Bug 2701: Fixed dependencies for features-netconf.

[controller.git] / opendaylight / netconf / config-netconf-connector / src / main / java / org / opendaylight / controller / netconf / confignetconfconnector / osgi / YangStoreServiceImpl.java

diff --git a/opendaylight/netconf/config-netconf-connector/src/main/java/org/opendaylight/controller/netconf/confignetconfconnector/osgi/YangStoreServiceImpl.java b/opendaylight/netconf/config-netconf-connector/src/main/java/org/opendaylight/controller/netconf/confignetconfconnector/osgi/YangStoreServiceImpl.java
index 5840c9dbd14d1aa3b3fe050d079e426d8ec21f20..958af54e3fbf7c63c5bc6d543a030d8cf9bbae2b 100644 (file)
--- a/opendaylight/netconf/config-netconf-connector/src/main/java/org/opendaylight/controller/netconf/confignetconfconnector/osgi/YangStoreServiceImpl.java
+++ b/opendaylight/netconf/config-netconf-connector/src/main/java/org/opendaylight/controller/netconf/confignetconfconnector/osgi/YangStoreServiceImpl.java
@@ -9,33 +9,64 @@
 package org.opendaylight.controller.netconf.confignetconfconnector.osgi;
 
 import java.lang.ref.SoftReference;
+import java.util.concurrent.atomic.AtomicReference;
 import org.opendaylight.yangtools.yang.model.api.SchemaContextProvider;
-
-import javax.annotation.concurrent.GuardedBy;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 public class YangStoreServiceImpl implements YangStoreService {
+    private static final Logger LOG = LoggerFactory.getLogger(YangStoreServiceImpl.class);
+
+    /**
+     * This is a rather interesting locking model. We need to guard against both the
+     * cache expiring from GC and being invalidated by schema context change. The
+     * context can change while we are doing processing, so we do not want to block
+     * it, so no synchronization can happen on the methods.
+     *
+     * So what we are doing is the following:
+     *
+     * We synchronize with GC as usual, using a SoftReference.
+     *
+     * The atomic reference is used to synchronize with {@link #refresh()}, e.g. when
+     * refresh happens, it will push a SoftReference(null), e.g. simulate the GC. Now
+     * that may happen while the getter is already busy acting on the old schema context,
+     * so it needs to understand that a refresh has happened and retry. To do that, it
+     * attempts a CAS operation -- if it fails, in knows that the SoftReference has
+     * been replaced and thus it needs to retry.
+     *
+     * Note that {@link #getYangStoreSnapshot()} will still use synchronize() internally
+     * to stop multiple threads doing the same work.
+     */
+    private final AtomicReference<SoftReference<YangStoreSnapshotImpl>> ref = new AtomicReference<>(new SoftReference<YangStoreSnapshotImpl>(null));
     private final SchemaContextProvider service;
-    @GuardedBy("this")
-    private SoftReference<YangStoreSnapshotImpl> cache = new SoftReference<>(null);
 
-    public YangStoreServiceImpl(SchemaContextProvider service) {
+    public YangStoreServiceImpl(final SchemaContextProvider service) {
         this.service = service;
     }
 
     @Override
     public synchronized YangStoreSnapshotImpl getYangStoreSnapshot() throws YangStoreException {
-        YangStoreSnapshotImpl yangStoreSnapshot = cache.get();
-        if (yangStoreSnapshot == null) {
-            yangStoreSnapshot = new YangStoreSnapshotImpl(service.getSchemaContext());
-            cache = new SoftReference<>(yangStoreSnapshot);
+        SoftReference<YangStoreSnapshotImpl> r = ref.get();
+        YangStoreSnapshotImpl ret = r.get();
+
+        while (ret == null) {
+            // We need to be compute a new value
+            ret = new YangStoreSnapshotImpl(service.getSchemaContext());
+
+            if (!ref.compareAndSet(r, new SoftReference<>(ret))) {
+                LOG.debug("Concurrent refresh detected, recomputing snapshot");
+                r = ref.get();
+                ret = null;
+            }
         }
-        return yangStoreSnapshot;
+
+        return ret;
     }
 
     /**
      * Called when schema context changes, invalidates cache.
      */
-    public synchronized void refresh() {
-        cache.clear();
+    public void refresh() {
+        ref.set(new SoftReference<YangStoreSnapshotImpl>(null));
     }
 }