Merge pull request #1650 from h2database/race-close

Fix race in MVStore.close()

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

@@ -138,11 +138,6 @@ public class MVMap<K, V> extends AbstractMap<K, V>
         return "map." + Integer.toHexString(mapId);
     }
 
-    /**
-     * Initialize this map.
-     */
-    protected void init() {}
-
     /**
      * Add or replace a key-value pair.
      *

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

@@ -28,6 +28,7 @@ import java.util.concurrent.ThreadPoolExecutor;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicInteger;
 import java.util.concurrent.atomic.AtomicLong;
+import java.util.concurrent.atomic.AtomicReference;
 import java.util.concurrent.locks.ReentrantLock;
 import org.h2.compress.CompressDeflate;
 import org.h2.compress.CompressLZF;
@@ -148,23 +149,24 @@ public class MVStore implements AutoCloseable {
     private static final int MARKED_FREE = 10_000_000;
 
     /**
-     * Storage is open.
+     * Store is open.
      */
     private static final int STATE_OPEN = 0;
 
     /**
-     * Storage is stopping now. Background writer thread finishes its work
-     * during this process.
+     * Store is about to close now, but is still operational.
+     * Outstanding store operation by background writer or other thread may be in progress.
+     * New updates must not be initiated, unless they are part of a closing procedure itself.
      */
     private static final int STATE_STOPPING = 1;
 
     /**
-     * Storage is closing now.
+     * Store is closing now, and any operation on it may fail.
      */
     private static final int STATE_CLOSING = 2;
 
     /**
-     * Storage is closed.
+     * Store is closed.
      */
     private static final int STATE_CLOSED = 3;
 
@@ -177,13 +179,13 @@ public class MVStore implements AutoCloseable {
     private final ReentrantLock storeLock = new ReentrantLock(true);
 
     /**
-     * The background thread, if any.
+     * Reference to a background thread, which is expected to be running, if any.
      */
-    volatile BackgroundWriterThread backgroundWriterThread;
+    private final AtomicReference<BackgroundWriterThread> backgroundWriterThread = new AtomicReference<>();
 
     private volatile boolean reuseSpace = true;
 
-    private volatile int state;
+    private final AtomicInteger state = new AtomicInteger();
 
     private final FileStore fileStore;
 
@@ -268,7 +270,8 @@ public class MVStore implements AutoCloseable {
     private final AtomicLong oldestVersionToKeep = new AtomicLong();
 
     /**
-     * Collection of all versions used by currently open transactions.
+     * Ordered collection of all version usage counters for all versions starting
+     * from oldestVersionToKeep and up to current.
      */
     private final Deque<TxCounter> versions = new LinkedList<>();
 
@@ -373,7 +376,6 @@ public class MVStore implements AutoCloseable {
         backgroundExceptionHandler =
                 (UncaughtExceptionHandler)config.get("backgroundExceptionHandler");
         meta = new MVMap<>(this);
-        meta.init();
         if (this.fileStore != null) {
             retentionTime = this.fileStore.getDefaultRetentionTime();
             // 19 KB memory is about 1 KB storage
@@ -435,7 +437,7 @@ public class MVStore implements AutoCloseable {
     }
 
     private void panic(IllegalStateException e) {
-        if (state == STATE_OPEN) {
+        if (isOpen()) {
             handleException(e);
             panicException = e;
             closeImmediately();
@@ -509,7 +511,6 @@ public class MVStore implements AutoCloseable {
             c.put("id", id);
             c.put("createVersion", currentVersion);
             map = builder.create(this, c);
-            map.init();
             String x = Integer.toHexString(id);
             meta.put(MVMap.getMapKey(id), map.asString(name));
             meta.put("name." + name, x);
@@ -539,7 +540,6 @@ public class MVStore implements AutoCloseable {
                 }
                 config.put("id", id);
                 map = builder.create(this, config);
-                map.init();
                 long root = getRootPos(meta, id);
                 map.setRootPos(root, lastStoredVersion);
                 maps.put(id, map);
@@ -939,27 +939,13 @@ public class MVStore implements AutoCloseable {
      */
     @Override
     public void close() {
-        if (isClosed()) {
-            return;
-        }
-        FileStore f = fileStore;
-        if (f != null && !f.isReadOnly()) {
-            stopBackgroundThread();
-            for (MVMap<?, ?> map : maps.values()) {
-                if (map.isClosed()) {
-                    if (meta.remove(MVMap.getMapRootKey(map.getId())) != null) {
-                        markMetaChanged();
-                    }
-                }
-            }
-            commit();
-        }
         closeStore(true);
     }
 
     /**
-     * Close the file and the store, without writing anything. This will stop
-     * the background thread. This method ignores all errors.
+     * Close the file and the store, without writing anything.
+     * This will try to stop the background thread (without waiting for it).
+     * This method ignores all errors.
      */
     public void closeImmediately() {
         try {
@@ -969,20 +955,32 @@ public class MVStore implements AutoCloseable {
         }
     }
 
-    private void closeStore(boolean shrinkIfPossible) {
-        if (isClosed()) {
-            return;
-        }
-        state = STATE_STOPPING;
+    private void closeStore(boolean normalShutdown) {
+        // If any other thead have already initiated closure procedure,
+        // isClosed() would wait until closure is done and then  we jump out of the loop.
+        // This is a subtle difference between !isClosed() and isOpen().
+        while (!isClosed()) {
+            if (state.compareAndSet(STATE_OPEN, STATE_STOPPING)) {
                 try {
-            stopBackgroundThread();
-            state = STATE_CLOSING;
+                    stopBackgroundThread(normalShutdown);
                     storeLock.lock();
                     try {
                         try {
-                    if (fileStore != null && shrinkIfPossible) {
+                            if (normalShutdown && fileStore != null && !fileStore.isReadOnly()) {
+                                for (MVMap<?, ?> map : maps.values()) {
+                                    if (map.isClosed()) {
+                                        if (meta.remove(MVMap.getMapRootKey(map.getId())) != null) {
+                                            markMetaChanged();
+                                        }
+                                    }
+                                }
+                                commit();
+
                                 shrinkFileIfPossible(0);
                             }
+
+                            state.set(STATE_CLOSING);
+
                             // release memory early - this is important when called
                             // because of out of memory
                             if (cache != null) {
@@ -1005,7 +1003,9 @@ public class MVStore implements AutoCloseable {
                         storeLock.unlock();
                     }
                 } finally {
-            state = STATE_CLOSED;
+                    state.set(STATE_CLOSED);
+                }
+            }
         }
     }
 
@@ -2779,7 +2779,7 @@ public class MVStore implements AutoCloseable {
     private void handleException(Throwable ex) {
         if (backgroundExceptionHandler != null) {
             try {
-                backgroundExceptionHandler.uncaughtException(null, ex);
+                backgroundExceptionHandler.uncaughtException(Thread.currentThread(), ex);
             } catch(Throwable ignore) {
                 if (ex != ignore) { // OOME may be the same
                     ex.addSuppressed(ignore);
@@ -2805,12 +2805,20 @@ public class MVStore implements AutoCloseable {
         }
     }
 
+    private boolean isOpen() {
+        return state.get() == STATE_OPEN;
+    }
+
+    /**
+     * Determine that store is open, or wait for it to be closed (by other thread)
+     * @return true if store is open, false otherwise
+     */
     public boolean isClosed() {
-        if (state == STATE_OPEN) {
+        if (isOpen()) {
             return false;
         }
         int millis = 1;
-        while (state != STATE_CLOSED) {
+        while (state.get() != STATE_CLOSED) {
             /*
              * We need to wait for completion of close procedure. This is
              * required because otherwise database may be closed too early while
@@ -2829,29 +2837,33 @@ public class MVStore implements AutoCloseable {
     }
 
     private boolean isOpenOrStopping() {
-        return state <= STATE_STOPPING;
-    }
-
-    private void stopBackgroundThread() {
-        BackgroundWriterThread t = backgroundWriterThread;
-        if (t == null) {
-            return;
-        }
-        backgroundWriterThread = null;
-        if (Thread.currentThread() == t) {
-            // within the thread itself - can not join
-            return;
-        }
+        return state.get() <= STATE_STOPPING;
+    }
+
+    private void stopBackgroundThread(boolean waitForIt) {
+        // Loop here is not strictly necessary, except for case of a spurious failure,
+        // which should not happen with non-weak flavour of CAS operation,
+        // but I've seen it, so just to be safe...
+        BackgroundWriterThread t;
+        while ((t = backgroundWriterThread.get()) != null &&
+                // if called from within the thread itself - can not join
+                t != Thread.currentThread()) {
+            if (backgroundWriterThread.compareAndSet(t, null)) {
                 synchronized (t.sync) {
                     t.sync.notifyAll();
                 }
 
+                if (waitForIt) {
                     try {
                         t.join();
                     } catch (Exception e) {
                         // ignore
                     }
                 }
+                break;
+            }
+        }
+    }
 
     /**
      * Set the maximum delay in milliseconds to auto-commit changes.
@@ -2872,18 +2884,23 @@ public class MVStore implements AutoCloseable {
         if (fileStore == null || fileStore.isReadOnly()) {
             return;
         }
-        stopBackgroundThread();
+        stopBackgroundThread(true);
         // start the background thread if needed
-        if (millis > 0 && state == STATE_OPEN) {
+        if (millis > 0 && isOpen()) {
             int sleep = Math.max(1, millis / 10);
             BackgroundWriterThread t =
                     new BackgroundWriterThread(this, sleep,
                             fileStore.toString());
+            if (backgroundWriterThread.compareAndSet(null, t)) {
                 t.start();
-            backgroundWriterThread = t;
+            }
         }
     }
 
+    boolean isBackgroundThread() {
+        return Thread.currentThread() == backgroundWriterThread.get();
+    }
+
     /**
      * Get the auto-commit delay.
      *
@@ -3097,20 +3114,19 @@ public class MVStore implements AutoCloseable {
 
         @Override
         public void run() {
-            while (store.backgroundWriterThread != null) {
+            while (store.isBackgroundThread()) {
                 synchronized (sync) {
                     try {
                         sync.wait(sleep);
                     } catch (InterruptedException ignore) {
                     }
                 }
-                if (store.backgroundWriterThread == null) {
+                if (!store.isBackgroundThread()) {
                     break;
                 }
                 store.writeInBackground();
             }
         }
-
     }
 
     /**

h2/src/tools/org/h2/build/doc/dictionary.txt

@@ -77,7 +77,7 @@ called caller calling calls cally caload came camel can cancel canceled cancelin
 cancellation cancelled cancels candidates cannot canonical cap capabilities
 capability capacity capitalization capitalize capitalized capone caps capture
 captured car card cardinal cardinality care careful carriage carrier cars cartesian
-cascade cascading case cases casesensitive casewhen cash casing casqueiro cast
+cas cascade cascading case cases casesensitive casewhen cash casing casqueiro cast
 casting castore cat catalina catalog catalogs cataloguing catch catcher catches
 catching category catlog caucho caught cause caused causes causing cavestro
 cayenne cbc cbtree ccedil cdata cdd cddl cdo cdup cease cedil ceil ceiling cell
@@ -251,7 +251,7 @@ filo filter filtered filtering filters fin final finalization finalize finalizer
 finally find finder finding finds fine finer finish finished finishes finland fire
 firebird firebirdsql fired firefox firewall first firstname fish fit fitness fits
 fitting five fix fixed fixes fixing fkcolumn fktable flag flags flash flashback
-flat fle fletcher flexibility flexible flexive flip flipped fload float floating
+flat flavour fle fletcher flexibility flexible flexive flip flipped fload float floating
 flooding floor florent flow flower flows fluent fluid flush flushed flushes
 flushing flux fly flyway fmb fmc fml fmrn fmt fmul fmxx fmxxx fneg focus focusable
 fog fogh folder follow followed following follows font fontes foo footer footers
@@ -321,7 +321,7 @@ inform information informational informed informix informs informtn infos
 infrastructure infringe infringed infringement infringements infringes infringing
 inherent inherit inheritance inherited inherits ini init initial initialization
 initialize initialized initializer initializes initializing initially initiate
-initiation inits inject injection injections injury inline inlined inliner
+initiated initiation inits inject injection injections injury inline inlined inliner
 inlining inner inno innodb inplace input inputs ins insecure insensitive insert
 inserted inserting insertion inserts insets inside insists inspect inspected
 inspector inspectors inst install installation installations installed installer
@@ -440,8 +440,8 @@ omitted omitting once onchange onclick one ones onfocus ongoing onkeydown onkeyu
 online onload only onmousedown onmousemove onmouseout onmouseover onmouseup
 onreadystatechange onresize onscroll onsubmit onto ontology ontoprise oome oops
 ooq open opened openfire opening openjpa opens opera operand operands operate
-operates operating operation operations operator operators oplus opposite ops opt
-optimal optimisation optimised optimistic optimizable optimization optimizations
+operates operating operation operational operations operator operators oplus opposite
+ops opt optimal optimisation optimised optimistic optimizable optimization optimizations
 optimize optimized optimizer optimizing option optional optionally options ora
 oracle orange oranges orchestration order orderable ordered orderid ordering
 orders ordf ordinal ordinary ordinate ordm ordplugins ordsys oren org organic