Backport PR#14605 to 7.17: Fix DLQ fails to start due to read 1 byte file (#14752)

Note this is a manual cherry-pick backport, as it did not backport cleanly. This backport includes some changed/additional code to
the original PR:

* Added additional null check in seekToNextEvent that was previously not present in 7.17, but is required for this PR
* Filter is added, but surrounding code is slightly different, but the intent is the same.

**Backport PR #14605 to 8.5 branch, original message:**

---

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Fix DLQ fails to start due to read 1 byte file

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  being an environment variable to a proper yaml config.

  Additionally, this PR exposes this setting for both xpack monitoring & management to the logstash.yml file.
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This commit ignores DLQ files that contain only the version number. These files have no content and should be skipped.

Mapping 1 byte DLQ files to buffer causes java.lang.IllegalArgumentException: newPosition < 0: (-1 < 0)
User is unable to start the pipeline using dead_letter_queue input

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  This PR fixes an issue that was preventing the docker image from using the proxy setting when sending xpack monitoring information.
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  This PR now allows the user to define the xpack monitoring proxy setting in the docker container.
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- [ ] I have added tests that prove my fix is effective or that my feature works

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- [x] manually test the 1 byte file and can start Logstash

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follow the reproducer of #14599

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- Fixed: #14599

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

@@ -78,7 +78,9 @@ public final class DeadLetterQueueReader implements Closeable {
             return id.apply(p1).compareTo(id.apply(p2));
         });
 
-        segments.addAll(getSegmentPaths(queuePath).collect(Collectors.toList()));
+        segments.addAll(getSegmentPaths(queuePath)
+                .filter(p -> p.toFile().length() > 1) // take the files that have content to process
+                .collect(Collectors.toList()));
     }
 
     public void seekToNextEvent(Timestamp timestamp) throws IOException {
@@ -95,9 +97,11 @@ public final class DeadLetterQueueReader implements Closeable {
                 return;
             }
         }
+        if (currentReader != null) {
             currentReader.close();
             currentReader = null;
         }
+    }
 
     private long pollNewSegments(long timeout) throws IOException, InterruptedException {
         long startTime = System.currentTimeMillis();

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

@@ -34,6 +34,7 @@ import java.io.IOException;
 import java.net.URISyntaxException;
 import java.net.URL;
 import java.nio.charset.StandardCharsets;
+import java.nio.file.Files;
 import java.nio.file.Path;
 import java.nio.file.Paths;
 import java.time.Duration;
@@ -51,6 +52,7 @@ import static org.hamcrest.MatcherAssert.assertThat;
 import static org.hamcrest.Matchers.containsString;
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertNull;
 import static org.logstash.common.io.RecordIOWriter.BLOCK_SIZE;
 import static org.logstash.common.io.RecordIOWriter.RECORD_HEADER_SIZE;
 import static org.logstash.common.io.RecordIOWriter.VERSION_SIZE;
@@ -420,6 +422,22 @@ public class DeadLetterQueueReaderTest {
         }
     }
 
+    @Test
+    public void testSeekByTimestampWhenSegmentIs1Byte() throws IOException, InterruptedException {
+        final long startTime = System.currentTimeMillis();
+        Files.write(dir.resolve("1.log"), "1".getBytes());
+
+        try (DeadLetterQueueReader reader = new DeadLetterQueueReader(dir)) {
+
+            //Exercise
+            final Timestamp seekTarget = new Timestamp(startTime);
+            reader.seekToNextEvent(seekTarget);
+
+            // Verify, no entry is available, reader should seek without exception
+            DLQEntry readEntry = reader.pollEntry(100);
+            assertNull("No entry is available after all segments are deleted", readEntry);
+        }
+    }
     @Test
     public void testWriteReadRandomEventSize() throws Exception {
         Event event = new Event(Collections.emptyMap());