SERVER-13489 move possible disk access out of index cursor ctor

(cherry picked from commit ae2dc0a937cd66c249d0d98802fe8229e1a32df8)

3 files changed, 15 insertions, 4 deletions

diff --git a/src/mongo/db/index/btree_based_access_method.cpp b/src/mongo/db/index/btree_based_access_method.cpp
index 729d1226be0..9b7ed5effd1 100644
--- a/src/mongo/db/index/btree_based_access_method.cpp
+++ b/src/mongo/db/index/btree_based_access_method.cpp
@@ -138,7 +138,7 @@ namespace mongo {
     }
 
     Status BtreeBasedAccessMethod::newCursor(IndexCursor **out) const {
-        *out = new BtreeIndexCursor(_btreeState, _interface);
+        *out = new BtreeIndexCursor(_btreeState, _btreeState->head(), _interface);
         return Status::OK();
     }
 
diff --git a/src/mongo/db/index/btree_index_cursor.cpp b/src/mongo/db/index/btree_index_cursor.cpp
index 24b788f7f14..be1472241f3 100644
--- a/src/mongo/db/index/btree_index_cursor.cpp
+++ b/src/mongo/db/index/btree_index_cursor.cpp
@@ -44,11 +44,12 @@ namespace mongo {
 
     // Go forward by default.
     BtreeIndexCursor::BtreeIndexCursor(const IndexCatalogEntry* btreeState,
+                                       const DiskLoc head,
                                        BtreeInterface *interface)
         : _direction(1),
           _btreeState(btreeState),
           _interface(interface),
-          _bucket(btreeState->head()),
+          _bucket(head),
           _keyOffset(0) {
 
         SimpleMutex::scoped_lock lock(_activeCursorsMutex);
diff --git a/src/mongo/db/index/btree_index_cursor.h b/src/mongo/db/index/btree_index_cursor.h
index 00be494f7ca..73a7923095e 100644
--- a/src/mongo/db/index/btree_index_cursor.h
+++ b/src/mongo/db/index/btree_index_cursor.h
@@ -95,8 +95,18 @@ namespace mongo {
         static unordered_set<BtreeIndexCursor*> _activeCursors;
         static SimpleMutex _activeCursorsMutex;
 
-        // Go forward by default.
-        BtreeIndexCursor(const IndexCatalogEntry* btreeState, BtreeInterface *interface);
+        /**
+         * btreeState is the ICE of the Btree that we're going to traverse.
+         * head is the head of the Btree.
+         * interface is an abstraction to hide the fact that we have two types of Btrees.
+         *
+         * Go forward by default.
+         *
+         * Intentionally private, we're friends with the only class allowed to call it.
+         */
+        BtreeIndexCursor(const IndexCatalogEntry* btreeState,
+                         const DiskLoc head,
+                         BtreeInterface *interface);
 
         void skipUnusedKeys();