Fix TranslogTests#testStats (#85828) (#85857)

Today these tests assume that the last-modified time on the translog
file is at least one millisecond before the time at which stats are
computed, but this isn't always true. With this commit we add a
(bounded) wait for the clock to advance beyond the file's last-modified
time to ensure that we observe a nonzero age.

Closes #85717

server/src/test/java/org/elasticsearch/index/translog/TranslogTests.java

@@ -446,17 +446,23 @@ public void testFindEarliestLastModifiedAge() throws IOException {
         assertThat(Translog.findEarliestLastModifiedAge(fixedTime, readers, w), equalTo(LongStream.of(periods).max().orElse(0L)));
     }
 
-    public void testStats() throws IOException {
+    private void waitForPositiveAge() throws Exception {
+        final var lastModifiedTime = translog.getCurrent().getLastModifiedTime();
+        assertBusy(() -> assertThat(System.currentTimeMillis(), greaterThan(lastModifiedTime)));
+    }
+
+    public void testStats() throws Exception {
         // self control cleaning for test
         final long firstOperationPosition = translog.getFirstOperationPosition();
         {
             final TranslogStats stats = stats();
             assertThat(stats.estimatedNumberOfOperations(), equalTo(0));
         }
         assertThat((int) firstOperationPosition, greaterThan(CodecUtil.headerLength(TranslogHeader.TRANSLOG_CODEC)));
-        translog.add(new Translog.Index("1", 0, primaryTerm.get(), new byte[] { 1 }));
 
+        translog.add(new Translog.Index("1", 0, primaryTerm.get(), new byte[] { 1 }));
         {
+            waitForPositiveAge();
             final TranslogStats stats = stats();
             assertThat(stats.estimatedNumberOfOperations(), equalTo(1));
             assertThat(stats.getTranslogSizeInBytes(), equalTo(157L));
@@ -467,6 +473,7 @@ public void testStats() throws IOException {
 
         translog.add(new Translog.Delete("2", 1, primaryTerm.get()));
         {
+            waitForPositiveAge();
             final TranslogStats stats = stats();
             assertThat(stats.estimatedNumberOfOperations(), equalTo(2));
             assertThat(stats.getTranslogSizeInBytes(), equalTo(193L));
@@ -477,6 +484,7 @@ public void testStats() throws IOException {
 
         translog.add(new Translog.Delete("3", 2, primaryTerm.get()));
         {
+            waitForPositiveAge();
             final TranslogStats stats = stats();
             assertThat(stats.estimatedNumberOfOperations(), equalTo(3));
             assertThat(stats.getTranslogSizeInBytes(), equalTo(229L));
@@ -487,6 +495,7 @@ public void testStats() throws IOException {
 
         translog.add(new Translog.NoOp(3, 1, randomAlphaOfLength(16)));
         {
+            waitForPositiveAge();
             final TranslogStats stats = stats();
             assertThat(stats.estimatedNumberOfOperations(), equalTo(4));
             assertThat(stats.getTranslogSizeInBytes(), equalTo(271L));
@@ -497,6 +506,7 @@ public void testStats() throws IOException {
 
         translog.rollGeneration();
         {
+            waitForPositiveAge();
             final TranslogStats stats = stats();
             assertThat(stats.estimatedNumberOfOperations(), equalTo(4));
             assertThat(stats.getTranslogSizeInBytes(), equalTo(326L));
@@ -532,13 +542,13 @@ public void testStats() throws IOException {
         translog.getDeletionPolicy().setLocalCheckpointOfSafeCommit(randomLongBetween(3, Long.MAX_VALUE));
         translog.trimUnreferencedReaders();
         {
-            long lastModifiedAge = System.currentTimeMillis() - translog.getCurrent().getLastModifiedTime();
+            waitForPositiveAge();
             final TranslogStats stats = stats();
             assertThat(stats.estimatedNumberOfOperations(), equalTo(0));
             assertThat(stats.getTranslogSizeInBytes(), equalTo(firstOperationPosition));
             assertThat(stats.getUncommittedOperations(), equalTo(0));
             assertThat(stats.getUncommittedSizeInBytes(), equalTo(firstOperationPosition));
-            assertThat(stats.getEarliestLastModifiedAge(), greaterThanOrEqualTo(lastModifiedAge));
+            assertThat(stats.getEarliestLastModifiedAge(), greaterThan(0L));
         }
     }