SERVER-41994 correctly create and show type 2 binary elements

1 files changed, 12 insertions, 3 deletions

diff --git a/src/mongo/bson/bsonelement.h b/src/mongo/bson/bsonelement.h
index 892be6df831..d7733b7966a 100644
--- a/src/mongo/bson/bsonelement.h
+++ b/src/mongo/bson/bsonelement.h
@@ -494,9 +494,18 @@ public:
         if (binDataType() != ByteArrayDeprecated) {
             return binData(len);
         } else {
-            // Skip extra size
-            len = valuestrsize() - 4;
-            return value() + 5 + 4;
+            // Because, for some time, the shell has incorrectly created type 2 binary objects
+            // without the extra length, we try to identify if this object does or doesn't start
+            // with a length and skip past it when its present. See SERVER-41994
+            if (valuestrsize() >= 4 &&
+                ConstDataView(value() + 5).read<LittleEndian<int>>() == valuestrsize() - 4) {
+                // Skip extra size
+                len = valuestrsize() - 4;
+                return value() + 5 + 4;
+            } else {
+                len = valuestrsize();
+                return value() + 5;
+            }
         }
     }