Merge pull request #17108 from s1monw/issues/17019

Try to renew sync ID if `flush=true` on forceMerge

Today we do a force flush which wipes the sync ID if there is one which
can cause the lost of all benefits of the sync ID ie. fast recovery.
This commit adds a check to renew the sync ID if possible. The flush call
is now also not forced since the IW will show pending changes if the forceMerge added new segments.
if we keep using force we will wipe the sync ID even if no renew was actually needed.

Closes #17019

core/src/main/java/org/elasticsearch/index/engine/InternalEngine.java

@@ -743,7 +743,9 @@ public class InternalEngine extends Engine {
                     indexWriter.forceMerge(maxNumSegments, true /* blocks and waits for merges*/);
                 }
                 if (flush) {
-                    flush(true, true);
+                    if (tryRenewSyncCommit() == false) {
+                        flush(false, true);
+                    }
                 }
                 if (upgrade) {
                     logger.info("finished segment upgrade");

core/src/test/java/org/elasticsearch/index/engine/InternalEngineTests.java

@@ -485,7 +485,7 @@ public class InternalEngineTests extends ESTestCase {
 
             if (flush) {
                 // we should have had just 1 merge, so last generation should be exact
-                assertEquals(gen2 + 1, store.readLastCommittedSegmentsInfo().getLastGeneration());
+                assertEquals(gen2, store.readLastCommittedSegmentsInfo().getLastGeneration());
             }
         }
     }
@@ -843,7 +843,7 @@ public class InternalEngineTests extends ESTestCase {
                         Engine.SyncedFlushResult.SUCCESS);
                 assertEquals(3, engine.segments(false).size());
 
-                engine.forceMerge(false, 1, false, false, false);
+                engine.forceMerge(forceMergeFlushes, 1, false, false, false);
                 if (forceMergeFlushes == false) {
                     engine.refresh("make all segments visible");
                     assertEquals(4, engine.segments(false).size());