move-only: Move settings to the common library

The background of this commit is an ongoing effort to decouple the
libbitcoinkernel library from code that is not strictly required by it.
The settings code belongs into the common library and namespace, since
the kernel library should not depend on it. See doc/design/libraries.md
for more information on this rationale.

Changing the namespace of the moved functions is scripted in the
following commit.

1 files changed, 115 insertions, 0 deletions

diff --git a/src/common/settings.h b/src/common/settings.h
new file mode 100644
index 0000000000..4cec85ea1e
--- /dev/null
+++ b/src/common/settings.h
@@ -0,0 +1,115 @@
+// Copyright (c) 2019-2022 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_COMMON_SETTINGS_H
+#define BITCOIN_COMMON_SETTINGS_H
+
+#include <util/fs.h>
+
+#include <cstddef>
+#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(),
+//!       getInt<int64_t>(), 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 settings. This struct combines settings from the command line, a
+//! read-only configuration file, and a read-write runtime settings 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 setting name to read-write file setting value.
+    std::map<std::string, SettingsValue> rw_settings;
+    //! 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;
+};
+
+//! Read settings file.
+bool ReadSettings(const fs::path& path,
+    std::map<std::string, SettingsValue>& values,
+    std::vector<std::string>& errors);
+
+//! Write settings file.
+bool WriteSettings(const fs::path& path,
+    const std::map<std::string, SettingsValue>& values,
+    std::vector<std::string>& errors);
+
+//! Get settings value from combined sources: forced settings, command line
+//! arguments, runtime read-write settings, 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 ignore_nonpersistent - ignore non-persistent settings values (forced
+//!                               settings values and values specified on the
+//!                               command line). Only return settings in the
+//!                               read-only config and read-write settings
+//!                               files.
+//! @param get_chain_type - enable special backwards compatible behavior
+//!                         for GetChainType
+SettingsValue GetSetting(const Settings& settings,
+    const std::string& section,
+    const std::string& name,
+    bool ignore_default_section_config,
+    bool ignore_nonpersistent,
+    bool get_chain_type);
+
+//! 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_COMMON_SETTINGS_H