Don't rely on test code execution time span.

Current implementation of [`RemoteSegmentTransferTrackerTests.testComputeTimeLagOnUpdate()`](https://github.com/opensearch-project/OpenSearch/blob/2b17902643738f0d2a75ade7c85cbca94d18ce49/server/src/test/java/org/opensearch/index/remote/RemoteSegmentTransferTrackerTests.java#L139) test rely on some assumptions about how fast the testing code will finish in JVM. Moreover it does not precisely control boundaries of the time span, specifically the start of the span because it is determined by internal implementation of [`RemoteSegmentTransferTracker.getTimeMsLag()`](https://github.com/opensearch-project/OpenSearch/blob/2b17902643738f0d2a75ade7c85cbca94d18ce49/server/src/main/java/org/opensearch/index/remote/RemoteSegmentTransferTracker.java#L262) which indirectly makes call to `System.nanoTime()`.

This commit loosens the assumption that the test code execution will finish within +/-20ms. Instead it only assumes that the execution time span won't be shorter than predefined (and controlled) thread sleep interval and any larger interval value is considered a success.

The whole point of this test is not to verify execution speed with defined precision. Instead the point is that the [`getTimeMsLag()`](https://github.com/opensearch-project/OpenSearch/blob/2b17902643738f0d2a75ade7c85cbca94d18ce49/server/src/main/java/org/opensearch/index/remote/RemoteSegmentTransferTracker.java#L262) method returns either 0 (for specific conditions) or possitive number (assuming that `remoteRefreshStartTimeMs` is not greater than `System.nanoTime()`).

Closes: opensearch-project#14325

Signed-off-by: Lukáš Vlček <[email protected]>

Author: lukas-vlcek

server/src/main/java/org/opensearch/index/remote/RemoteSegmentTransferTracker.java

@@ -65,7 +65,7 @@ public class RemoteSegmentTransferTracker extends RemoteTransferTracker {
     private volatile long remoteRefreshSeqNo;
 
     /**
-     * The refresh time of most recent remote refresh.
+     * The refresh time of the most recent remote refresh.
      */
     private volatile long remoteRefreshTimeMs;
 
@@ -76,7 +76,7 @@ public class RemoteSegmentTransferTracker extends RemoteTransferTracker {
     private volatile long remoteRefreshStartTimeMs = -1;
 
     /**
-     * The refresh time(clock) of most recent remote refresh.
+     * The refresh time(clock) of the most recent remote refresh.
      */
     private volatile long remoteRefreshClockTimeMs;
 

server/src/test/java/org/opensearch/index/remote/RemoteSegmentTransferTrackerTests.java

@@ -152,15 +152,16 @@ public void testComputeTimeLagOnUpdate() throws InterruptedException {
         transferTracker.updateLocalRefreshTimeMs(currentTimeMsUsingSystemNanos());
 
         transferTracker.updateLatestLocalFileNameLengthMap(List.of("test"), k -> 1L);
-        // Sleep for 100ms and then the lag should be within 100ms +/- 20ms
-        Thread.sleep(100);
-        assertTrue(Math.abs(transferTracker.getTimeMsLag() - 100) <= 20);
+        // Sleep for 100ms and then the lag should not be shorter
+        long span = 100;
+        Thread.sleep(span);
+        assertTrue("Lag is not expected to be shorter than span [" + span + "ms]", transferTracker.getTimeMsLag() >= span);
 
         transferTracker.updateRemoteRefreshTimeMs(transferTracker.getLocalRefreshTimeMs());
         transferTracker.updateLocalRefreshTimeMs(currentTimeMsUsingSystemNanos());
-        long random = randomIntBetween(50, 200);
-        Thread.sleep(random);
-        assertTrue(Math.abs(transferTracker.getTimeMsLag() - random) <= 20);
+        long randomSpan = randomIntBetween(50, 200);
+        Thread.sleep(randomSpan);
+        assertTrue("Lag is not expected to be shorter than span [" + randomSpan + "ms]", transferTracker.getTimeMsLag() >= randomSpan);
     }
 
     public void testAddUploadBytesStarted() {