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

Persistent settings are used in followup PRs #15936 to unify gui settings
between bitcoin-qt and bitcoind, and #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.

test/functional/feature_settings.py

@@ -0,0 +1,84 @@
+#!/usr/bin/env python3
+# Copyright (c) 2017-2020 The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+"""Test various command line arguments and configuration file parameters."""
+
+import json
+
+from pathlib import Path
+
+from test_framework.test_framework import BitcoinTestFramework
+from test_framework.test_node import ErrorMatch
+from test_framework.util import assert_equal
+
+
+class SettingsTest(BitcoinTestFramework):
+    def set_test_params(self):
+        self.setup_clean_chain = True
+        self.num_nodes = 1
+
+    def run_test(self):
+        node, = self.nodes
+        settings = Path(node.datadir, self.chain, "settings.json")
+        conf = Path(node.datadir, "bitcoin.conf")
+
+        # Assert empty settings file was created
+        self.stop_node(0)
+        with settings.open() as fp:
+            assert_equal(json.load(fp), {})
+
+        # Assert settings are parsed and logged
+        with settings.open("w") as fp:
+            json.dump({"string": "string", "num": 5, "bool": True, "null": None, "list": [6,7]}, fp)
+        with node.assert_debug_log(expected_msgs=[
+                'Setting file arg: string = "string"',
+                'Setting file arg: num = 5',
+                'Setting file arg: bool = true',
+                'Setting file arg: null = null',
+                'Setting file arg: list = [6,7]']):
+            self.start_node(0)
+            self.stop_node(0)
+
+        # Assert settings are unchanged after shutdown
+        with settings.open() as fp:
+            assert_equal(json.load(fp), {"string": "string", "num": 5, "bool": True, "null": None, "list": [6,7]})
+
+        # Test invalid json
+        with settings.open("w") as fp:
+            fp.write("invalid json")
+        node.assert_start_raises_init_error(expected_msg='Unable to parse settings file', match=ErrorMatch.PARTIAL_REGEX)
+
+        # Test invalid json object
+        with settings.open("w") as fp:
+            fp.write('"string"')
+        node.assert_start_raises_init_error(expected_msg='Found non-object value "string" in settings file', match=ErrorMatch.PARTIAL_REGEX)
+
+        # Test invalid settings file containing duplicate keys
+        with settings.open("w") as fp:
+            fp.write('{"key": 1, "key": 2}')
+        node.assert_start_raises_init_error(expected_msg='Found duplicate key key in settings file', match=ErrorMatch.PARTIAL_REGEX)
+
+        # Test invalid settings file is ignored with command line -nosettings
+        with node.assert_debug_log(expected_msgs=['Command-line arg: settings=false']):
+            self.start_node(0, extra_args=["-nosettings"])
+            self.stop_node(0)
+
+        # Test invalid settings file is ignored with config file -nosettings
+        with conf.open('a') as conf:
+            conf.write('nosettings=1\n')
+        with node.assert_debug_log(expected_msgs=['Config file arg: [regtest] settings=false']):
+            self.start_node(0)
+            self.stop_node(0)
+
+        # Test alternate settings path
+        altsettings = Path(node.datadir, "altsettings.json")
+        with altsettings.open("w") as fp:
+            fp.write('{"key": "value"}')
+        with node.assert_debug_log(expected_msgs=['Setting file arg: key = "value"']):
+            self.start_node(0, extra_args=["-settings={}".format(altsettings)])
+            self.stop_node(0)
+
+
+if __name__ == '__main__':
+    SettingsTest().main()