1 files changed, 53 insertions, 2 deletions

diff --git a/src/mongo/db/s/config/configsvr_reshard_collection_cmd.cpp b/src/mongo/db/s/config/configsvr_reshard_collection_cmd.cpp
index 5586c71a25d..9830b44c5db 100644
--- a/src/mongo/db/s/config/configsvr_reshard_collection_cmd.cpp
+++ b/src/mongo/db/s/config/configsvr_reshard_collection_cmd.cpp
@@ -48,8 +48,35 @@
 #include "mongo/s/resharding/resharding_feature_flag_gen.h"
 
 namespace mongo {
+
+MONGO_FAIL_POINT_DEFINE(reshardCollectionJoinedExistingOperation);
+
 namespace {
 
+// Returns a resharding instance if one exists for the same collection and shard key already.
+boost::optional<std::shared_ptr<ReshardingCoordinatorService::ReshardingCoordinator>>
+getExistingInstanceToJoin(OperationContext* opCtx,
+                          const NamespaceString& nss,
+                          const BSONObj& newShardKey) {
+    auto reshardingCoordinatorService = checked_cast<ReshardingCoordinatorService*>(
+        repl::PrimaryOnlyServiceRegistry::get(opCtx->getServiceContext())
+            ->lookupServiceByName(ReshardingCoordinatorService::kServiceName));
+    auto instances = reshardingCoordinatorService->getAllReshardingInstances(opCtx);
+    for (auto& instance : instances) {
+        auto reshardingCoordinator =
+            checked_pointer_cast<ReshardingCoordinatorService::ReshardingCoordinator>(instance);
+
+        auto instanceMetadata = reshardingCoordinator->getMetadata();
+        if (SimpleBSONObjComparator::kInstance.evaluate(
+                instanceMetadata.getReshardingKey().toBSON() == newShardKey) &&
+            instanceMetadata.getSourceNss() == nss) {
+            return reshardingCoordinator;
+        };
+    }
+
+    return boost::none;
+}
+
 class ConfigsvrReshardCollectionCommand final
     : public TypedCommand<ConfigsvrReshardCollectionCommand> {
 public:
@@ -109,7 +136,9 @@ public:
                     *presetChunks, opCtx, ShardKeyPattern(request().getKey()).getKeyPattern());
             }
 
-            auto instance = ([&] {
+            // Returns boost::none if there isn't any work to be done by the resharding operation.
+            auto instance = ([&]() -> boost::optional<std::shared_ptr<
+                                       ReshardingCoordinatorService::ReshardingCoordinator>> {
                 FixedFCVRegion fixedFcv(opCtx);
 
                 uassert(ErrorCodes::CommandNotSupported,
@@ -117,10 +146,28 @@ public:
                         resharding::gFeatureFlagResharding.isEnabled(
                             serverGlobalParams.featureCompatibility));
 
+                if (auto existingInstance =
+                        getExistingInstanceToJoin(opCtx, nss, request().getKey())) {
+                    // Join the existing resharding operation to prevent generating a new resharding
+                    // instance if the same command is issued consecutively due to client disconnect
+                    // etc.
+                    reshardCollectionJoinedExistingOperation.pauseWhileSet(opCtx);
+                    existingInstance.get()->getCoordinatorDocWrittenFuture().get(opCtx);
+                    return existingInstance;
+                }
+
                 const auto cm = uassertStatusOK(
                     Grid::get(opCtx)->catalogCache()->getShardedCollectionRoutingInfoWithRefresh(
                         opCtx, nss));
 
+                const auto currentShardKey = cm.getShardKeyPattern().getKeyPattern();
+                if (SimpleBSONObjComparator::kInstance.evaluate(currentShardKey.toBSON() ==
+                                                                request().getKey())) {
+                    // There isn't any work to be done by the resharding operation since the
+                    // existing shard key matches the desired new shard key.
+                    return boost::none;
+                }
+
                 auto tempReshardingNss = constructTemporaryReshardingNss(
                     nss.db(), getCollectionUUIDFromChunkManger(nss, cm));
 
@@ -158,7 +205,11 @@ public:
                 return instance;
             })();
 
-            instance->getCompletionFuture().get(opCtx);
+            if (instance) {
+                // There is work to be done in order to have the collection's shard key match the
+                // requested shard key. Wait until the work is complete.
+                instance.get()->getCompletionFuture().get(opCtx);
+            }
         }
 
     private: