Merge bitcoin#27236: util: fix argsman dupe key error

8fcbdad util: fix argsman dupe key error (willcl-ark)

Pull request description:

  fixes bitcoin#22638

  Make GUI "Settings file could not be read. Do you want to reset settings to default values?" dialog actually clear all settings instead of partially keeping them when `settings.json` contains duplicate keys. This change has no effect on `bitcoind` because it treats a corrupt `settings.json` file as a hard error and doesn't attempt to modify it.

  If we find a duplicate key and error, clear `values` before returning so that `WriteSettings()` will write an empty file, therefore clearing it.

  This aligns with GUI behaviour added in 1ee6d0b.

  The test added only checks that `values` is empty after a duplicate key is detected. This paves the way for the `abort` option in the GUI to properly clear `settings.json`, if the user selects the option, but the test does not currently check this entire mechanism (e.g. the file contents).

ACKs for top commit:
  TheCharlatan:
    Code review ACK 8fcbdad
  ryanofsky:
    Code review ACK 8fcbdad. Thanks for the fix! I would maybe update the PR description or add to it to describe behavior change of this PR:

Tree-SHA512: a5fd49b30ede0a24188623192825bccb952e427cc35f96ff9bfdc737361dcc35ac6480589ddf7f0ddeaebd34361bdaee31e7a91f2c0d857e4ff682614bb6bc04

src/test/settings_tests.cpp

@@ -80,14 +80,15 @@ BOOST_AUTO_TEST_CASE(ReadWrite)
     BOOST_CHECK(values.empty());
     BOOST_CHECK(errors.empty());
 
-    // Check duplicate keys not allowed
+    // Check duplicate keys not allowed and that values returns empty if a duplicate is found.
     WriteText(path, R"({
         "dupe": "string",
         "dupe": "dupe"
     })");
     BOOST_CHECK(!util::ReadSettings(path, values, errors));
     std::vector<std::string> dup_keys = {strprintf("Found duplicate key dupe in settings file %s", fs::PathToString(path))};
     BOOST_CHECK_EQUAL_COLLECTIONS(errors.begin(), errors.end(), dup_keys.begin(), dup_keys.end());
+    BOOST_CHECK(values.empty());
 
     // Check non-kv json files not allowed
     WriteText(path, R"("non-kv")");

src/util/settings.cpp

@@ -111,6 +111,8 @@ bool ReadSettings(const fs::path& path, std::map<std::string, SettingsValue>& va
         auto inserted = values.emplace(in_keys[i], in_values[i]);
         if (!inserted.second) {
             errors.emplace_back(strprintf("Found duplicate key %s in settings file %s", in_keys[i], fs::PathToString(path)));
+            values.clear();
+            break;
         }
     }
     return errors.empty();