SERVER-40355 Handle RS config with _id larger than set sizer4.0.8-rc0r4.0.8

2 files changed, 38 insertions, 1 deletions

diff --git a/jstests/noPassthrough/member_id_too_large.js b/jstests/noPassthrough/member_id_too_large.js
new file mode 100644
index 00000000000..9e514d49a8e
--- /dev/null
+++ b/jstests/noPassthrough/member_id_too_large.js
@@ -0,0 +1,37 @@
+// Tests replSetInitiate and replSetReconfig with member _id values greater than the number of
+// members in the set, followed by waiting for writeConcern with "w" values equal to size of set.
+// @tags: [requires_replication]
+(function() {
+    "use strict";
+
+    const rst = new ReplSetTest({nodes: 2});
+    rst.startSet();
+
+    jsTestLog("replSetInitiate with member _id greater than number of members");
+
+    let conf = rst.getReplSetConfig();
+    conf.members[1]._id = 2;
+
+    rst.initiate(conf);
+
+    const dbName = "test";
+    const collName = "test";
+    const primary = rst.getPrimary();
+    const testColl = primary.getDB(dbName).getCollection(collName);
+    const doc = {a: 1};
+
+    assert.commandWorked(testColl.insert(doc, {writeConcern: {w: 2}}));
+
+    jsTestLog("replSetReconfig with member _id greater than number of members");
+
+    let secondary2 = MongoRunner.runMongod({replSet: rst.name});
+    conf = rst.getReplSetConfigFromNode();
+    conf.version++;
+    conf.members.push({_id: 5, host: secondary2.host});
+    assert.commandWorked(primary.getDB("admin").runCommand({replSetReconfig: conf}));
+    assert.commandWorked(testColl.insert(doc, {writeConcern: {w: 2}}));
+    assert.commandWorked(testColl.insert(doc, {writeConcern: {w: 3}}));
+
+    MongoRunner.stopMongod(secondary2);
+    rst.stopSet();
+})();
diff --git a/src/mongo/db/repl/topology_coordinator.cpp b/src/mongo/db/repl/topology_coordinator.cpp
index 1d1d242ed14..b6fb47ac5fa 100644
--- a/src/mongo/db/repl/topology_coordinator.cpp
+++ b/src/mongo/db/repl/topology_coordinator.cpp
@@ -1174,7 +1174,7 @@ bool TopologyCoordinator::haveNumNodesReachedOpTime(const OpTime& targetOpTime,
     }
 
     for (auto&& memberData : _memberData) {
-        const auto isArbiter = _rsConfig.getMemberAt(memberData.getMemberId()).isArbiter();
+        const auto isArbiter = _rsConfig.getMemberAt(memberData.getConfigIndex()).isArbiter();
 
         // We do not count arbiters towards the write concern.
         if (isArbiter) {