DeadLetterQueueUtils#extractSegmentId improvement: replace split with index of and substring methods. (#18874)

* DeadLetterQueueUtils: avoid using heavy split operation, replace it with simple index of and substring methods.

* Handle exception case if file name doesn't end with .log. Add unit test cases.

(cherry picked from commit 8d7a5a3)

Author: mashhurs

logstash-core/src/main/java/org/logstash/common/io/DeadLetterQueueUtils.java

@@ -37,8 +37,13 @@ class DeadLetterQueueUtils {
 
     private static final Logger logger = LogManager.getLogger(DeadLetterQueueUtils.class);
 
-    static int extractSegmentId(Path p) {
-        return Integer.parseInt(p.getFileName().toString().split("\\.log")[0]);
+    static int extractSegmentId(final Path p) {
+        final String fileName = p.getFileName().toString();
+        final int dotIndex = fileName.indexOf(".log");
+        if (dotIndex <= 0) {
+            throw new IllegalArgumentException("Invalid segment file name: " + fileName);
+        }
+        return Integer.parseInt(fileName.substring(0, dotIndex));
     }
 
     static Stream<Path> listFiles(Path path, String suffix) throws IOException {

logstash-core/src/test/java/org/logstash/common/io/DeadLetterQueueReaderTest.java

@@ -1124,4 +1124,26 @@ public void testReaderLockProhibitMultipleInstances() throws IOException {
             }
         }
     }
+
+    @Test
+    public void testExtractSegmentIdWithValidFileName() {
+        Path validPath = Paths.get("123.log");
+        assertEquals(123, DeadLetterQueueUtils.extractSegmentId(validPath));
+
+        Path singleDigitPath = Paths.get("1.log");
+        assertEquals(1, DeadLetterQueueUtils.extractSegmentId(singleDigitPath));
+
+        Path largeNumberPath = Paths.get("999999.log");
+        assertEquals(999999, DeadLetterQueueUtils.extractSegmentId(largeNumberPath));
+    }
+
+    @Test
+    public void testExtractSegmentIdWithNoLogExtensionThrowsException() {
+        Path noExtensionPath = Paths.get("123.txt");
+        IllegalArgumentException exception = Assert.assertThrows(
+                IllegalArgumentException.class,
+                () -> DeadLetterQueueUtils.extractSegmentId(noExtensionPath)
+        );
+        assertThat(exception.getMessage(), containsString("Invalid segment file name"));
+    }
 }