[Sync] Don't notify the transaction observer of huge mutation sets.

The transaction observer is currently only used for debug information sent to
chrome://sync_internals. When we have large numbers of changes within a single
transaction, our mutation set can grow exceedingly large, and the unnecessary
copying we perform in sending the data to the transaction observer can result
in out of memory crashes. This change limits the size of the mutation sets
we copy to 1000 elements. If the size is larger, we send a dummy empty mutation
set instead.

BUG=90169
TEST=turn on encryption on an account with >1000 items. Ensure none are printed in the events pane of about:sync


Review URL: http://codereview.chromium.org/7861005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@100515 0039d316-1c4b-4281-b951-d872f2087c98

1 files changed, 18 insertions, 3 deletions

diff --git a/chrome/browser/sync/syncable/syncable.cc b/chrome/browser/sync/syncable/syncable.cc
index 4ece5bd..288c3dd 100644
--- a/chrome/browser/sync/syncable/syncable.cc
+++ b/chrome/browser/sync/syncable/syncable.cc
@@ -64,6 +64,9 @@ static const InvariantCheckLevel kInvariantCheckLevel = VERIFY_IN_MEMORY;
 
 // Max number of milliseconds to spend checking syncable entry invariants
 static const int kInvariantCheckMaxMs = 50;
+
+// Max number of mutations in a mutation set we permit passing to observers.
+static const size_t kMutationObserverLimit = 1000;
 }  // namespace
 
 using std::string;
@@ -1257,9 +1260,21 @@ ModelTypeBitSet WriteTransaction::NotifyTransactionChangingAndEnding(
   ModelTypeBitSet models_with_changes =
       delegate->HandleTransactionEndingChangeEvent(this);
 
-  dirkernel_->observers->Notify(
-      &TransactionObserver::OnTransactionMutate,
-      from_here_, writer_, mutations, models_with_changes);
+  // These notifications pass the mutation list around by value, which when we
+  // rewrite all sync data can be extremely large and result in out of memory
+  // errors (see crbug.com/90169). As a result, when there are more than
+  // kMutationObserverLimit mutations, we pass around an empty mutation set.
+  if (mutations.size() < kMutationObserverLimit) {
+    dirkernel_->observers->Notify(
+        &TransactionObserver::OnTransactionMutate,
+        from_here_, writer_, mutations, models_with_changes);
+  } else {
+    EntryKernelMutationSet dummy_mutations;  // Empty set.
+    dirkernel_->observers->Notify(
+        &TransactionObserver::OnTransactionMutate,
+        from_here_, writer_, dummy_mutations, models_with_changes);
+  }
+
   return models_with_changes;
 }