SERVER-25717 Negative push index out of bounds should default to beginning of array

2 files changed, 9 insertions, 6 deletions

diff --git a/src/mongo/db/ops/modifier_push.cpp b/src/mongo/db/ops/modifier_push.cpp
index e2298b25ba8..d2d816776fa 100644
--- a/src/mongo/db/ops/modifier_push.cpp
+++ b/src/mongo/db/ops/modifier_push.cpp
@@ -557,11 +557,14 @@ Status ModifierPush::apply() const {
     if (actualPosition < 0) {
         actualPosition = _preparedState->arrayPreModSize + _position;
     }
-    // Default to adding to the end of the array, even if we're out of bounds because a negative
-    // position was too negative.
-    if (actualPosition < 0 || actualPosition > int32_t(_preparedState->arrayPreModSize)) {
+    // Default to adding to the end of the array if the position was too high.
+    if (actualPosition > int32_t(_preparedState->arrayPreModSize)) {
         actualPosition = _preparedState->arrayPreModSize;
     }
+    // Default to adding to the beginning of the array if the position was too low.
+    if (actualPosition < 0) {
+        actualPosition = 0;
+    }
     _preparedState->actualPosition = actualPosition;
 
     // 2. Add new elements to the array either by going over the $each array or by
diff --git a/src/mongo/db/ops/modifier_push_test.cpp b/src/mongo/db/ops/modifier_push_test.cpp
index 8ef8e1488cd..c4a05aafa7f 100644
--- a/src/mongo/db/ops/modifier_push_test.cpp
+++ b/src/mongo/db/ops/modifier_push_test.cpp
@@ -1535,7 +1535,7 @@ TEST(ToPosition, NegativePositionMultiElementArray) {
     ASSERT_EQUALS(fromjson("{$set: {'a': [0, 1, 2, 5, 3, 4]}}"), logDoc);
 }
 
-TEST(ToPosition, NegativePositionOutOfBoundsDefaultsToEnd) {
+TEST(ToPosition, NegativePositionOutOfBoundsDefaultsToBeginning) {
     Document doc(fromjson("{a: [0]}"));
     Mod pushMod(fromjson("{$push: {a: {$each: [1], $position: -2}}}"));
 
@@ -1547,13 +1547,13 @@ TEST(ToPosition, NegativePositionOutOfBoundsDefaultsToEnd) {
 
     ASSERT_OK(pushMod.apply());
     ASSERT_FALSE(doc.isInPlaceModeEnabled());
-    ASSERT_EQUALS(fromjson("{a: [0, 1]}"), doc);
+    ASSERT_EQUALS(fromjson("{a: [1, 0]}"), doc);
 
     Document logDoc;
     LogBuilder logBuilder(logDoc.root());
     ASSERT_OK(pushMod.log(&logBuilder));
     ASSERT_EQUALS(countChildren(logDoc.root()), 1u);
-    ASSERT_EQUALS(fromjson("{$set: {'a.1': 1}}"), logDoc);
+    ASSERT_EQUALS(fromjson("{$set: {'a': [1, 0]}}"), logDoc);
 }
 
 TEST(ToPosition, NegativePositionPushMultipleElements) {