SERVER-32635 an old mongos server communicating with a fully upgraded cluster should crash

2 files changed, 26 insertions, 10 deletions

diff --git a/src/mongo/rpc/protocol.cpp b/src/mongo/rpc/protocol.cpp
index fe0f9cebfcc..8d4823da8e8 100644
--- a/src/mongo/rpc/protocol.cpp
+++ b/src/mongo/rpc/protocol.cpp
@@ -211,16 +211,14 @@ Status validateWireVersion(const WireVersionInfo client, const WireVersionInfo s
     // We assert the invariant that min < max above.
     if (!(client.minWireVersion <= server.maxWireVersion &&
           client.maxWireVersion >= server.minWireVersion)) {
-        return Status(ErrorCodes::IncompatibleServerVersion,
-                      str::stream() << "Server min and max wire version are incompatible ("
-                                    << server.minWireVersion
-                                    << ","
-                                    << server.maxWireVersion
-                                    << ") with client min wire version ("
-                                    << client.minWireVersion
-                                    << ","
-                                    << client.maxWireVersion
-                                    << ")");
+        std::string errmsg = str::stream()
+            << "Server min and max wire version (" << server.minWireVersion << ","
+            << server.maxWireVersion << ") is incompatible with client min wire version ("
+            << client.minWireVersion << "," << client.maxWireVersion << ").";
+        if (client.maxWireVersion < server.minWireVersion) {
+            return Status(ErrorCodes::IncompatibleWithUpgradedServer, errmsg);
+        }
+        return Status(ErrorCodes::IncompatibleServerVersion, errmsg);
     }
 
     return Status::OK();
diff --git a/src/mongo/rpc/protocol_test.cpp b/src/mongo/rpc/protocol_test.cpp
index 1781d9fab2c..ab92e3eeae1 100644
--- a/src/mongo/rpc/protocol_test.cpp
+++ b/src/mongo/rpc/protocol_test.cpp
@@ -281,4 +281,22 @@ TEST(Protocol, validateWireVersion) {
                           WireVersion::LATEST_WIRE_VERSION - 1);
 }
 
+// A mongos is unable to communicate with a fully upgraded cluster with a higher wire version.
+TEST(Protocol, validateWireVersionFailsForUpgradedServerNode) {
+    // Server is fully upgraded higher than the latest wire version.
+    auto msg =
+        BSON("minWireVersion" << static_cast<int>(WireVersion::FUTURE_WIRE_VERSION_FOR_TESTING)
+                              << "maxWireVersion"
+                              << static_cast<int>(WireVersion::FUTURE_WIRE_VERSION_FOR_TESTING));
+    auto swReply = parseProtocolSetFromIsMasterReply(msg);
+    ASSERT_OK(swReply.getStatus());
+
+    // The client (this mongos server) only has latest wire version.
+    ASSERT_EQUALS(
+        mongo::ErrorCodes::IncompatibleWithUpgradedServer,
+        validateWireVersion({WireVersion::LATEST_WIRE_VERSION, WireVersion::LATEST_WIRE_VERSION},
+                            swReply.getValue().version)
+            .code());
+}
+
 }  // namespace