[fuchsia] Cache the package configuration in LoadPackageConfig().

Package configurations do not change at run-time, so cache it the first
time it is read, to avoid the cost of repeatedly re-reading it from the
filesystem.

Change-Id: I53c6ea55be5fca48bf3c81f189e251d8a11a55a5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2135607
Commit-Queue: Wez <wez@chromium.org>
Auto-Submit: Wez <wez@chromium.org>
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#756349}

fuchsia/base/config_reader.cc

@@ -6,10 +6,13 @@
 
 #include "base/files/file_util.h"
 #include "base/json/json_reader.h"
+#include "base/no_destructor.h"
 
 namespace cr_fuchsia {
 
-base::Optional<base::Value> LoadPackageConfig() {
+namespace {
+
+base::Optional<base::Value> ReadPackageConfig() {
   constexpr char kConfigPath[] = "/config/data/config.json";
 
   base::FilePath path(kConfigPath);
@@ -35,4 +38,14 @@ base::Optional<base::Value> LoadPackageConfig() {
   return std::move(parsed.value());
 }
 
+}  // namespace
+
+const base::Optional<base::Value>& LoadPackageConfig() {
+  // Package configurations do not change at run-time, so read the configuration
+  // on the first call and cache the result.
+  static base::NoDestructor<base::Optional<base::Value>> config(
+      ReadPackageConfig());
+  return *config;
+}
+
 }  // namespace cr_fuchsia

fuchsia/base/config_reader.h

@@ -13,7 +13,7 @@ namespace cr_fuchsia {
 // Loads and parses configuration data from the environment.
 // Returns a null value if the file(s) do not exist.
 // CHECK-fails if the file(s) are present but not parseable.
-base::Optional<base::Value> LoadPackageConfig();
+const base::Optional<base::Value>& LoadPackageConfig();
 
 }  // namespace cr_fuchsia
 

fuchsia/engine/context_provider_impl.cc

@@ -255,9 +255,10 @@ const uint32_t ContextProviderImpl::kContextRequestHandleId =
     PA_HND(PA_USER0, 0);
 
 ContextProviderImpl::ContextProviderImpl() {
-  base::Optional<base::Value> default_config = cr_fuchsia::LoadPackageConfig();
+  const base::Optional<base::Value>& default_config =
+      cr_fuchsia::LoadPackageConfig();
   if (default_config) {
-    config_default_ = std::move(default_config.value());
+    config_default_ = default_config->Clone();
   } else {
     config_default_ = base::Value(base::Value::Type::DICTIONARY);
   }
@@ -571,14 +572,14 @@ base::Value ContextProviderImpl::LoadConfig() {
   if (!config_override_.is_none())
     return config_override_.Clone();
 
-  base::Optional<base::Value> config = cr_fuchsia::LoadPackageConfig();
+  const base::Optional<base::Value>& config = cr_fuchsia::LoadPackageConfig();
   if (!config) {
     DLOG(WARNING) << "Configuration data not found. Using default "
                      "WebEngine configuration.";
     return base::Value(base::Value::Type::DICTIONARY);
   }
 
-  return std::move(*config);
+  return config->Clone();
 }
 
 void ContextProviderImpl::EnableDevTools(

fuchsia/runners/cast/main.cc

@@ -24,7 +24,7 @@ namespace {
 bool IsHeadless() {
   constexpr char kHeadlessConfigKey[] = "headless";
 
-  base::Optional<base::Value> config = cr_fuchsia::LoadPackageConfig();
+  const base::Optional<base::Value>& config = cr_fuchsia::LoadPackageConfig();
   if (config) {
     base::Optional<bool> headless = config->FindBoolPath(kHeadlessConfigKey);
     return headless && *headless;