2 files changed, 40 insertions, 9 deletions

diff --git a/src/mongo/db/ops/path_support.cpp b/src/mongo/db/ops/path_support.cpp
index 0523ef6acbb..e430c7d57cc 100644
--- a/src/mongo/db/ops/path_support.cpp
+++ b/src/mongo/db/ops/path_support.cpp
@@ -59,15 +59,16 @@ bool isNumeric(StringData str, size_t* num) {
 Status maybePadTo(mutablebson::Element* elemArray, size_t sizeRequired) {
     dassert(elemArray->getType() == Array);
 
-    if (sizeRequired > kMaxPaddingAllowed) {
-        return Status(ErrorCodes::CannotBackfillArray,
-                      mongoutils::str::stream() << "can't backfill array to larger than "
-                                                << kMaxPaddingAllowed << " elements");
-    }
-
     size_t currSize = mutablebson::countChildren(*elemArray);
     if (sizeRequired > currSize) {
         size_t toPad = sizeRequired - currSize;
+
+        if (toPad > kMaxPaddingAllowed) {
+            return Status(ErrorCodes::CannotBackfillArray,
+                          mongoutils::str::stream() << "can't backfill more than "
+                                                    << kMaxPaddingAllowed << " elements");
+        }
+
         for (size_t i = 0; i < toPad; i++) {
             Status status = elemArray->appendNull("");
             if (!status.isOK()) {
diff --git a/src/mongo/db/ops/path_support_test.cpp b/src/mongo/db/ops/path_support_test.cpp
index 475c6552fc3..310a6eb2f72 100644
--- a/src/mongo/db/ops/path_support_test.cpp
+++ b/src/mongo/db/ops/path_support_test.cpp
@@ -455,9 +455,9 @@ TEST_F(ArrayDoc, ArrayPaddingNecessary) {
 }
 
 TEST_F(ArrayDoc, ExcessivePaddingRequested) {
-    // Try to create an array item beyond what we're allowed to pad.
-    string paddedField = stream() << "b." << mongo::pathsupport::kMaxPaddingAllowed + 1;
-    ;
+    // Try to create an array item beyond what we're allowed to pad. The index is two beyond the max
+    // padding since the array already has one element.
+    string paddedField = stream() << "b." << mongo::pathsupport::kMaxPaddingAllowed + 2;
     setField(paddedField);
 
     size_t idxFound;
@@ -473,6 +473,36 @@ TEST_F(ArrayDoc, ExcessivePaddingRequested) {
     ASSERT_EQUALS(status.code(), ErrorCodes::CannotBackfillArray);
 }
 
+TEST_F(ArrayDoc, ExcessivePaddingNotRequestedIfArrayAlreadyPadded) {
+    // We will try to set an array element whose index is 5 beyond the max padding.
+    string paddedField = stream() << "a." << mongo::pathsupport::kMaxPaddingAllowed + 5;
+    setField(paddedField);
+
+    // Add 5 elements to the array.
+    for (size_t i = 0; i < 5; ++i) {
+        Element arrayA = doc().root().leftChild();
+        ASSERT_EQ(arrayA.getFieldName(), "a");
+        ASSERT_EQ(arrayA.getType(), mongo::Array);
+        arrayA.appendInt("", 1);
+    }
+
+    size_t idxFound;
+    Element elemFound = root();
+    ASSERT_OK(findLongestPrefix(field(), root(), &idxFound, &elemFound));
+    ASSERT_TRUE(elemFound.ok());
+    ASSERT_EQUALS(countChildren(elemFound), 5u);
+
+    Element newElem = doc().makeElementInt("", 99);
+    ASSERT_TRUE(newElem.ok());
+
+    ASSERT_OK(createPathAt(field(), idxFound + 1, elemFound, newElem));
+
+    // Array should now have maxPadding + 6 elements, since the highest array index is maxPadding +
+    // 5. maxPadding of these elements are nulls adding as padding, 5 were appended at the
+    // beginning, and 1 was added by createPathAt().
+    ASSERT_EQ(countChildren(doc().root().leftChild()), mongo::pathsupport::kMaxPaddingAllowed + 6);
+}
+
 TEST_F(ArrayDoc, NonNumericPathInArray) {
     setField("b.z");