SERVER-26462 Check if _buffer is allocated in DocumentStorage::clone()

jstests/aggregation/bugs/server26462.js

@@ -0,0 +1,29 @@
+// Tests that adding a field that only contains metadata does not cause a segmentation fault when
+// grouping on the added field.
+(function() {
+    "use strict";
+
+    // Drop the old test collection, if any.
+    db.server26462.drop();
+
+    // Insert some test documents into the collection.
+    assert.writeOK(db.server26462.insert({"_id": 1, "title": "cakes and ale"}));
+    assert.writeOK(db.server26462.insert({"_id": 2, "title": "more cakes"}));
+    assert.writeOK(db.server26462.insert({"_id": 3, "title": "bread"}));
+    assert.writeOK(db.server26462.insert({"_id": 4, "title": "some cakes"}));
+
+    // Create a text index on the documents.
+    assert.commandWorked(db.server26462.createIndex({title: "text"}));
+
+    // Add a metadata only field in the aggregation pipeline and use that field in the $group _id.
+    let res = db.server26462
+                  .aggregate([
+                      {$match: {$text: {$search: "cake"}}},
+                      {$addFields: {fooScore: {$meta: "textScore"}}},
+                      {$group: {_id: "$fooScore", count: {$sum: 1}}}
+                  ])
+                  .itcount();
+
+    // Assert that the command worked.
+    assert.eq(2, res);
+})();

src/mongo/db/pipeline/document.cpp

@@ -186,10 +186,12 @@ intrusive_ptr<DocumentStorage> DocumentStorage::clone() const {
 
     // Make a copy of the buffer.
     // It is very important that the positions of each field are the same after cloning.
-    const size_t bufferBytes = (_bufferEnd + hashTabBytes()) - _buffer;
+    const size_t bufferBytes = allocatedBytes();
     out->_buffer = new char[bufferBytes];
     out->_bufferEnd = out->_buffer + (_bufferEnd - _buffer);
-    memcpy(out->_buffer, _buffer, bufferBytes);
+    if (bufferBytes > 0) {
+        memcpy(out->_buffer, _buffer, bufferBytes);
+    }
 
     // Copy remaining fields
     out->_usedBytes = _usedBytes;

src/mongo/db/pipeline/document_value_test.cpp

@@ -104,6 +104,14 @@ TEST(DocumentConstruction, FromInitializerList) {
     ASSERT_EQUALS("q", getNthField(document, 1).second.getString());
 }
 
+TEST(DocumentConstruction, FromEmptyDocumentClone) {
+    Document document;
+    ASSERT_EQUALS(0U, document.size());
+    // Prior to SERVER-26462, cloning an empty document would cause a segmentation fault.
+    Document documentClone = document.clone();
+    ASSERT_DOCUMENT_EQ(document, documentClone);
+}
+
 /** Add Document fields. */
 class AddField {
 public: