SERVER-34218 Always reinitialize fields in FieldRef::parse().

Previously, calling FieldRef::FieldRef(StringData) never initialized
_cachedSize, resulting in unpredictable build failures.

Additionally, calling FieldRef::parse(StringData) with the empty
string did not reinitialize the FieldRef.

(cherry picked from commit c22357338a6f192deb3e24e2a07672419cc7dacb)

2 files changed, 12 insertions, 5 deletions

diff --git a/src/mongo/db/field_ref.cpp b/src/mongo/db/field_ref.cpp
index 3a759febe3c..3070aafebb3 100644
--- a/src/mongo/db/field_ref.cpp
+++ b/src/mongo/db/field_ref.cpp
@@ -38,19 +38,17 @@ const size_t FieldRef::kReserveAhead;
 
 FieldRef::FieldRef() : _size(0), _cachedSize(0) {}
 
-FieldRef::FieldRef(StringData path) : _size(0) {
+FieldRef::FieldRef(StringData path) {
     parse(path);
 }
 
 void FieldRef::parse(StringData path) {
+    clear();
+
     if (path.size() == 0) {
         return;
     }
 
-    if (_size != 0) {
-        clear();
-    }
-
     // We guarantee that accesses through getPart() will be valid while 'this' is. So we
     // keep a copy in a local sting.
 
diff --git a/src/mongo/db/field_ref_test.cpp b/src/mongo/db/field_ref_test.cpp
index fe28c5fc5e1..60c143d5da8 100644
--- a/src/mongo/db/field_ref_test.cpp
+++ b/src/mongo/db/field_ref_test.cpp
@@ -76,6 +76,15 @@ TEST(Empty, EmptyFieldName) {
     ASSERT_EQUALS(fieldRef.dottedField(), field);
 }
 
+TEST(Empty, ReinitializeWithEmptyString) {
+    FieldRef fieldRef("a.b.c.d.e");
+    ASSERT_EQUALS(fieldRef.numParts(), 5U);
+
+    fieldRef.parse("");
+    ASSERT_EQUALS(fieldRef.numParts(), 0U);
+    ASSERT_EQUALS(fieldRef.dottedField(), "");
+}
+
 TEST(Normal, SinglePart) {
     string field = "a";
     FieldRef fieldRef(field);