SERVER-37557 Add startup warning about enableMajorityReadConcern and usage of arbiters

(cherry picked from commit 1f40ed56b27d9c98c23eb601d6b06116b194dfe7)

3 files changed, 25 insertions, 0 deletions

diff --git a/src/mongo/db/repl/repl_set_config.cpp b/src/mongo/db/repl/repl_set_config.cpp
index 6917253e196..1c6cedbdbda 100644
--- a/src/mongo/db/repl/repl_set_config.cpp
+++ b/src/mongo/db/repl/repl_set_config.cpp
@@ -869,5 +869,14 @@ int ReplSetConfig::calculatePriorityRank(double priority) const {
     return count;
 }
 
+bool ReplSetConfig::containsArbiter() const {
+    for (MemberIterator mem = membersBegin(); mem != membersEnd(); mem++) {
+        if (mem->isArbiter()) {
+            return true;
+        }
+    }
+    return false;
+}
+
 }  // namespace repl
 }  // namespace mongo
diff --git a/src/mongo/db/repl/repl_set_config.h b/src/mongo/db/repl/repl_set_config.h
index 53621d2e6df..bdcbe58d3ab 100644
--- a/src/mongo/db/repl/repl_set_config.h
+++ b/src/mongo/db/repl/repl_set_config.h
@@ -358,6 +358,11 @@ public:
      */
     int calculatePriorityRank(double priority) const;
 
+    /**
+     * Returns true if this replica set has at least one arbiter.
+     */
+    bool containsArbiter() const;
+
 private:
     /**
      * Parses the "settings" subdocument of a replica set configuration.
diff --git a/src/mongo/db/repl/replication_coordinator_impl.cpp b/src/mongo/db/repl/replication_coordinator_impl.cpp
index be73f59bacf..5ddda1bc260 100644
--- a/src/mongo/db/repl/replication_coordinator_impl.cpp
+++ b/src/mongo/db/repl/replication_coordinator_impl.cpp
@@ -572,6 +572,17 @@ void ReplicationCoordinatorImpl::_finishLoadLocalConfig(
         myIndex = StatusWith<int>(-1);
     }
 
+    if (serverGlobalParams.enableMajorityReadConcern && localConfig.containsArbiter()) {
+        log() << startupWarningsLog;
+        log() << "** WARNING: This replica set uses arbiters, but readConcern:majority is enabled "
+              << startupWarningsLog;
+        log() << "**          for this node. This is not a recommended configuration. Please see "
+              << startupWarningsLog;
+        log() << "**          https://dochub.mongodb.org/core/psa-disable-rc-majority"
+              << startupWarningsLog;
+        log() << startupWarningsLog;
+    }
+
     // Do not check optime, if this node is an arbiter.
     bool isArbiter =
         myIndex.getValue() != -1 && localConfig.getMemberAt(myIndex.getValue()).isArbiter();