[DOMStorage] Offset the session storage namespace ids generated by different...

[DOMStorage] Offset the session storage namespace ids generated by different contexts to help identify when an id from one is mistakenly used in another.

Review URL: https://codereview.chromium.org/881253004

Cr-Commit-Position: refs/heads/master@{#314256}

content/browser/dom_storage/dom_storage_context_impl.cc

@@ -4,6 +4,8 @@
 
 #include "content/browser/dom_storage/dom_storage_context_impl.h"
 
+#include <stdlib.h>
+
 #include "base/bind.h"
 #include "base/bind_helpers.h"
 #include "base/files/file_enumerator.h"
@@ -26,6 +28,10 @@ namespace content {
 
 static const int kSessionStoraceScavengingSeconds = 60;
 
+// Offset the session storage namespace ids generated by different contexts
+// to help identify when an id from one is mistakenly used in another.
+static int g_session_id_offset_sequence = 0;
+
 DOMStorageContextImpl::DOMStorageContextImpl(
     const base::FilePath& localstorage_directory,
     const base::FilePath& sessionstorage_directory,
@@ -34,6 +40,7 @@ DOMStorageContextImpl::DOMStorageContextImpl(
     : localstorage_directory_(localstorage_directory),
       sessionstorage_directory_(sessionstorage_directory),
       task_runner_(task_runner),
+      session_id_offset_(abs((g_session_id_offset_sequence++ % 10)) * 1000),
       is_shutdown_(false),
       force_keep_session_state_(false),
       special_storage_policy_(special_storage_policy),
@@ -227,6 +234,10 @@ void DOMStorageContextImpl::NotifyAreaCleared(
       OnDOMStorageAreaCleared(area, page_url));
 }
 
+int64 DOMStorageContextImpl::AllocateSessionId() {
+  return session_id_sequence_.GetNext() + session_id_offset_;
+}
+
 std::string DOMStorageContextImpl::AllocatePersistentSessionId() {
   std::string guid = base::GenerateGUID();
   std::replace(guid.begin(), guid.end(), '-', '_');

content/browser/dom_storage/dom_storage_context_impl.h

@@ -147,10 +147,7 @@ class CONTENT_EXPORT DOMStorageContextImpl
       const GURL& page_url);
 
   // May be called on any thread.
-  int64 AllocateSessionId() {
-    return session_id_sequence_.GetNext();
-  }
-
+  int64 AllocateSessionId();
   std::string AllocatePersistentSessionId();
 
   // Must be called on the background thread.
@@ -207,7 +204,10 @@ class CONTENT_EXPORT DOMStorageContextImpl
 
   // We use a 32 bit identifier for per tab storage sessions.
   // At a tab per second, this range is large enough for 68 years.
+  // The offset is to more quickly detect the error condition where
+  // an id related to one context is mistakenly used in another.
   base::AtomicSequenceNumber session_id_sequence_;
+  const int session_id_offset_;
 
   bool is_shutdown_;
   bool force_keep_session_state_;