Do not set path.data in environment if not set

When preparing the final settings in the environment, we unconditionally
set path.data even if path.data was not explicitly set. This confounds
detection for whether or not path.data was explicitly set, and this is
trappy. This commit adds logic to only set path.data in the final
settings if path.data was explicitly set, and provides a test case that
fails without this logic.

Relates #24132

core/src/main/java/org/elasticsearch/env/Environment.java

@@ -183,7 +183,9 @@ public Environment(Settings settings) {
 
         Settings.Builder finalSettings = Settings.builder().put(settings);
         finalSettings.put(PATH_HOME_SETTING.getKey(), homeFile);
-        finalSettings.putArray(PATH_DATA_SETTING.getKey(), dataPaths);
+        if (PATH_DATA_SETTING.exists(settings)) {
+            finalSettings.putArray(PATH_DATA_SETTING.getKey(), dataPaths);
+        }
         finalSettings.put(PATH_LOGS_SETTING.getKey(), logsFile);
         this.settings = finalSettings.build();
 

core/src/test/java/org/elasticsearch/env/EnvironmentTests.java

@@ -102,6 +102,19 @@ public void testPathDataWhenNotSet() {
         assertThat(environment.dataFiles(), equalTo(new Path[]{pathHome.resolve("data")}));
     }
 
+    public void testPathDataNotSetInEnvironmentIfNotSet() {
+        final Path defaultPathData = createTempDir().toAbsolutePath();
+        final Settings settings = Settings.builder()
+                .put("path.home", createTempDir().toAbsolutePath())
+                .put("default.path.data", defaultPathData)
+                .build();
+        assertFalse(Environment.PATH_DATA_SETTING.exists(settings));
+        assertTrue(Environment.DEFAULT_PATH_DATA_SETTING.exists(settings));
+        final Environment environment = new Environment(settings);
+        assertFalse(Environment.PATH_DATA_SETTING.exists(environment.settings()));
+        assertTrue(Environment.DEFAULT_PATH_DATA_SETTING.exists(environment.settings()));
+    }
+
     public void testDefaultPathLogs() {
         final Path defaultPathLogs = createTempDir().toAbsolutePath();
         final Settings settings = Settings.builder()