Added the RequestContext for the scope.

The RequestContext attempts to make data that is normally RequestScoped
available in a non-RequestScoped context. Request scoping of the
RequestContext is facilitated by the ThreadLocalRequestContext, using
a ThreadLocal to setup the context, similar to the normal way
request scopes work. However, if there is no RequestContext set, the
a default FallbackRequestContext is used, instead of throwing an
OutOfScope exception.

The motivation for this change was prompted by the new GroupBackend
system needing to know the current user in a Singleton context, without
passing the current user through the method.

Change-Id: Ifd7745bec0d4b22dfc98b9a0d18cfd7138bc3375

gerrit-server/src/main/java/com/google/gerrit/server/util/FallbackRequestContext.java

@@ -0,0 +1,40 @@
+// Copyright (C) 2012 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.server.util;
+
+import com.google.gerrit.server.AnonymousUser;
+import com.google.gerrit.server.CurrentUser;
+import com.google.inject.Inject;
+import com.google.inject.Singleton;
+
+/**
+ * The default RequestContext to use when not in a request scope e.g.
+ * ThreadLocalRequestContext is not set.
+ */
+@Singleton
+public class FallbackRequestContext implements RequestContext {
+
+  private final AnonymousUser user;
+
+  @Inject
+  FallbackRequestContext(AnonymousUser user) {
+    this.user = user;
+  }
+
+  @Override
+  public CurrentUser getCurrentUser() {
+    return user;
+  }
+}

gerrit-server/src/main/java/com/google/gerrit/server/util/RequestContext.java

@@ -0,0 +1,26 @@
+// Copyright (C) 2012 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.server.util;
+
+import com.google.gerrit.server.CurrentUser;
+
+/**
+ * The RequestContext is an interface exposing the fields that are needed
+ * by the GerritGlobalModule scope.
+ */
+public interface RequestContext {
+
+  CurrentUser getCurrentUser();
+}

gerrit-server/src/main/java/com/google/gerrit/server/util/ThreadLocalRequestContext.java

@@ -0,0 +1,88 @@
+// Copyright (C) 2012 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.server.util;
+
+import com.google.common.base.Objects;
+import com.google.gerrit.common.errors.NotSignedInException;
+import com.google.gerrit.server.CurrentUser;
+import com.google.gerrit.server.IdentifiedUser;
+import com.google.inject.AbstractModule;
+import com.google.inject.Inject;
+import com.google.inject.Module;
+import com.google.inject.Provider;
+import com.google.inject.Provides;
+import com.google.inject.ProvisionException;
+import com.google.inject.name.Named;
+import com.google.inject.name.Names;
+
+import javax.annotation.Nullable;
+
+/**
+ * ThreadLocalRequestContext manages the current RequestContext using a
+ * ThreadLocal. When the context is set, the fields exposed by the context
+ * are considered in scope. Otherwise, the FallbackRequestContext is used.
+ */
+public class ThreadLocalRequestContext {
+  private static final String FALLBACK = "FALLBACK";
+
+  public static Module module() {
+    return new AbstractModule() {
+      @Override
+      protected void configure() {
+        bind(ThreadLocalRequestContext.class);
+        bind(RequestContext.class).annotatedWith(Names.named(FALLBACK))
+            .to(FallbackRequestContext.class);
+      }
+
+      @Provides
+      RequestContext provideRequestContext(
+          @Named(FALLBACK) RequestContext fallback) {
+        return Objects.firstNonNull(local.get(), fallback);
+      }
+
+      @Provides
+      CurrentUser provideCurrentUser(RequestContext ctx) {
+        return ctx.getCurrentUser();
+      }
+
+      @Provides
+      IdentifiedUser provideCurrentUser(CurrentUser user) {
+        if (user instanceof IdentifiedUser) {
+          return (IdentifiedUser) user;
+        }
+        throw new ProvisionException(NotSignedInException.MESSAGE,
+            new NotSignedInException());
+      }
+    };
+  }
+
+  private static final ThreadLocal<RequestContext> local =
+      new ThreadLocal<RequestContext>();
+
+  @Inject
+  ThreadLocalRequestContext() {
+  }
+
+  public RequestContext setContext(@Nullable RequestContext ctx) {
+    RequestContext old = getContext();
+    local.set(ctx);
+    return old;
+  }
+
+  @Nullable
+  public RequestContext getContext() {
+    return local.get();
+  }
+}