SERVER-50412 implement helloOk protocol negotiation on mongod

(cherry picked from commit 1118ad680f76ac29795c8396beb11b0b50322f20)
(cherry picked from commit 4dbefcfc9304a9ffa1fec8390dc2bd30e0dc9a54)

3 files changed, 46 insertions, 3 deletions

diff --git a/src/mongo/db/client.h b/src/mongo/db/client.h
index 2c06e0e1453..7d63bb39b9f 100644
--- a/src/mongo/db/client.h
+++ b/src/mongo/db/client.h
@@ -243,6 +243,22 @@ public:
         return _killed.loadRelaxed();
     }
 
+    /**
+     * Whether this client supports the hello command, which indicates that the server
+     * can return "not primary" error messages.
+     */
+    bool supportsHello() const {
+        return _supportsHello;
+    }
+
+    /**
+     * Will be set to true if the client sent { helloOk: true } when opening a
+     * connection to the server. Defaults to false.
+     */
+    void setSupportsHello(bool newVal) {
+        _supportsHello = newVal;
+    }
+
 private:
     friend class ServiceContext;
     friend class ThreadClient;
@@ -274,6 +290,10 @@ private:
     PseudoRandom _prng;
 
     AtomicWord<bool> _killed{false};
+
+    // Whether this client used { helloOk: true } when opening its connection, indicating that
+    // it supports the hello command.
+    bool _supportsHello = false;
 };
 
 /**
diff --git a/src/mongo/db/repl/replication_info.cpp b/src/mongo/db/repl/replication_info.cpp
index 594baf38b59..2446336e382 100644
--- a/src/mongo/db/repl/replication_info.cpp
+++ b/src/mongo/db/repl/replication_info.cpp
@@ -34,6 +34,7 @@
 #include <vector>
 
 #include "mongo/base/string_data.h"
+#include "mongo/bson/util/bson_extract.h"
 #include "mongo/client/connpool.h"
 #include "mongo/client/dbclient_connection.h"
 #include "mongo/db/auth/sasl_mechanism_registry.h"
@@ -370,6 +371,23 @@ public:
 
         appendReplicationInfo(opCtx, result, 0, useLegacyResponseFields());
 
+        // Try to parse the optional 'helloOk' field. This should be provided on the initial
+        // handshake for an incoming connection if the client supports the hello command. Clients
+        // that specify 'helloOk' do not rely on "not master" error message parsing, which means
+        // that we can safely return "not primary" error messages instead.
+        bool helloOk = opCtx->getClient()->supportsHello();
+        Status status = bsonExtractBooleanField(cmdObj, "helloOk", &helloOk);
+        if (status.isOK()) {
+            // If the hello request contains a "helloOk" field, set _supportsHello on the Client
+            // to the value.
+            opCtx->getClient()->setSupportsHello(helloOk);
+            // Attach helloOk: true to the response so that the client knows the server supports
+            // the hello command.
+            result.append("helloOk", true);
+        } else if (status.code() != ErrorCodes::NoSuchKey) {
+            uassertStatusOK(status);
+        }
+
         if (serverGlobalParams.clusterRole == ClusterRole::ConfigServer) {
             const int configServerModeNumber = 2;
             result.append("configsvr", configServerModeNumber);
diff --git a/src/mongo/db/service_entry_point_common.cpp b/src/mongo/db/service_entry_point_common.cpp
index 9a4b200d37a..e6997ed7877 100644
--- a/src/mongo/db/service_entry_point_common.cpp
+++ b/src/mongo/db/service_entry_point_common.cpp
@@ -775,13 +775,18 @@ void execCommandDatabase(OperationContext* opCtx,
                 couldHaveOptedIn && ReadPreferenceSetting::get(opCtx).canRunOnSecondary();
             bool canRunHere = commandCanRunHere(opCtx, dbname, command, inMultiDocumentTransaction);
             if (!canRunHere && couldHaveOptedIn) {
-                uasserted(ErrorCodes::NotPrimaryNoSecondaryOk, "not master and slaveOk=false");
+                const auto msg = opCtx->getClient()->supportsHello()
+                    ? "not primary and secondaryOk=false"_sd
+                    : "not master and slaveOk=false"_sd;
+                uasserted(ErrorCodes::NotPrimaryNoSecondaryOk, msg);
             }
 
             if (MONGO_FAIL_POINT(respondWithNotPrimaryInCommandDispatch)) {
                 uassert(ErrorCodes::NotWritablePrimary, "not primary", canRunHere);
             } else {
-                uassert(ErrorCodes::NotWritablePrimary, "not master", canRunHere);
+                const auto msg =
+                    opCtx->getClient()->supportsHello() ? "not primary"_sd : "not master"_sd;
+                uassert(ErrorCodes::NotWritablePrimary, msg, canRunHere);
             }
 
             if (!command->maintenanceOk() &&
@@ -1074,7 +1079,7 @@ DbResponse receivedCommands(OperationContext* opCtx,
             notPrimaryUnackWrites.increment();
             uasserted(ErrorCodes::NotWritablePrimary,
                       str::stream()
-                          << "Not-master error while processing '" << request.getCommandName()
+                          << "Not-primary error while processing '" << request.getCommandName()
                           << "' operation  on '" << request.getDatabase() << "' database via "
                           << "fire-and-forget command execution.");
         }