SERVER-38178 - Fix bug with data_type_terminated that caused seg-faults on data_builder resizing

2 files changed, 15 insertions, 1 deletions

diff --git a/src/mongo/base/data_builder_test.cpp b/src/mongo/base/data_builder_test.cpp
index 6b7e0655f61..d497e78622e 100644
--- a/src/mongo/base/data_builder_test.cpp
+++ b/src/mongo/base/data_builder_test.cpp
@@ -29,6 +29,7 @@
  */
 
 #include "mongo/base/data_builder.h"
+#include "mongo/base/data_type_terminated.h"
 
 #include "mongo/platform/endian.h"
 #include "mongo/unittest/unittest.h"
@@ -169,4 +170,15 @@ TEST(DataBuilder, Move) {
     ASSERT(!db.getCursor().data());
 }
 
+TEST(DataBuilder, TerminatedStringDatas) {
+    DataBuilder db{10};
+    StringData sample{"abcdefgh"};
+
+    auto status2 = db.writeAndAdvance<Terminated<'\0', StringData>>(sample);
+    ASSERT_EQUALS(true, status2.isOK());
+
+    auto status3 = db.writeAndAdvance<Terminated<'\0', StringData>>(sample);
+    ASSERT_EQUALS(true, status3.isOK());
+}
+
 }  // namespace mongo
diff --git a/src/mongo/base/data_type_terminated.h b/src/mongo/base/data_type_terminated.h
index b16bc6ae30d..cfe00236b83 100644
--- a/src/mongo/base/data_type_terminated.h
+++ b/src/mongo/base/data_type_terminated.h
@@ -109,7 +109,9 @@ struct DataType::Handler<Terminated<C, T>> {
             return TerminatedHelper::makeStoreStatus(C, length, debug_offset + local_advanced);
         }
 
-        ptr[local_advanced] = C;
+        if (ptr) {
+            ptr[local_advanced] = C;
+        }
 
         if (advanced) {
             *advanced = local_advanced + 1;