Info

MongoDB config server will crash and can't  start normally in a  cluster which is upgrade from 6.0 version。The crash log is as follows: with 7.0+ version: {"t":{"$date":"2026-03-19T17:15:03.886+08:00"},"s":"F", "c":"ASSERT", "id":23079, "ctx":"ReplWriterWorker-2","msg":"Invariant failure","attr":{"expr":"erased","file":"src/mongo/db/s/query_analysis_coordinator.cpp","line":164}} with 8.0+ version: {"t":{"$date":"2026-03-17T21:21:24.897+08:00"},"s":"F", "c":"ASSERT", "id":23079, "svc":"S", "ctx":"ReplWriterWorker-3","msg":"Invariant failure","attr":{"expr":"erased","file":"src/mongo/db/s/query_analysis_coordinator.cpp","line":189}} The main reason for this is that QueryAnalysisCoordinator records `_samplers` when inserting documents into `config.mongos` and cleans up `_samplers` when deleting records. There's also an invariant check after `QueryAnalysisCoordinator::onSamplerDelete _samplers.erase`. However, for clusters upgraded from local versions, `config.mongos` retains information from older versions. These records haven't been inserted after the upgrade, so they're not recorded in `_samplers`. This causes `_samplers.erase` to return 0 during deletion, leading to invariant failure and process crash. void QueryAnalysisCoordinator::onSamplerDelete(const MongosType& doc) { invariant(serverGlobalParams.clusterRole.has(ClusterRole::ConfigServer)); stdx::lock_guard lk(_mutex); auto erased = _samplers.erase(doc.getName()); invariant(erased); } I think we need to optimize the logic and remove `invariant(erased)`.

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Steps to Reproduce

create an cluster of version  6.0.27  with a mongos Ensure that config.mongos contains a mongos record. Upgrade the cluster to version 7.0 in the order described Connect to the cluster using mongosh, execute db.getSiblingDB("config").mongos.remove({}), and then check your config server . The config servers will likely crash and remain unrecoverable until we intervene and correct the oplog data.