rolled back to a temporary thread pool use

h2/src/main/org/h2/mvstore/MVStore.java

@@ -305,8 +305,6 @@ public class MVStore {
 
     private long lastFreeUnusedChunks;
 
-    private final ThreadPoolExecutor executorService;
-
     /**
      * Create and open the store.
      *
@@ -355,8 +353,6 @@ public class MVStore {
         }
         pageSplitSize = pgSplitSize;
         keysPerPage = DataUtils.getConfigParam(config, "keysPerPage", 48);
-        executorService = new ThreadPoolExecutor(10, 10, 10L, TimeUnit.SECONDS,
-                                            new ArrayBlockingQueue<Runnable>(keysPerPage + 1));
         backgroundExceptionHandler =
                 (UncaughtExceptionHandler)config.get("backgroundExceptionHandler");
         meta = new MVMap<>(this);
@@ -945,11 +941,6 @@ public class MVStore {
             return;
         }
         stopBackgroundThread();
-        if (shrinkIfPossible) {
-            executorService.shutdown();
-        } else {
-            executorService.shutdownNow();
-        }
         closed = true;
         storeLock.lock();
         try {
@@ -1395,9 +1386,18 @@ public class MVStore {
         }
     }
 
+    /**
+     * Collect ids for chunks that are no longer in use.
+     * @param fast if true, simplified version is used, which assumes that recent chunks
+     *            are still in-use and do not scan recent versions of the store.
+     *            Also is this case only oldest available version of the store is scanned.
+     */
     private Set<Integer> collectReferencedChunks(boolean fast) {
         assert lastChunk != null;
+        final ThreadPoolExecutor executorService = new ThreadPoolExecutor(10, 10, 10L, TimeUnit.SECONDS,
+                new ArrayBlockingQueue<Runnable>(keysPerPage + 1));
         final AtomicInteger executingThreadCounter = new AtomicInteger(0);
+        try {
             ChunkIdsCollector collector = new ChunkIdsCollector(meta.getId());
             long oldestVersionToKeep = getOldestVersionToKeep();
             MVMap.RootReference rootReference = meta.getRoot();
@@ -1406,7 +1406,7 @@ public class MVStore {
                 while (rootReference.version >= oldestVersionToKeep && (previous = rootReference.previous) != null) {
                     rootReference = previous;
                 }
-            inspectVersion(rootReference, collector, executingThreadCounter, null);
+                inspectVersion(rootReference, collector, executorService, executingThreadCounter, null);
 
                 Page rootPage = rootReference.root;
                 long pos = rootPage.getPos();
@@ -1418,13 +1418,17 @@ public class MVStore {
             } else {
                 Set<Long> inspectedRoots = new HashSet<>();
                 do {
-                inspectVersion(rootReference, collector, executingThreadCounter, inspectedRoots);
+                    inspectVersion(rootReference, collector, executorService, executingThreadCounter, inspectedRoots);
                 } while (rootReference.version >= oldestVersionToKeep && (rootReference = rootReference.previous) != null);
             }
             return collector.getReferenced();
+        } finally {
+            executorService.shutdownNow();
+        }
     }
 
     private void inspectVersion(MVMap.RootReference rootReference, ChunkIdsCollector collector,
+                                ThreadPoolExecutor executorService,
                                 AtomicInteger executingThreadCounter,
                                 Set<Long> inspectedRoots) {
         Page rootPage = rootReference.root;