Fix NullFileSystem & LocalFileSystem getInstance() implementation

getInstance() should always return same value, but that was not always
the case.

Public constructors of both file systems allowed clients to override
static instance field, causing potential and hard to find errors at at
runtime.

Fixes #1114

resources/bundles/org.eclipse.core.filesystem/src/org/eclipse/core/internal/filesystem/InternalFileSystemCore.java

@@ -15,9 +15,18 @@
 
 import java.net.URI;
 import java.util.HashMap;
-import org.eclipse.core.filesystem.*;
+import org.eclipse.core.filesystem.EFS;
+import org.eclipse.core.filesystem.IFileStore;
+import org.eclipse.core.filesystem.IFileSystem;
 import org.eclipse.core.filesystem.provider.FileSystem;
-import org.eclipse.core.runtime.*;
+import org.eclipse.core.runtime.CoreException;
+import org.eclipse.core.runtime.IConfigurationElement;
+import org.eclipse.core.runtime.IExtension;
+import org.eclipse.core.runtime.IExtensionDelta;
+import org.eclipse.core.runtime.IExtensionPoint;
+import org.eclipse.core.runtime.IRegistryChangeEvent;
+import org.eclipse.core.runtime.IRegistryChangeListener;
+import org.eclipse.core.runtime.RegistryFactory;
 import org.eclipse.osgi.util.NLS;
 
 /**
@@ -72,9 +81,15 @@ public IFileSystem getFileSystem(String scheme) throws CoreException {
 			IConfigurationElement element = (IConfigurationElement) result;
 			FileSystem fs = (FileSystem) element.createExecutableExtension("run"); //$NON-NLS-1$
 			fs.initialize(scheme);
-			//store the file system instance so we don't have to keep recreating it
-			registry.put(scheme, fs);
-			return fs;
+			synchronized (this) {
+				result = registry.get(scheme);
+				if (result instanceof IFileSystem) {
+					return (IFileSystem) result;
+				}
+				//store the file system instance so we don't have to keep recreating it
+				registry.put(scheme, fs);
+				return fs;
+			}
 		} catch (CoreException e) {
 			//remove this invalid file system from the registry
 			registry.remove(scheme);

resources/bundles/org.eclipse.core.filesystem/src/org/eclipse/core/internal/filesystem/NullFileSystem.java

@@ -29,23 +29,22 @@ public class NullFileSystem extends FileSystem {
 	/**
 	 * The singleton instance of this file system.
 	 */
-	private static IFileSystem instance;
+	private static final IFileSystem INSTANCE = EFS.getNullFileSystem();
 
 	/**
 	 * Returns the instance of this file system
 	 *
 	 * @return The instance of this file system.
 	 */
 	public static IFileSystem getInstance() {
-		return instance;
+		return INSTANCE;
 	}
 
 	/**
 	 * Creates the null file system.
 	 */
 	public NullFileSystem() {
 		super();
-		instance = this;
 	}
 
 	@Override

resources/bundles/org.eclipse.core.filesystem/src/org/eclipse/core/internal/filesystem/local/LocalFileSystem.java

@@ -19,7 +19,9 @@
 
 import java.io.File;
 import java.net.URI;
-import org.eclipse.core.filesystem.*;
+import org.eclipse.core.filesystem.EFS;
+import org.eclipse.core.filesystem.IFileStore;
+import org.eclipse.core.filesystem.IFileSystem;
 import org.eclipse.core.filesystem.provider.FileSystem;
 import org.eclipse.core.runtime.IPath;
 import org.eclipse.osgi.service.environment.Constants;
@@ -47,15 +49,15 @@ public class LocalFileSystem extends FileSystem {
 	/**
 	 * The singleton instance of this file system.
 	 */
-	private static IFileSystem instance;
+	private static final IFileSystem INSTANCE = EFS.getLocalFileSystem();
 
 	/**
 	 * Returns the instance of this file system
 	 *
 	 * @return The instance of this file system.
 	 */
 	public static IFileSystem getInstance() {
-		return instance;
+		return INSTANCE;
 	}
 
 	/**
@@ -71,7 +73,6 @@ static String getOS() {
 	 */
 	public LocalFileSystem() {
 		super();
-		instance = this;
 	}
 
 	@Override