Add util::Settings struct and helper functions.

Implement merging of settings from different sources (command line and config
file) separately from parsing code in system.cpp, so it is easier to add new
sources.

Document current inconsistent merging behavior without changing it.

This commit only adds new settings code without using it. The next commit calls
the new code to replace existing code in system.cpp.

Co-authored-by: John Newbery <john@johnnewbery.com>

1 files changed, 87 insertions, 0 deletions

diff --git a/src/util/settings.h b/src/util/settings.h
new file mode 100644
index 0000000000..17832e4d2c
--- /dev/null
+++ b/src/util/settings.h
@@ -0,0 +1,87 @@
+// Copyright (c) 2019 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_UTIL_SETTINGS_H
+#define BITCOIN_UTIL_SETTINGS_H
+
+#include <map>
+#include <string>
+#include <vector>
+
+class UniValue;
+
+namespace util {
+
+//! Settings value type (string/integer/boolean/null variant).
+//!
+//! @note UniValue is used here for convenience and because it can be easily
+//!       serialized in a readable format. But any other variant type that can
+//!       be assigned strings, int64_t, and bool values and has get_str(),
+//!       get_int64(), get_bool(), isNum(), isBool(), isFalse(), isTrue() and
+//!       isNull() methods can be substituted if there's a need to move away
+//!       from UniValue. (An implementation with boost::variant was posted at
+//!       https://github.com/bitcoin/bitcoin/pull/15934/files#r337691812)
+using SettingsValue = UniValue;
+
+//! Stored bitcoin settings. This struct combines settings from the command line
+//! and a read-only configuration file.
+struct Settings {
+    //! Map of setting name to forced setting value.
+    std::map<std::string, SettingsValue> forced_settings;
+    //! Map of setting name to list of command line values.
+    std::map<std::string, std::vector<SettingsValue>> command_line_options;
+    //! Map of config section name and setting name to list of config file values.
+    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.
+//!
+//! @param ignore_default_section_config - ignore values in the default section
+//!                                        of the config file (part before any
+//!                                        [section] keywords)
+//! @param get_chain_name - enable special backwards compatible behavior
+//!                         for GetChainName
+SettingsValue GetSetting(const Settings& settings, const std::string& section, const std::string& name, bool ignore_default_section_config, bool get_chain_name);
+
+//! Get combined setting value similar to GetSetting(), except if setting was
+//! specified multiple times, return a list of all the values specified.
+std::vector<SettingsValue> GetSettingsList(const Settings& settings, const std::string& section, const std::string& name, bool ignore_default_section_config);
+
+//! Return true if a setting is set in the default config file section, and not
+//! overridden by a higher priority command-line or network section value.
+//!
+//! This is used to provide user warnings about values that might be getting
+//! ignored unintentionally.
+bool OnlyHasDefaultSectionSetting(const Settings& settings, const std::string& section, const std::string& name);
+
+//! Accessor for list of settings that skips negated values when iterated over.
+//! The last boolean `false` value in the list and all earlier values are
+//! considered negated.
+struct SettingsSpan {
+    explicit SettingsSpan() = default;
+    explicit SettingsSpan(const SettingsValue& value) noexcept : SettingsSpan(&value, 1) {}
+    explicit SettingsSpan(const SettingsValue* data, size_t size) noexcept : data(data), size(size) {}
+    explicit SettingsSpan(const std::vector<SettingsValue>& vec) noexcept;
+    const SettingsValue* begin() const; //<! Pointer to first non-negated value.
+    const SettingsValue* end() const;   //<! Pointer to end of values.
+    bool empty() const;                 //<! True if there are any non-negated values.
+    bool last_negated() const;          //<! True if the last value is negated.
+    size_t negated() const;             //<! Number of negated values.
+
+    const SettingsValue* data = nullptr;
+    size_t size = 0;
+};
+
+//! Map lookup helper.
+template <typename Map, typename Key>
+auto FindKey(Map&& map, Key&& key) -> decltype(&map.at(key))
+{
+    auto it = map.find(key);
+    return it == map.end() ? nullptr : &it->second;
+}
+
+} // namespace util
+
+#endif // BITCOIN_UTIL_SETTINGS_H