Fix windows empty line in logging capture (#52162)

This commit fixes another edge case in handling windows newlines in our
capture of stdout/stderr to log4j. The case is that the \r appears at
the beginning of the buffer when flushing, which would unintentionally
be emitted as an empty string. This commit skips the flush if only a \r
was found.

closes #51838

server/src/main/java/org/elasticsearch/common/logging/LoggingOutputStream.java

@@ -96,6 +96,10 @@ public void flush() {
             // windows case: remove the first part of newlines there too
             --used;
         }
+        if (used == 0) {
+            // only windows \r was in the buffer
+            return;
+        }
         log(new String(buffer.bytes, 0, used, StandardCharsets.UTF_8));
         if (buffer.bytes.length != DEFAULT_BUFFER_LENGTH) {
             threadLocal.set(new Buffer()); // reset size

server/src/test/java/org/elasticsearch/common/logging/LoggingOutputStreamTests.java

@@ -58,9 +58,15 @@ public void createStream() throws Exception {
         printStream = new PrintStream(loggingStream, false, StandardCharsets.UTF_8.name());
     }
 
-    @AwaitsFix(bugUrl = "https://github.com/elastic/elasticsearch/issues/51838")
-    public void testEmptyLine() {
-        printStream.println("");
+    public void testEmptyLineUnix() {
+        printStream.print("\n");
+        assertTrue(loggingStream.lines.isEmpty());
+        printStream.flush();
+        assertTrue(loggingStream.lines.isEmpty());
+    }
+
+    public void testEmptyLineWindows() {
+        printStream.print("\r\n");
         assertTrue(loggingStream.lines.isEmpty());
         printStream.flush();
         assertTrue(loggingStream.lines.isEmpty());
@@ -96,7 +102,6 @@ public void testBufferExtension() {
         assertThat(loggingStream.threadLocal.get().bytes.length, equalTo(DEFAULT_BUFFER_LENGTH));
     }
 
-    @AwaitsFix(bugUrl = "https://github.com/elastic/elasticsearch/issues/51838")
     public void testMaxBuffer() {
         String longStr = randomAlphaOfLength(MAX_BUFFER_LENGTH);
         String extraLongStr = longStr + "OVERFLOW";