Sync: support implicit permanent folders in commits.

Traversal::AddUncommittedParentsAndTheirPredecessors walks the
hierarchy to sort commits which is not necessary for any types
except bookmarks.
We've seen some crashes here when implicit permanent folders
were turned on alpha server. I reproduced the crash by planting
an empty parent ID in the database.

The fix avoids traversing the hierarchy for committed items with
unset parent IDs. Verified that this has stopped the crash
locally and successfully committed the item.

BUG=438313

Review URL: https://codereview.chromium.org/1023843002

Cr-Commit-Position: refs/heads/master@{#321472}

sync/engine/directory_commit_contribution_unittest.cc

@@ -38,13 +38,15 @@ class DirectoryCommitContributionTest : public ::testing::Test {
       ModelType type,
       const std::string& tag,
       const sync_pb::AttachmentMetadata& attachment_metadata) {
+    // For bookmarks specify the Bookmarks root folder as the parent;
+    // for other types leave the parent ID empty
+    syncable::Id parent_id;
+    if (type == BOOKMARKS) {
       syncable::Entry parent_entry(trans, syncable::GET_TYPE_ROOT, type);
-    syncable::MutableEntry entry(
-        trans,
-        syncable::CREATE,
-        type,
-        parent_entry.GetId(),
-        tag);
+      parent_id = parent_entry.GetId();
+    }
+
+    syncable::MutableEntry entry(trans, syncable::CREATE, type, parent_id, tag);
     if (attachment_metadata.record_size() > 0) {
       entry.PutAttachmentMetadata(attachment_metadata);
     }

sync/engine/get_commit_ids.cc

@@ -254,6 +254,8 @@ class Traversal {
                          const syncable::Directory::Metahandles& traversed,
                          syncable::Directory::Metahandles* result) const;
 
+  bool SupportsHierarchy(const syncable::Entry& item) const;
+
   // Returns true if we've collected enough items.
   bool IsFull() const;
 
@@ -288,6 +290,7 @@ bool Traversal::AddUncommittedParentsAndTheirPredecessors(
     const std::set<int64>& ready_unsynced_set,
     const syncable::Entry& item,
     syncable::Directory::Metahandles* result) const {
+  DCHECK(SupportsHierarchy(item));
   syncable::Directory::Metahandles dependencies;
   syncable::Id parent_id = item.GetParentId();
 
@@ -393,6 +396,7 @@ bool Traversal::AddDeletedParents(
     const syncable::Entry& item,
     const syncable::Directory::Metahandles& traversed,
     syncable::Directory::Metahandles* result) const {
+  DCHECK(SupportsHierarchy(item));
   syncable::Directory::Metahandles dependencies;
   syncable::Id parent_id = item.GetParentId();
 
@@ -448,6 +452,10 @@ bool Traversal::HaveItem(int64 handle) const {
   return added_handles_.find(handle) != added_handles_.end();
 }
 
+bool Traversal::SupportsHierarchy(const syncable::Entry& item) const {
+  return !item.GetParentId().IsNull();
+}
+
 void Traversal::AppendManyToTraversal(
     const syncable::Directory::Metahandles& handles) {
   out_->insert(out_->end(), handles.begin(), handles.end());
@@ -472,18 +480,20 @@ void Traversal::AddCreatesAndMoves(
                           syncable::GET_BY_HANDLE,
                           metahandle);
     if (!entry.GetIsDel()) {
+      if (SupportsHierarchy(entry)) {
         // We only commit an item + its dependencies if it and all its
         // dependencies are not in conflict.
         syncable::Directory::Metahandles item_dependencies;
-      if (AddUncommittedParentsAndTheirPredecessors(
-              ready_unsynced_set,
-              entry,
+        if (AddUncommittedParentsAndTheirPredecessors(ready_unsynced_set, entry,
                                                       &item_dependencies)) {
-        AddPredecessorsThenItem(ready_unsynced_set,
-                                entry,
+          AddPredecessorsThenItem(ready_unsynced_set, entry,
                                   &item_dependencies);
           AppendManyToTraversal(item_dependencies);
         }
+      } else {
+        // No hierarchy dependencies, just commit the item itself.
+        AppendToTraversal(metahandle);
+      }
     }
   }
 
@@ -515,15 +525,18 @@ void Traversal::AddDeletes(const std::set<int64>& ready_unsynced_set) {
                           metahandle);
 
     if (entry.GetIsDel()) {
+      if (SupportsHierarchy(entry)) {
         syncable::Directory::Metahandles parents;
-      if (AddDeletedParents(
-              ready_unsynced_set, entry, deletion_list, &parents)) {
+        if (AddDeletedParents(ready_unsynced_set, entry, deletion_list,
+                              &parents)) {
           // Append parents and chilren in top to bottom order.
-        deletion_list.insert(deletion_list.end(),
-                             parents.begin(),
+          deletion_list.insert(deletion_list.end(), parents.begin(),
                                parents.end());
           deletion_list.push_back(metahandle);
         }
+      } else {
+        deletion_list.push_back(metahandle);
+      }
     }
   }