Fix testKeepTranslogAfterGlobalCheckpoint (#55868)

If we advance the global checkpoint during commit and sync that
checkpoint after commit, then the assertions in the test won't hold
because the deletion policy did not see the latest global checkpoint
but only the value before committing.

Closes #55680

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

@@ -4886,9 +4886,11 @@ public void testKeepTranslogAfterGlobalCheckpoint() throws Exception {
 
         final EngineConfig engineConfig = config(indexSettings, store, translogPath,
             NoMergePolicy.INSTANCE, null, null, () -> globalCheckpoint.get());
+        final AtomicLong lastSyncedGlobalCheckpointBeforeCommit = new AtomicLong(Translog.readGlobalCheckpoint(translogPath, translogUUID));
         try (InternalEngine engine = new InternalEngine(engineConfig) {
                 @Override
                 protected void commitIndexWriter(IndexWriter writer, Translog translog, String syncId) throws IOException {
+                    lastSyncedGlobalCheckpointBeforeCommit.set(Translog.readGlobalCheckpoint(translogPath, translogUUID));
                     // Advance the global checkpoint during the flush to create a lag between a persisted global checkpoint in the translog
                     // (this value is visible to the deletion policy) and an in memory global checkpoint in the SequenceNumbersService.
                     if (rarely()) {
@@ -4909,21 +4911,20 @@ protected void commitIndexWriter(IndexWriter writer, Translog translog, String s
                 }
                 if (frequently()) {
                     engine.flush(randomBoolean(), true);
-                    final long lastSyncedGlobalCheckpoint = Translog.readGlobalCheckpoint(translogPath, translogUUID);
                     final List<IndexCommit> commits = DirectoryReader.listCommits(store.directory());
                     // Keep only one safe commit as the oldest commit.
                     final IndexCommit safeCommit = commits.get(0);
-                    if (lastSyncedGlobalCheckpoint == UNASSIGNED_SEQ_NO) {
+                    if (lastSyncedGlobalCheckpointBeforeCommit.get() == UNASSIGNED_SEQ_NO) {
                         // If the global checkpoint is still unassigned, we keep an empty(eg. initial) commit as a safe commit.
                         assertThat(Long.parseLong(safeCommit.getUserData().get(SequenceNumbers.MAX_SEQ_NO)),
                             equalTo(SequenceNumbers.NO_OPS_PERFORMED));
                     } else {
                         assertThat(Long.parseLong(safeCommit.getUserData().get(SequenceNumbers.MAX_SEQ_NO)),
-                            lessThanOrEqualTo(lastSyncedGlobalCheckpoint));
+                            lessThanOrEqualTo(lastSyncedGlobalCheckpointBeforeCommit.get()));
                     }
                     for (int i = 1; i < commits.size(); i++) {
                         assertThat(Long.parseLong(commits.get(i).getUserData().get(SequenceNumbers.MAX_SEQ_NO)),
-                            greaterThan(lastSyncedGlobalCheckpoint));
+                            greaterThan(lastSyncedGlobalCheckpointBeforeCommit.get()));
                     }
                     // Make sure we keep all translog operations after the local checkpoint of the safe commit.
                     long localCheckpointFromSafeCommit = Long.parseLong(safeCommit.getUserData().get(SequenceNumbers.LOCAL_CHECKPOINT_KEY));