Defensively guard against uninitialized startedRelativeTimeInNanos

Author: PeteGillinElastic

server/src/main/java/org/elasticsearch/index/shard/IndexShard.java

@@ -298,7 +298,7 @@ public class IndexShard extends AbstractIndexShardComponent implements IndicesCl
     private final RefreshFieldHasValueListener refreshFieldHasValueListener;
     private volatile boolean useRetentionLeasesInPeerRecovery;
     private final LongSupplier relativeTimeInNanosSupplier;
-    private volatile long startedRelativeTimeInNanos;
+    private volatile long startedRelativeTimeInNanos = -1L; // use -1 to indicate this has not yet been set to its true value
     private volatile long indexingTimeBeforeShardStartedInNanos;
     private volatile double recentIndexingLoadAtShardStarted;
     private final SubscribableListener<Void> waitForEngineOrClosedShardListeners = new SubscribableListener<>();
@@ -557,7 +557,10 @@ public void updateShardState(
                     : "a primary relocation is completed by the master, but primary mode is not active " + currentRouting;
 
                 changeState(IndexShardState.STARTED, "global state is [" + newRouting.state() + "]");
-                startedRelativeTimeInNanos = getRelativeTimeInNanos();
+                long relativeTimeInNanos = getRelativeTimeInNanos();
+                // We use -1 to indicate that startedRelativeTimeInNanos has yet not been set to its true value. So in the vanishingly
+                // unlikely case that getRelativeTimeInNanos() returns exactly -1, we advance by 1ns to avoid that special value.
+                startedRelativeTimeInNanos = (relativeTimeInNanos != -1L) ? relativeTimeInNanos : 0L;
                 indexingTimeBeforeShardStartedInNanos = internalIndexingStats.totalIndexingTimeInNanos();
                 recentIndexingLoadAtShardStarted = internalIndexingStats.recentIndexingLoad(startedRelativeTimeInNanos);
             } else if (currentRouting.primary()
@@ -1370,11 +1373,14 @@ public IndexingStats indexingStats() {
         }
 
         long currentTimeInNanos = getRelativeTimeInNanos();
+        // We use -1 to indicate that startedRelativeTimeInNanos has yet not been set to its true value, i.e the shard has not started.
+        // In that case, we set timeSinceShardStartedInNanos to zero (which will result in all load metrics definitely being zero).
+        long timeSinceShardStartedInNanos = (startedRelativeTimeInNanos != -1L) ? (currentTimeInNanos - startedRelativeTimeInNanos) : 0L;
         return internalIndexingStats.stats(
             throttled,
             throttleTimeInMillis,
             indexingTimeBeforeShardStartedInNanos,
-            currentTimeInNanos - startedRelativeTimeInNanos,
+            timeSinceShardStartedInNanos,
             currentTimeInNanos,
             recentIndexingLoadAtShardStarted
         );