SERVER-62827 Prevent rpc::upconvertRequest from erroneously duplicating top-level $readPreference field in command object

2 files changed, 16 insertions, 0 deletions

diff --git a/src/mongo/rpc/metadata.cpp b/src/mongo/rpc/metadata.cpp
index ba002cdc748..666e0584583 100644
--- a/src/mongo/rpc/metadata.cpp
+++ b/src/mongo/rpc/metadata.cpp
@@ -158,6 +158,9 @@ OpMsgRequest upconvertRequest(StringData db, BSONObj cmdObj, int queryFlags) {
     }
 
     if (!readPrefContainer.isEmpty()) {
+        uassert(ErrorCodes::InvalidOptions,
+                "Duplicate readPreference found in command object.",
+                !cmdObj.hasField("$readPreference"));
         cmdObj = BSONObjBuilder(std::move(cmdObj)).appendElements(readPrefContainer).obj();
     } else if (!cmdObj.hasField("$readPreference") && (queryFlags & QueryOption_SecondaryOk)) {
         BSONObjBuilder bodyBuilder(std::move(cmdObj));
diff --git a/src/mongo/rpc/metadata_test.cpp b/src/mongo/rpc/metadata_test.cpp
index d35a37d1b8d..898b98c745a 100644
--- a/src/mongo/rpc/metadata_test.cpp
+++ b/src/mongo/rpc/metadata_test.cpp
@@ -149,6 +149,19 @@ TEST(Metadata, UpconvertInvalidMetadata) {
                   AssertionException);
 }
 
+TEST(Metadata, UpconvertDuplicateReadPreference) {
+    auto secondaryReadPref = BSON("mode"
+                                  << "secondary");
+    auto nearestReadPref = BSON("mode"
+                                << "nearest");
+
+    BSONObjBuilder bob;
+    bob.append("query", BSON("$readPreference" << secondaryReadPref));
+    bob.append("$readPreference", nearestReadPref);
+
+    ASSERT_THROWS_CODE(
+        rpc::upconvertRequest("db", bob.obj(), 0), AssertionException, ErrorCodes::InvalidOptions);
+}
 
 TEST(Metadata, UpconvertUsesDocumentSequecesCorrectly) {
     // These are cases where it is valid to use document sequences.