model: prevent simultan access of singleton (#773)

The singleton instance of "Diagnostician" is used without
synchronization.
The singleton "Diasnostician" instance is using the singleton
"EValidator.Registry" instance (without synchronization).
In very rare high load situations there has been CME detected.

My first "solution" has been to synchronize the access of
Diagnostician's instance method by using

```java
synchronized (Diagnostician.INSTANCE) {
    ...
```

But after realize that EMF is using internally other singletons I tried
to find any information about EMF and thread safety.

I found this one: https://javahacks.net/2016/07/13/emf-thread-safety/

  EMF models are not thread-safe by default and writing multithreaded
  applications is not that simple.
  The more complex our application became, the more often we got
  concurrent modification exceptions and had problems with filtering and
  sorting operations.

So, I assume instead of adding synchronizations to our code that is
using EMF (IIRC this has been already done on the ESH hosted code base
long time ago) we should try to execute EMF code in a safe manner.

This implementation adds a "SafeEMF" OSGi service that should be used to
execute EMF code to ensure none code of EMF (or at least the ones that
calls has been migrated to the SafeEMF usage) is accessed by separate
threads at the same time.

Related to: https://github.com/openhab/openhab-core/issues/772

Signed-off-by: Markus Rathgeb <maggu2810@gmail.com>

bundles/org.openhab.core.model.core/src/main/java/org/eclipse/smarthome/model/core/SafeEMF.java

@@ -0,0 +1,40 @@
+/**
+ * Copyright (c) 2010-2019 Contributors to the openHAB project
+ *
+ * See the NOTICE file(s) distributed with this work for additional
+ * information.
+ *
+ * This program and the accompanying materials are made available under the
+ * terms of the Eclipse Public License 2.0 which is available at
+ * http://www.eclipse.org/legal/epl-2.0
+ *
+ * SPDX-License-Identifier: EPL-2.0
+ */
+package org.eclipse.smarthome.model.core;
+
+import java.util.function.Supplier;
+
+/**
+ * Service interface to execute EMF methods in a single based thread.
+ *
+ * @author Markus Rathgeb - Initial contribution
+ */
+public interface SafeEMF {
+
+    /**
+     * Calls the given function.
+     *
+     * @param <T> the return type of the calling function
+     * @param func the function to call
+     * @return the return value of the called function
+     */
+    <T> T call(Supplier<T> func);
+
+    /**
+     * Calls the given function.
+     *
+     * @param func the function to call
+     */
+    void call(Runnable func);
+
+}

bundles/org.openhab.core.model.core/src/main/java/org/eclipse/smarthome/model/core/internal/ModelRepositoryImpl.java

@@ -38,10 +38,13 @@ import org.eclipse.emf.ecore.util.Diagnostician;
 import org.eclipse.smarthome.model.core.EventType;
 import org.eclipse.smarthome.model.core.ModelRepository;
 import org.eclipse.smarthome.model.core.ModelRepositoryChangeListener;
+import org.eclipse.smarthome.model.core.SafeEMF;
 import org.eclipse.xtext.resource.SynchronizedXtextResourceSet;
 import org.eclipse.xtext.resource.XtextResource;
 import org.eclipse.xtext.resource.XtextResourceSet;
+import org.osgi.service.component.annotations.Activate;
 import org.osgi.service.component.annotations.Component;
+import org.osgi.service.component.annotations.Reference;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -58,7 +61,12 @@ public class ModelRepositoryImpl implements ModelRepository {
 
     private final List<ModelRepositoryChangeListener> listeners = new CopyOnWriteArrayList<>();
 
-    public ModelRepositoryImpl() {
+    private final SafeEMF safeEmf;
+
+    @Activate
+    public ModelRepositoryImpl(final @Reference SafeEMF safeEmf) {
+        this.safeEmf = safeEmf;
+
         XtextResourceSet xtextResourceSet = new SynchronizedXtextResourceSet();
         xtextResourceSet.addLoadOption(XtextResource.OPTION_RESOLVE_ALL, Boolean.TRUE);
         this.resourceSet = xtextResourceSet;
@@ -278,8 +286,8 @@ public class ModelRepositoryImpl implements ModelRepository {
 
                 // Check for validation errors, but log them only
                 try {
-                    org.eclipse.emf.common.util.Diagnostic diagnostic = Diagnostician.INSTANCE
-                            .validate(resource.getContents().get(0));
+                    final org.eclipse.emf.common.util.Diagnostic diagnostic = safeEmf
+                            .call(() -> Diagnostician.INSTANCE.validate(resource.getContents().get(0)));
                     for (org.eclipse.emf.common.util.Diagnostic d : diagnostic.getChildren()) {
                         warnings.add(d.getMessage());
                     }

bundles/org.openhab.core.model.core/src/main/java/org/eclipse/smarthome/model/core/internal/SafeEMFImpl.java

@@ -0,0 +1,38 @@
+/**
+ * Copyright (c) 2010-2019 Contributors to the openHAB project
+ *
+ * See the NOTICE file(s) distributed with this work for additional
+ * information.
+ *
+ * This program and the accompanying materials are made available under the
+ * terms of the Eclipse Public License 2.0 which is available at
+ * http://www.eclipse.org/legal/epl-2.0
+ *
+ * SPDX-License-Identifier: EPL-2.0
+ */
+package org.eclipse.smarthome.model.core.internal;
+
+import java.util.function.Supplier;
+
+import org.eclipse.smarthome.model.core.SafeEMF;
+import org.osgi.service.component.annotations.Component;
+
+/**
+ * Implementation of a safe EMF caller..
+ *
+ * @author Markus Rathgeb - Initial contribution
+ */
+@Component
+public class SafeEMFImpl implements SafeEMF {
+
+    @Override
+    public synchronized <T> T call(Supplier<T> func) {
+        return func.get();
+    }
+
+    @Override
+    public synchronized void call(Runnable func) {
+        func.run();
+    }
+
+}