From cc923b4e66e0c76714f877b33ce9fd5f28e17f62 Mon Sep 17 00:00:00 2001
From: Matthew Saltz <matthew.saltz@mongodb.com>
Date: Tue, 7 May 2019 17:39:06 -0400
Subject: SERVER-39498 Make rollback trigger a lazy (rather than blocking)
 reload of the ShardRegistry

(cherry picked from commit d3ee35d6e3ac7a42cd5ad106c3ecb9fb554900c7)
---
 src/mongo/db/op_observer_impl.cpp     | 11 +++--------
 src/mongo/db/repl/rs_rollback.cpp     | 11 +++--------
 src/mongo/s/client/shard_registry.cpp | 10 ++++++++--
 src/mongo/s/client/shard_registry.h   |  6 ++++++
 4 files changed, 20 insertions(+), 18 deletions(-)

diff --git a/src/mongo/db/op_observer_impl.cpp b/src/mongo/db/op_observer_impl.cpp
index 7b106277c61..cb6fd810ee7 100644
--- a/src/mongo/db/op_observer_impl.cpp
+++ b/src/mongo/db/op_observer_impl.cpp
@@ -985,16 +985,11 @@ void OpObserverImpl::onReplicationRollback(OperationContext* opCtx,
         fassertFailedNoTrace(50712);
     }
 
-    // The code below will force the config server to update its shard registry.
-    // Otherwise it may have the stale data that has been just rolled back.
+    // Force the config server to update its shard registry on next access. Otherwise it may have
+    // the stale data that has been just rolled back.
     if (serverGlobalParams.clusterRole == ClusterRole::ConfigServer) {
         if (auto shardRegistry = Grid::get(opCtx)->shardRegistry()) {
-            auto& readConcernArgs = repl::ReadConcernArgs::get(opCtx);
-            ON_BLOCK_EXIT([ argsCopy = readConcernArgs, &readConcernArgs ] {
-                readConcernArgs = std::move(argsCopy);
-            });
-            readConcernArgs = repl::ReadConcernArgs(repl::ReadConcernLevel::kLocalReadConcern);
-            shardRegistry->reload(opCtx);
+            shardRegistry->clearEntries();
         }
     }
 }
diff --git a/src/mongo/db/repl/rs_rollback.cpp b/src/mongo/db/repl/rs_rollback.cpp
index 65ba8fff86b..e95fcaf4044 100644
--- a/src/mongo/db/repl/rs_rollback.cpp
+++ b/src/mongo/db/repl/rs_rollback.cpp
@@ -1467,16 +1467,11 @@ void rollback_internal::syncFixUp(OperationContext* opCtx,
         validator->resetKeyManagerCache();
     }
 
-    // The code below will force the config server to update its shard registry.
-    // Otherwise it may have the stale data that has been just rolled back.
+    // Force the config server to update its shard registry on next access. Otherwise it may have
+    // the stale data that has been just rolled back.
     if (serverGlobalParams.clusterRole == ClusterRole::ConfigServer) {
         if (auto shardRegistry = Grid::get(opCtx)->shardRegistry()) {
-            auto& readConcernArgs = repl::ReadConcernArgs::get(opCtx);
-            ON_BLOCK_EXIT([ argsCopy = readConcernArgs, &readConcernArgs ] {
-                readConcernArgs = std::move(argsCopy);
-            });
-            readConcernArgs = repl::ReadConcernArgs(repl::ReadConcernLevel::kLocalReadConcern);
-            shardRegistry->reload(opCtx);
+            shardRegistry->clearEntries();
         }
     }
 
diff --git a/src/mongo/s/client/shard_registry.cpp b/src/mongo/s/client/shard_registry.cpp
index e2c5ec5573f..265c8aed6c6 100644
--- a/src/mongo/s/client/shard_registry.cpp
+++ b/src/mongo/s/client/shard_registry.cpp
@@ -66,17 +66,17 @@
 
 namespace mongo {
 
-using std::shared_ptr;
 using std::set;
+using std::shared_ptr;
 using std::string;
 using std::unique_ptr;
 using std::vector;
 
 using executor::NetworkInterface;
 using executor::NetworkInterfaceThreadPool;
+using executor::TaskExecutor;
 using executor::TaskExecutorPool;
 using executor::ThreadPoolTaskExecutor;
-using executor::TaskExecutor;
 using CallbackArgs = TaskExecutor::CallbackArgs;
 using CallbackHandle = TaskExecutor::CallbackHandle;
 
@@ -350,6 +350,12 @@ void ShardRegistry::replicaSetChangeShardRegistryUpdateHook(
     Grid::get(getGlobalServiceContext())->shardRegistry()->updateReplSetHosts(connString);
 }
 
+void ShardRegistry::clearEntries() {
+    ShardRegistryData empty;
+    empty.addConfigShard(_data.getConfigShard());
+    _data.swap(empty);
+}
+
 void ShardRegistry::replicaSetChangeConfigServerUpdateHook(const std::string& setName,
                                                            const std::string& newConnectionString) {
     // This is run in it's own thread. Exceptions escaping would result in a call to terminate.
diff --git a/src/mongo/s/client/shard_registry.h b/src/mongo/s/client/shard_registry.h
index 5004b271a90..8371b1aff8a 100644
--- a/src/mongo/s/client/shard_registry.h
+++ b/src/mongo/s/client/shard_registry.h
@@ -181,6 +181,12 @@ public:
      */
     bool reload(OperationContext* opCtx);
 
+    /**
+     * Clears all entries from the shard registry entries, which will force the registry to do a
+     * reload on next access.
+     */
+    void clearEntries();
+
     /**
      * Takes a connection string describing either a shard or config server replica set, looks
      * up the corresponding Shard object based on the replica set name, then updates the
-- 
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