[feature] mongod: add auto promote hidden

Author: honchowchen

src/mongo/db/repl/repl_set_config.h

@@ -439,6 +439,14 @@ class ReplSetConfig : private MutableReplSetConfig {
         return getSettings()->getChainingAllowed();
     }
 
+    /**
+     * Returns true if hidden nodes should be automatically promoted when all non-hidden 
+     * secondary nodes are unhealthy.
+     */
+     bool getAutoPromoteHidden() const {
+        return getSettings()->getAutoPromoteHidden();
+    }
+
     /**
      * Returns whether all members of this replica set have hostname localhost.
      */

src/mongo/db/repl/repl_set_config.idl

@@ -137,6 +137,11 @@ structs:
                 type: objectid
                 optional: true
                 validator: { callback: "validateReplicaSetIdNotNull"}
+            autoPromoteHidden:
+                type: safeBool
+                default: false
+                description: "When true, allows hidden nodes to be automatically promoted to appear
+                              in the hosts list when all non-hidden secondary nodes are unhealthy"
 
     ReplSetConfigBase:
         description: "The complete configuration for the replica set"

src/mongo/db/repl/topology_coordinator.cpp

@@ -2321,13 +2321,43 @@ void TopologyCoordinator::fillHelloForReplSet(std::shared_ptr<HelloResponse> res
 
     invariant(!_rsConfig.members().empty());
 
+    bool promoteHidden = true;
+    if (_rsConfig.getAutoPromoteHidden()) {
+        for (int i = 0; i < _rsConfig.getNumMembers(); i++) {
+            const auto& memberCfg = _rsConfig.getMemberAt(i);
+            if (memberCfg.isHidden()) {
+                // Only consider non-hidden secondaries for health check
+                continue;
+            }
+            const auto& memberData = _memberData.at(i);
+            // Check if this is a healthy secondary
+            bool isHealthySecondary = false;
+            if (i == _selfIndex) {
+                // For self, we are always "up", just check if we are secondary
+                isHealthySecondary = myState.secondary();
+            } else {
+                // For other members, check both state and health from heartbeat data
+                isHealthySecondary = memberData.getState().secondary() && memberData.up();
+            }
+            
+            if (isHealthySecondary) {
+                promoteHidden = false;
+                break;
+            }
+        }
+    } else {
+        promoteHidden = false;
+    }
+
     for (const auto& member : _rsConfig.members()) {
-        if (member.isHidden() || member.getSecondaryDelay() > Seconds{0}) {
+        if (member.getSecondaryDelay() > Seconds{0}) {
             continue;
         }
         auto hostView = member.getHostAndPort(horizonString);
 
-        if (member.isElectable()) {
+        if (member.isHidden() && promoteHidden) {
+            response->addHost(std::move(hostView));
+        } else if (member.isElectable()) {
             response->addHost(std::move(hostView));
         } else if (member.isArbiter()) {
             response->addArbiter(std::move(hostView));
@@ -2359,7 +2389,7 @@ void TopologyCoordinator::fillHelloForReplSet(std::shared_ptr<HelloResponse> res
     if (selfConfig.getSecondaryDelay() > Seconds(0)) {
         response->setSecondaryDelaySecs(selfConfig.getSecondaryDelay());
     }
-    if (selfConfig.isHidden()) {
+    if (selfConfig.isHidden() && !promoteHidden) {
         response->setIsHidden(true);
     }
     if (!selfConfig.shouldBuildIndexes()) {