Fix path.data as comma separated string (#76202) (#79272)

For multiple data paths, there are several allowed varieties of
specifcying the paths. The yaml list forms, both single and multi line,
are allowed, but another form is a string containing commas that is
parsed as a list. This latter form was broken recently by the
refactoring of path.data parsing to emit deprecation warnings for MDP.
This commit fixes the comma separated string case and adds a test.

closes #76181

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

@@ -144,7 +144,13 @@ public Environment(final Settings settings, final Path configPath) {
 
         final Settings.Builder finalSettings = Settings.builder().put(settings);
         if (PATH_DATA_SETTING.exists(settings)) {
-            finalSettings.putList(PATH_DATA_SETTING.getKey(), Arrays.stream(dataFiles).map(Path::toString).collect(Collectors.toList()));
+            if (dataPathUsesList(settings)) {
+                finalSettings.putList(PATH_DATA_SETTING.getKey(),
+                    Arrays.stream(dataFiles).map(Path::toString).collect(Collectors.toList()));
+            } else {
+                assert dataFiles.length == 1;
+                finalSettings.put(PATH_DATA_SETTING.getKey(), dataFiles[0]);
+            }
         }
         finalSettings.put(PATH_HOME_SETTING.getKey(), homeFile);
         finalSettings.put(PATH_LOGS_SETTING.getKey(), logsFile.toString());
@@ -303,7 +309,7 @@ public static boolean dataPathUsesList(Settings settings) {
             return false;
         }
         String rawDataPath = settings.get(PATH_DATA_SETTING.getKey());
-        return rawDataPath.startsWith("[");
+        return rawDataPath.startsWith("[") || rawDataPath.contains(",");
     }
 
     public static FileStore getFileStore(final Path path) throws IOException {

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

@@ -21,6 +21,7 @@
 import static org.hamcrest.CoreMatchers.notNullValue;
 import static org.hamcrest.CoreMatchers.nullValue;
 import static org.hamcrest.CoreMatchers.startsWith;
+import static org.hamcrest.Matchers.arrayWithSize;
 import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.hasSize;
 import static org.hamcrest.Matchers.is;
@@ -72,6 +73,15 @@ public void testPathDataNotSetInEnvironmentIfNotSet() {
         assertFalse(Environment.PATH_DATA_SETTING.exists(environment.settings()));
     }
 
+    public void testPathDataLegacyCommaList() {
+        final Settings settings = Settings.builder()
+            .put("path.home", createTempDir().toAbsolutePath())
+            .put("path.data", createTempDir().toAbsolutePath() + "," + createTempDir().toAbsolutePath())
+            .build();
+        final Environment environment = new Environment(settings, null);
+        assertThat(environment.dataFiles(), arrayWithSize(2));
+    }
+
     public void testPathLogsWhenNotSet() {
         final Path pathHome = createTempDir().toAbsolutePath();
         final Settings settings = Settings.builder().put("path.home", pathHome).build();
@@ -173,6 +183,11 @@ public void testSingleDataPathListCheck() {
                 .putList(Environment.PATH_DATA_SETTING.getKey(), createTempDir().toString()).build();
             assertThat(Environment.dataPathUsesList(settings), is(true));
         }
+        {
+            final Settings settings = Settings.builder()
+                .put(Environment.PATH_DATA_SETTING.getKey(), createTempDir().toString() + "," + createTempDir().toString()).build();
+            assertThat(Environment.dataPathUsesList(settings), is(true));
+        }
     }
 
     private void assertPath(final String actual, final Path expected) {