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, 169 insertions, 0 deletions

diff --git a/src/util/settings.cpp b/src/util/settings.cpp
new file mode 100644
index 0000000000..af75fef310
--- /dev/null
+++ b/src/util/settings.cpp
@@ -0,0 +1,169 @@
+// 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.
+
+#include <util/settings.h>
+
+#include <univalue.h>
+
+namespace util {
+namespace {
+
+enum class Source {
+   FORCED,
+   COMMAND_LINE,
+   CONFIG_FILE_NETWORK_SECTION,
+   CONFIG_FILE_DEFAULT_SECTION
+};
+
+//! Merge settings from multiple sources in precedence order:
+//! Forced config > command line > config file network-specific section > config file default section
+//!
+//! This function is provided with a callback function fn that contains
+//! specific logic for how to merge the sources.
+template <typename Fn>
+static void MergeSettings(const Settings& settings, const std::string& section, const std::string& name, Fn&& fn)
+{
+    // Merge in the forced settings
+    if (auto* value = FindKey(settings.forced_settings, name)) {
+        fn(SettingsSpan(*value), Source::FORCED);
+    }
+    // Merge in the command-line options
+    if (auto* values = FindKey(settings.command_line_options, name)) {
+        fn(SettingsSpan(*values), Source::COMMAND_LINE);
+    }
+    // Merge in the network-specific section of the config file
+    if (!section.empty()) {
+        if (auto* map = FindKey(settings.ro_config, section)) {
+            if (auto* values = FindKey(*map, name)) {
+                fn(SettingsSpan(*values), Source::CONFIG_FILE_NETWORK_SECTION);
+            }
+        }
+    }
+    // Merge in the default section of the config file
+    if (auto* map = FindKey(settings.ro_config, "")) {
+        if (auto* values = FindKey(*map, name)) {
+            fn(SettingsSpan(*values), Source::CONFIG_FILE_DEFAULT_SECTION);
+        }
+    }
+}
+} // namespace
+
+SettingsValue GetSetting(const Settings& settings,
+    const std::string& section,
+    const std::string& name,
+    bool ignore_default_section_config,
+    bool get_chain_name)
+{
+    SettingsValue result;
+    MergeSettings(settings, section, name, [&](SettingsSpan span, Source source) {
+        // Weird behavior preserved for backwards compatibility: Apply negated
+        // setting even if non-negated setting would be ignored. A negated
+        // value in the default section is applied to network specific options,
+        // even though normal non-negated values there would be ignored.
+        const bool never_ignore_negated_setting = span.last_negated();
+
+        // Weird behavior preserved for backwards compatibility: Take first
+        // assigned value instead of last. In general, later settings take
+        // precedence over early settings, but for backwards compatibility in
+        // the config file the precedence is reversed for all settings except
+        // chain name settings.
+        const bool reverse_precedence = (source == Source::CONFIG_FILE_NETWORK_SECTION || source == Source::CONFIG_FILE_DEFAULT_SECTION) && !get_chain_name;
+
+        // Weird behavior preserved for backwards compatibility: Negated
+        // -regtest and -testnet arguments which you would expect to override
+        // values set in the configuration file are currently accepted but
+        // silently ignored. It would be better to apply these just like other
+        // negated values, or at least warn they are ignored.
+        const bool skip_negated_command_line = get_chain_name;
+
+        // Ignore settings in default config section if requested.
+        if (ignore_default_section_config && source == Source::CONFIG_FILE_DEFAULT_SECTION && !never_ignore_negated_setting) return;
+
+        // Skip negated command line settings.
+        if (skip_negated_command_line && span.last_negated()) return;
+
+        // Stick with highest priority value, keeping result if already set.
+        if (!result.isNull()) return;
+
+        if (!span.empty()) {
+            result = reverse_precedence ? span.begin()[0] : span.end()[-1];
+        } else if (span.last_negated()) {
+            result = false;
+        }
+    });
+    return result;
+}
+
+std::vector<SettingsValue> GetSettingsList(const Settings& settings,
+    const std::string& section,
+    const std::string& name,
+    bool ignore_default_section_config)
+{
+    std::vector<SettingsValue> result;
+    bool result_complete = false;
+    bool prev_negated_empty = false;
+    MergeSettings(settings, section, name, [&](SettingsSpan span, Source source) {
+        // Weird behavior preserved for backwards compatibility: Apply config
+        // file settings even if negated on command line. Negating a setting on
+        // command line will ignore earlier settings on the command line and
+        // ignore settings in the config file, unless the negated command line
+        // value is followed by non-negated value, in which case config file
+        // settings will be brought back from the dead (but earlier command
+        // line settings will still be ignored).
+        const bool add_zombie_config_values = (source == Source::CONFIG_FILE_NETWORK_SECTION || source == Source::CONFIG_FILE_DEFAULT_SECTION) && !prev_negated_empty;
+
+        // Ignore settings in default config section if requested.
+        if (ignore_default_section_config && source == Source::CONFIG_FILE_DEFAULT_SECTION) return;
+
+        // Add new settings to the result if isn't already complete, or if the
+        // values are zombies.
+        if (!result_complete || add_zombie_config_values) {
+            for (const auto& value : span) {
+                if (value.isArray()) {
+                    result.insert(result.end(), value.getValues().begin(), value.getValues().end());
+                } else {
+                    result.push_back(value);
+                }
+            }
+        }
+
+        // If a setting was negated, or if a setting was forced, set
+        // result_complete to true to ignore any later lower priority settings.
+        result_complete |= span.negated() > 0 || source == Source::FORCED;
+
+        // Update the negated and empty state used for the zombie values check.
+        prev_negated_empty |= span.last_negated() && result.empty();
+    });
+    return result;
+}
+
+bool OnlyHasDefaultSectionSetting(const Settings& settings, const std::string& section, const std::string& name)
+{
+    bool has_default_section_setting = false;
+    bool has_other_setting = false;
+    MergeSettings(settings, section, name, [&](SettingsSpan span, Source source) {
+        if (span.empty()) return;
+        else if (source == Source::CONFIG_FILE_DEFAULT_SECTION) has_default_section_setting = true;
+        else has_other_setting = true;
+    });
+    // If a value is set in the default section and not explicitly overwritten by the
+    // user on the command line or in a different section, then we want to enable
+    // warnings about the value being ignored.
+    return has_default_section_setting && !has_other_setting;
+}
+
+SettingsSpan::SettingsSpan(const std::vector<SettingsValue>& vec) noexcept : SettingsSpan(vec.data(), vec.size()) {}
+const SettingsValue* SettingsSpan::begin() const { return data + negated(); }
+const SettingsValue* SettingsSpan::end() const { return data + size; }
+bool SettingsSpan::empty() const { return size == 0 || last_negated(); }
+bool SettingsSpan::last_negated() const { return size > 0 && data[size - 1].isFalse(); }
+size_t SettingsSpan::negated() const
+{
+    for (size_t i = size; i > 0; --i) {
+        if (data[i - 1].isFalse()) return i; // Return number of negated values (position of last false value)
+    }
+    return 0;
+}
+
+} // namespace util