Normalize handling of Undefined vs EOO/missing in agg

Related tickets:
SERVER-6571 replace use of Undefined as missing with EOO
SERVER-6471 ignore nullish values in $min and $max accumulators
SERVER-6144 divide by zero makes field disappear (this solution isn't final)

1 files changed, 8 insertions, 4 deletions

diff --git a/src/mongo/db/pipeline/accumulator_first.cpp b/src/mongo/db/pipeline/accumulator_first.cpp
index b226fb552d6..ce53cccd365 100755
--- a/src/mongo/db/pipeline/accumulator_first.cpp
+++ b/src/mongo/db/pipeline/accumulator_first.cpp
@@ -25,15 +25,19 @@ namespace mongo {
         verify(vpOperand.size() == 1);
 
         /* only remember the first value seen */
-        if (pValue.missing())
+        if (!_haveFirst) {
+            // can't use pValue.missing() since we want the first value even if missing
+            _haveFirst = true;
             pValue = vpOperand[0]->evaluate(pDocument);
+        }
 
         return pValue;
     }
 
-    AccumulatorFirst::AccumulatorFirst():
-        AccumulatorSingleValue() {
-    }
+    AccumulatorFirst::AccumulatorFirst()
+        : AccumulatorSingleValue()
+        , _haveFirst(false)
+    {}
 
     intrusive_ptr<Accumulator> AccumulatorFirst::create(
         const intrusive_ptr<ExpressionContext> &pCtx) {