CORE: Validate Type for String Settings

* Special handling for `String` in the generic
Setting class because we parse raw values
String values for all types:
  * If we parse out a `String` setting validate
that it came from a scalar raw value
* Closes elastic#33135

Author: original-brownbear

server/src/main/java/org/elasticsearch/common/settings/Setting.java

@@ -19,6 +19,7 @@
 
 package org.elasticsearch.common.settings;
 
+import java.util.Collection;
 import org.apache.logging.log4j.Logger;
 import org.elasticsearch.ElasticsearchException;
 import org.elasticsearch.ElasticsearchParseException;
@@ -385,6 +386,16 @@ private T get(Settings settings, boolean validate) {
         try {
             T parsed = parser.apply(value);
             if (validate) {
+                if (parsed instanceof String) {
+                    Class<?> rawValueClass = settings.getRawType(getKey());
+                    if (rawValueClass != null
+                        && (Map.class.isAssignableFrom(rawValueClass) || Collection.class.isAssignableFrom(rawValueClass))
+                    ) {
+                        throw new IllegalArgumentException(
+                            "Failed to parse [" + value + "] for string setting [" + getKey() + "] because it is not scalar."
+                        );
+                    }
+                }
                 final Iterator<Setting<T>> it = validator.settings();
                 final Map<Setting<T>, T> map;
                 if (it.hasNext()) {

server/src/main/java/org/elasticsearch/common/settings/Settings.java

@@ -245,6 +245,11 @@ public String get(String setting, String defaultValue) {
         return retVal == null ? defaultValue : retVal;
     }
 
+    Class<?> getRawType(String setting) {
+        Object value = settings.get(setting);
+        return value == null ? null : value.getClass();
+    }
+
     /**
      * Returns the setting value (as float) associated with the setting key. If it does not exists,
      * returns the default value provided.

server/src/test/java/org/elasticsearch/common/settings/SettingTests.java

@@ -180,6 +180,13 @@ public void testSimpleUpdate() {
         }
     }
 
+    public void testValidateStringSetting() {
+        Settings settings = Settings.builder().putList("foo.bar", Arrays.asList("bla-a", "bla-b")).build();
+        Setting<String> stringSetting = Setting.simpleString("foo.bar", Property.NodeScope);
+        IllegalArgumentException e = expectThrows(IllegalArgumentException.class, () -> stringSetting.get(settings));
+        assertEquals("Failed to parse [[bla-a, bla-b]] for string setting [foo.bar] because it is not scalar.", e.getMessage());
+    }
+
     private static final Setting<String> FOO_BAR_SETTING = new Setting<>(
             "foo.bar",
             "foobar",