From 191e9e47d8c211acdf7bef6850c261c2a40e2a8c Mon Sep 17 00:00:00 2001
From: Scott Hernandez <scotthernandez@gmail.com>
Date: Fri, 20 Dec 2013 12:44:40 -0500
Subject: SERVER-12126: Improve update validation performance

---
 src/mongo/db/field_ref_set.cpp | 11 +++++++++--
 src/mongo/db/field_ref_set.h   |  4 +++-
 src/mongo/db/ops/update.cpp    | 19 +++++++++++++------
 3 files changed, 25 insertions(+), 9 deletions(-)

diff --git a/src/mongo/db/field_ref_set.cpp b/src/mongo/db/field_ref_set.cpp
index 64c5e178b7a..46b09e8dd4b 100644
--- a/src/mongo/db/field_ref_set.cpp
+++ b/src/mongo/db/field_ref_set.cpp
@@ -59,11 +59,12 @@ namespace mongo {
     FieldRefSet::FieldRefSet() {
     }
 
-    void FieldRefSet::getConflicts(const FieldRef* toCheck, FieldRefSet* conflicts) const {
+    bool FieldRefSet::findConflicts(const FieldRef* toCheck, FieldRefSet* conflicts) const {
+        bool foundConflict = false;
 
         // If the set is empty, there is no work to do.
         if (_fieldSet.empty())
-            return;
+            return foundConflict;
 
         StringData prefixStr = safeFirstPart(toCheck);
         FieldRef prefixField;
@@ -75,10 +76,16 @@ namespace mongo {
         while (it != _fieldSet.end() && safeFirstPart(*it) == prefixStr) {
             size_t common = (*it)->commonPrefixSize(*toCheck);
             if ((*it)->numParts() == common || toCheck->numParts() == common) {
+                if (!conflicts)
+                    return true;
+
                 conflicts->_fieldSet.insert(*it);
+                foundConflict = true;
             }
             ++it;
         }
+
+        return foundConflict;
     }
 
     void FieldRefSet::keepShortest(const FieldRef* toInsert) {
diff --git a/src/mongo/db/field_ref_set.h b/src/mongo/db/field_ref_set.h
index 71a48502739..e7258c2a184 100644
--- a/src/mongo/db/field_ref_set.h
+++ b/src/mongo/db/field_ref_set.h
@@ -94,8 +94,10 @@ namespace mongo {
         /**
          * Find all inserted fields which conflict with the FieldRef 'toCheck' by the semantics
          * of 'insert', and add those fields to the 'conflicts' set.
+         *
+         * Return true if conflicts were found.
          */
-        void getConflicts(const FieldRef* toCheck, FieldRefSet* conflicts) const;
+        bool findConflicts(const FieldRef* toCheck, FieldRefSet* conflicts) const;
 
         void clear() {
             _fieldSet.clear();
diff --git a/src/mongo/db/ops/update.cpp b/src/mongo/db/ops/update.cpp
index 009a74a494f..6d0f3b3be7c 100644
--- a/src/mongo/db/ops/update.cpp
+++ b/src/mongo/db/ops/update.cpp
@@ -266,20 +266,27 @@ namespace mongo {
                             return s;
                     }
                     // Check if the updated field conflicts with immutable fields
-                    immutableFieldRef.getConflicts(&current, &changedImmutableFields);
+                    immutableFieldRef.findConflicts(&current, &changedImmutableFields);
                 }
             }
 
+            FieldRef idFR;
+            idFR.parse(idFieldName);
+            const bool idChanged = updatedFields.findConflicts(&idFR, NULL);
+
+            // Add _id to fields to check since it too is immutable
+            if (idChanged)
+                changedImmutableFields.keepShortest(&idFR);
+            else if (changedImmutableFields.empty()) {
+                // Return early if nothing changed which is immutable
+                return Status::OK();
+            }
+
             LOG(4) << "Changed immutable fields: " << changedImmutableFields;
             // 2.) Now compare values of the changed immutable fields (to make sure they haven't)
 
             const mutablebson::ConstElement newIdElem = updated.root()[idFieldName];
 
-            // Add _id to fields to check since it too is immutable
-            FieldRef idFR;
-            idFR.parse(idFieldName);
-            changedImmutableFields.keepShortest(&idFR);
-
             FieldRefSet::const_iterator where = changedImmutableFields.begin();
             const FieldRefSet::const_iterator end = changedImmutableFields.end();
             for( ; where != end; ++where ) {
-- 
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