[ios] Allow removing file protection for local state file

According to Stability.iOS.UTE.MobileSessionAppState, a large number
of Unexplained Termination Events happen in background. There is an
assumption that "clean exit beacon" does not get written because
prefs file is protected. This CL allows removing protection in the
experiment. The experiment will be run on Canary and Dev to test
the assumption. There is no intention to roll this out to Beta or Stable.

If experiment will confirm the assumption, then clean exit beacon
file will be decoupled from prefs to remove the protection.

Bug: None
Change-Id: If816c4b3ed28ba5b2649e63526d5cfea93f839c3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2506175Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Commit-Queue: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824585}

ios/chrome/browser/application_context_impl.mm

@@ -16,6 +16,7 @@
 #include "base/memory/ptr_util.h"
 #include "base/path_service.h"
 #include "base/sequenced_task_runner.h"
+#include "base/strings/sys_string_conversions.h"
 #include "base/task/post_task.h"
 #include "base/task/thread_pool.h"
 #include "base/time/default_clock.h"
@@ -83,6 +84,25 @@
 
 namespace {
 
+// If enabled local state file will have NSURLFileProtectionNone protection
+// level set for NSURLFileProtectionKey key. The purpose of this feature is to
+// understand if file protection interferes with "clean exit beacon" pref.
+const base::Feature kRemoveProtectionFromPrefFile{
+    "RemoveProtectionFromPrefFile", base::FEATURE_DISABLED_BY_DEFAULT};
+
+// Sets |level| value for NSURLFileProtectionKey key for the URL with given
+// |local_state_path|.
+void SetProtectionLevel(const base::FilePath& file_path, id level) {
+  NSString* file_path_string = base::SysUTF8ToNSString(file_path.value());
+  NSURL* file_path_url = [NSURL fileURLWithPath:file_path_string
+                                    isDirectory:NO];
+  NSError* error = nil;
+  BOOL protection_set = [file_path_url setResourceValue:level
+                                                 forKey:NSURLFileProtectionKey
+                                                  error:&error];
+  DCHECK(protection_set) << base::SysNSStringToUTF8(error.localizedDescription);
+}
+
 // Requests a network::mojom::ProxyResolvingSocketFactory on the UI thread.
 // Note that this cannot be called on a thread that is not the UI thread.
 void RequestProxyResolvingSocketFactoryOnUIThread(
@@ -484,6 +504,24 @@ void ApplicationContextImpl::CreateLocalState() {
 
   base::FilePath local_state_path;
   CHECK(base::PathService::Get(ios::FILE_LOCAL_STATE, &local_state_path));
+
+  NSString* const kRemoveProtectionFromPrefFileKey =
+      @"RemoveProtectionFromPrefKey";
+  if (base::FeatureList::IsEnabled(kRemoveProtectionFromPrefFile)) {
+    SetProtectionLevel(local_state_path, NSURLFileProtectionNone);
+    [NSUserDefaults.standardUserDefaults
+        setBool:YES
+         forKey:kRemoveProtectionFromPrefFileKey];
+  } else if ([NSUserDefaults.standardUserDefaults
+                 boolForKey:kRemoveProtectionFromPrefFileKey]) {
+    // Restore default protection level when user is no longer in the
+    // experimental group.
+    SetProtectionLevel(local_state_path,
+                       NSFileProtectionCompleteUntilFirstUserAuthentication);
+    [NSUserDefaults.standardUserDefaults
+        removeObjectForKey:kRemoveProtectionFromPrefFileKey];
+  }
+
   scoped_refptr<PrefRegistrySimple> pref_registry(new PrefRegistrySimple);
 
   // Register local state preferences.