings storage.

Persistent settings are used in followup PRs bitcoin#15936 to unify gui settings
between bitcoin-qt and bitcoind, and bitcoin#15937 to add a load_on_startup flag to
the loadwallet RPC and maintain a dynamic list of wallets that should be loaded
on startup that also can be shared between bitcoind and bitcoin-qt.

doc/files.md

@@ -1,7 +1,7 @@
 Filename            | Description
 --------------------|----------------------------------------------------------------------------------------------------------------------------
 banlist.dat         | stores the IPs/Subnets of banned nodes
-bitcoin.conf        | contains configuration settings for bitcoind or bitcoin-qt
+bitcoin.conf        | user-defined settings for bitcoind and bitcoin-qt; file is read-only and must be created manually
 bitcoind.pid        | stores the process id of bitcoind while running
 blocks/blk000??.dat | block data (custom, 128 MiB per file); since 0.8.0
 blocks/rev000??.dat | block undo data (custom); since 0.8.0 (format changed since pre-0.8)
@@ -14,6 +14,7 @@ fee_estimates.dat   | stores statistics used to estimate minimum transaction fee
 indexes/txindex/*   | optional transaction index database (LevelDB); since 0.17.0
 mempool.dat         | dump of the mempool's transactions; since 0.14.0
 peers.dat           | peer IP address database (custom format); since 0.7.0
+setting.json        | read-write settings set in GUI or RPC interfaces that augment manual settings from bitcoin.conf
 wallet.dat          | personal wallet (BDB) with keys and transactions; moved to wallets/ directory on new installs since 0.16.0
 wallets/database/*  | BDB database environment; used for wallets since 0.16.0
 wallets/db.log      | wallet database log file; since 0.16.0

src/bitcoind.cpp

@@ -123,6 +123,11 @@ static bool AppInit(int argc, char* argv[])
             }
         }
 
+        if (!gArgs.ReadSettingsFile()) {
+            fprintf(stderr, "Error reading settings file\n");
+            return false;
+        }
+
         // -server defaults to true for bitcoind but not for the GUI so do this here
         gArgs.SoftSetBoolArg("-server", true);
         // Set this early so that parameter interactions go to console

src/interfaces/node.cpp

@@ -63,6 +63,7 @@ class NodeImpl : public Node
     bool softSetArg(const std::string& arg, const std::string& value) override { return gArgs.SoftSetArg(arg, value); }
     bool softSetBoolArg(const std::string& arg, bool value) override { return gArgs.SoftSetBoolArg(arg, value); }
     void selectParams(const std::string& network) override { SelectParams(network); }
+    bool readSettingsFile() override { return gArgs.ReadSettingsFile(); }
     uint64_t getAssumedBlockchainSize() override { return Params().AssumedBlockchainSize(); }
     uint64_t getAssumedChainStateSize() override { return Params().AssumedChainStateSize(); }
     std::string getNetwork() override { return Params().NetworkIDString(); }

src/interfaces/node.h

@@ -53,6 +53,11 @@ class Node
     //! Choose network parameters.
     virtual void selectParams(const std::string& network) = 0;
 
+    //! Load <datadir>/settings.json file with saved settings. This needs to be
+    //! called after selectParams() because the settings file is
+    //! network-specific.
+    virtual bool readSettingsFile() = 0;
+
     //! Get the (assumed) blockchain size.
     virtual uint64_t getAssumedBlockchainSize() = 0;
 

src/qt/bitcoin.cpp

@@ -527,6 +527,11 @@ int GuiMain(int argc, char* argv[])
     // Parse URIs on command line -- this can affect Params()
     PaymentServer::ipcParseCommandLine(*node, argc, argv);
 #endif
+    if (!node->readSettingsFile()) {
+        QMessageBox::critical(nullptr, QObject::tr(PACKAGE_NAME),
+                              QObject::tr("Error: Cannot read settings file."));
+        return EXIT_FAILURE;
+    }
 
     QScopedPointer<const NetworkStyle> networkStyle(NetworkStyle::instantiate(QString::fromStdString(Params().NetworkIDString())));
     assert(!networkStyle.isNull());

src/util/settings.cpp

@@ -29,6 +29,9 @@ static void MergeSettings(const Settings& settings, const std::string& section,
     if (auto* value = FindKey(settings.command_line_options, name)) {
         fn(Source(SettingsSpan(*value)));
     }
+    if (auto* value = FindKey(settings.rw_settings, name)) {
+        fn(Source(SettingsSpan(value)));
+    }
     if (!section.empty()) {
         if (auto* map = FindKey(settings.ro_config, section)) {
             if (auto* value = FindKey(*map, name)) {

src/util/settings.h

@@ -20,16 +20,17 @@ namespace util {
 //!       substituted if there's a need to move away from UniValue.
 using SettingsValue = UniValue;
 
-//! Stored bitcoin settings. This struct combines settings from the command line
-//! and a read-only configuration file.
+//! Stored bitcoin settings. This struct combines settings from the command line,
+//! a read-only configuration file, and a read-write runtime settings file.
 struct Settings {
     std::map<std::string, SettingsValue> forced_settings;
     std::map<std::string, std::vector<SettingsValue>> command_line_options;
+    std::map<std::string, SettingsValue> rw_settings;
     std::map<std::string, std::map<std::string, std::vector<SettingsValue>>> ro_config;
 };
 
 //! Get settings value from combined sources: forced settings, command line
-//! arguments and the read-only config file.
+//! arguments, runtime read-write settings, and the read-only config file.
 //!
 //! @param ignore_default_section_config - ignore values set in the top-level
 //!                                        section of the config file.

src/util/system.cpp

@@ -381,6 +381,72 @@ bool ArgsManager::IsArgSet(const std::string& strArg) const
     return !ArgsManagerHelper::Get(*this, strArg).isNull();
 }
 
+static fs::path GetSettingsFile(bool temp = false)
+{
+    return fs::absolute(temp ? "settings.json.tmp" : "settings.json", GetDataDir(/* net_specific= */ true));
+}
+
+bool ArgsManager::ReadSettingsFile()
+{
+    fsbridge::ifstream file;
+    fs::path filepath = GetSettingsFile(/* temp= */ false);
+    file.open(filepath);
+    if (!file.is_open()) return true; // Ok for file not to exist.
+
+    util::SettingsValue in;
+    if (!in.read(std::string{std::istreambuf_iterator<char>(file), std::istreambuf_iterator<char>()})) {
+        LogPrintf("Error: Unable to parse settings file %s\n", filepath.string());
+        return false;
+    }
+    if (file.fail()) {
+        LogPrintf("Error reading settings file %s\n", filepath.string());
+        return false;
+    }
+    file.close();
+
+    if (!in.isObject()) {
+        LogPrintf("Error: Settings file %s is not in expected key-value format.\n", filepath.string());
+        return false;
+    }
+
+    LOCK(cs_args);
+    m_settings.rw_settings.clear();
+    const std::vector<std::string>& keys = in.getKeys();
+    const std::vector<UniValue>& values = in.getValues();
+    for (size_t i = 0; i < keys.size(); ++i) {
+        m_settings.rw_settings.emplace(keys[i], values[i]);
+    }
+    return true;
+}
+
+bool ArgsManager::WriteSettingsFile() const
+{
+    util::SettingsValue out(util::SettingsValue::VOBJ);
+    {
+        LOCK(cs_args);
+        for (const auto& value : m_settings.rw_settings) {
+            out.__pushKV(value.first, value.second);
+        }
+    }
+
+    fsbridge::ofstream file;
+    fs::path filepath_tmp = GetSettingsFile(/* temp= */ true);
+    file.open(filepath_tmp);
+    if (file.fail()) {
+        LogPrintf("Error: Unable to open settings file %s for writing\n", filepath_tmp.string());
+        return false;
+    }
+    file << out.write(/* prettyIndent= */ 1, /* indentLevel= */ 4) << std::endl;
+    file.close();
+
+    fs::path filepath = GetSettingsFile(/* temp= */ false);
+    if (!RenameOver(filepath_tmp, filepath)) {
+        LogPrintf("Error: Unable to rename settings file %s to %s\n", filepath_tmp.string(), filepath.string());
+        return false;
+    }
+    return true;
+}
+
 bool ArgsManager::IsArgNegated(const std::string& strArg) const
 {
     return ArgsManagerHelper::Get(*this, strArg).isFalse();

src/util/system.h

@@ -291,6 +291,27 @@ class ArgsManager
      * Check whether we know of this arg
      */
     bool IsArgKnown(const std::string& key) const;
+
+    /**
+     * Load <datadir>/settings.json file with saved settings. This needs to be
+     * called after SelectParams() because the settings file is network-specific.
+     */
+    bool ReadSettingsFile();
+
+    /**
+     * Save <datadir>/settings.json file with persistent settings.
+     */
+    bool WriteSettingsFile() const;
+
+    /**
+     * Access settings with lock held.
+     */
+    template <typename Fn>
+    void LockSettings(Fn&& fn)
+    {
+        LOCK(cs_args);
+        fn(m_settings);
+    }
 };
 
 extern ArgsManager gArgs;