SERVER-44132 DataBuilder move assignment computes size incorrectly

2 files changed, 6 insertions, 3 deletions

diff --git a/src/mongo/base/data_builder.h b/src/mongo/base/data_builder.h
index 92a0ba73d0a..aa7cb29154b 100644
--- a/src/mongo/base/data_builder.h
+++ b/src/mongo/base/data_builder.h
@@ -78,9 +78,12 @@ public:
     }
 
     DataBuilder& operator=(DataBuilder&& other) {
+        size_t size = other.size();
         _buf = std::move(other._buf);
         _capacity = other._capacity;
-        _unwrittenSpaceCursor = {_buf.get(), _buf.get() + other.size()};
+        char* start = _buf.get() + size;
+        char* end = _buf.get() + _capacity;
+        _unwrittenSpaceCursor = {start, end};
 
         other._capacity = 0;
         other._unwrittenSpaceCursor = {nullptr, nullptr};
diff --git a/src/mongo/base/data_builder_test.cpp b/src/mongo/base/data_builder_test.cpp
index ed001bd31b1..0133ae20273 100644
--- a/src/mongo/base/data_builder_test.cpp
+++ b/src/mongo/base/data_builder_test.cpp
@@ -151,7 +151,7 @@ TEST(DataBuilder, Clear) {
 }
 
 TEST(DataBuilder, Move) {
-    DataBuilder db(1);
+    DataBuilder db(42);
 
     ASSERT_EQUALS(true, db.writeAndAdvance<uint16_t>(1).isOK());
 
@@ -160,7 +160,7 @@ TEST(DataBuilder, Move) {
     ConstDataRangeCursor cdrc = db2.getCursor();
 
     ASSERT_EQUALS(static_cast<uint16_t>(1), cdrc.readAndAdvance<uint16_t>());
-    ASSERT_EQUALS(2u, db2.capacity());
+    ASSERT_EQUALS(42u, db2.capacity());
     ASSERT_EQUALS(2u, db2.size());
 
     ASSERT_EQUALS(0u, db.capacity());