[Fuchsia] Add ScopedDefaultComponentContext helper.

This can be used to replace the current default ComponentContext with
one backed by a supplied |service_root| channel, e.g. for testing.

Bug: 893229
Change-Id: Ib08caf7527066c5b269d756a1726e1843c6431f5
Reviewed-on: https://chromium-review.googlesource.com/c/1333113
Commit-Queue: Wez <wez@chromium.org>
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607788}

base/fuchsia/component_context.cc

@@ -9,6 +9,7 @@
 #include <utility>
 
 #include "base/fuchsia/fuchsia_logging.h"
+#include "base/logging.h"
 #include "base/no_destructor.h"
 
 namespace base {
@@ -27,6 +28,13 @@ zx::channel ConnectToServiceRoot() {
   return client_channel;
 }
 
+std::unique_ptr<ComponentContext>* DefaultComponentContext() {
+  static base::NoDestructor<std::unique_ptr<ComponentContext>>
+      component_context_ptr(
+          std::make_unique<ComponentContext>(ConnectToServiceRoot()));
+  return component_context_ptr.get();
+}
+
 }  // namespace
 
 ComponentContext::ComponentContext(zx::channel service_root)
@@ -38,9 +46,7 @@ ComponentContext::~ComponentContext() = default;
 
 // static
 ComponentContext* ComponentContext::GetDefault() {
-  static base::NoDestructor<ComponentContext> component_context(
-      ConnectToServiceRoot());
-  return component_context.get();
+  return DefaultComponentContext()->get();
 }
 
 zx_status_t ComponentContext::ConnectToService(FidlInterfaceRequest request) {
@@ -49,5 +55,18 @@ zx_status_t ComponentContext::ConnectToService(FidlInterfaceRequest request) {
                                  request.TakeChannel().release());
 }
 
+ScopedDefaultComponentContext::ScopedDefaultComponentContext(
+    zx::channel service_root)
+    : old_context_(std::move(*DefaultComponentContext())) {
+  *DefaultComponentContext() =
+      std::make_unique<ComponentContext>(std::move(service_root));
+  context_ = DefaultComponentContext()->get();
+}
+
+ScopedDefaultComponentContext::~ScopedDefaultComponentContext() {
+  DCHECK_EQ(DefaultComponentContext()->get(), context_);
+  *DefaultComponentContext() = std::move(old_context_);
+}
+
 }  // namespace fuchsia
 }  // namespace base

base/fuchsia/component_context.h

@@ -61,6 +61,20 @@ class BASE_EXPORT ComponentContext {
   DISALLOW_COPY_AND_ASSIGN(ComponentContext);
 };
 
+// Replaces the default ComponentContext with the supplied |service_root|, and
+// restores it when going out-of-scope.
+class BASE_EXPORT ScopedDefaultComponentContext {
+ public:
+  ScopedDefaultComponentContext(zx::channel service_root);
+  ~ScopedDefaultComponentContext();
+
+ private:
+  ComponentContext* context_;
+  std::unique_ptr<ComponentContext> old_context_;
+
+  DISALLOW_COPY_AND_ASSIGN(ScopedDefaultComponentContext);
+};
+
 }  // namespace fuchsia
 }  // namespace base