chore: almost mutex free replication

Signed-off-by: kostas <[email protected]>

Author: kostasrim

src/server/replica.cc

@@ -542,13 +542,6 @@ error_code Replica::InitiateDflySync(std::optional<LastMasterSyncData> last_mast
     JoinDflyFlows();
     if (sync_type == "full") {
       service_.RemoveLoadingState();
-    } else if (service_.IsLoadingExclusively()) {
-      // We need this check. We originally set the state unconditionally to LOADING
-      // when we call ReplicaOf command. If for some reason we fail to start full sync below
-      // or cancel the context, we still need to switch to ACTIVE state.
-      // TODO(kostasrim) we can remove this once my proposed changes for replication move forward
-      // as the state transitions for ReplicaOf command will be much clearer.
-      service_.SwitchState(GlobalState::LOADING, GlobalState::ACTIVE);
     }
     state_mask_ &= ~R_SYNCING;
     last_journal_LSNs_.reset();

src/server/server_family.cc

@@ -242,6 +242,11 @@ using detail::SaveStagesController;
 using http::StringResponse;
 using strings::HumanReadableNumBytes;
 
+// Initialized by REPLICAOF
+thread_local std::shared_ptr<Replica> tl_replica = nullptr;
+// Initialized by ADDREPLICAOF
+thread_local std::vector<std::shared_ptr<Replica>> tl_cluster_replicas;
+
 namespace {
 
 // TODO these should be configurable as command line flag and at runtime via config set
@@ -1228,6 +1233,11 @@ void ServerFamily::Shutdown() {
     dfly_cmd_->Shutdown();
     DebugCmd::Shutdown();
   });
+
+  service_.proactor_pool().AwaitFiberOnAll([](auto index, auto* cntx) {
+    tl_replica = nullptr;
+    tl_cluster_replicas.clear();
+  });
 }
 
 bool ServerFamily::HasPrivilegedInterface() {
@@ -3130,12 +3140,15 @@ string ServerFamily::FormatInfoMetrics(const Metrics& m, std::string_view sectio
         append("psync_attempts", rinfo.psync_attempts);
         append("psync_successes", rinfo.psync_successes);
       };
-      fb2::LockGuard lk(replicaof_mu_);
+      // Deep copy because tl_replica might be overwritten inbetween
+      auto replica = tl_replica;
 
-      replication_info_cb(replica_->GetSummary());
+      replication_info_cb(replica->GetSummary());
 
+      // Deep copy because tl_cluster_replicas might be overwritten inbetween
+      auto cluster_replicas = tl_cluster_replicas;
       // Special case, when multiple masters replicate to a single replica.
-      for (const auto& replica : cluster_replicas_) {
+      for (const auto& replica : cluster_replicas) {
         replication_info_cb(replica->GetSummary());
       }
     }
@@ -3417,7 +3430,7 @@ void ServerFamily::AddReplicaOf(CmdArgList args, const CommandContext& cmd_cntx)
   }
   LOG(INFO) << "Add Replica " << *replicaof_args;
 
-  auto add_replica = make_unique<Replica>(replicaof_args->host, replicaof_args->port, &service_,
+  auto add_replica = make_shared<Replica>(replicaof_args->host, replicaof_args->port, &service_,
                                           master_replid(), replicaof_args->slot_range);
   GenericError ec = add_replica->Start();
   if (ec) {
@@ -3426,77 +3439,76 @@ void ServerFamily::AddReplicaOf(CmdArgList args, const CommandContext& cmd_cntx)
   }
   add_replica->StartMainReplicationFiber(nullopt);
   cluster_replicas_.push_back(std::move(add_replica));
-  cmd_cntx.rb->SendOk();
-}
 
-void ServerFamily::ReplicaOfInternal(CmdArgList args, Transaction* tx, SinkReplyBuilder* builder,
-                                     ActionOnConnectionFail on_err) {
-  std::shared_ptr<Replica> new_replica;
-  std::optional<Replica::LastMasterSyncData> last_master_data;
-  {
-    util::fb2::LockGuard lk(replicaof_mu_);  // Only one REPLICAOF command can run at a time
+  service_.proactor_pool().AwaitFiberOnAll(
+      [this](auto index, auto* cntx)
+          ABSL_NO_THREAD_SAFETY_ANALYSIS { tl_cluster_replicas = cluster_replicas_; });
 
-    // We should not execute replica of command while loading from snapshot.
-    ServerState* ss = ServerState::tlocal();
-    if (ss->is_master && ss->gstate() == GlobalState::LOADING) {
-      builder->SendError(kLoadingErr);
-      return;
-    }
-
-    auto replicaof_args = ReplicaOfArgs::FromCmdArgs(args, builder);
-    if (!replicaof_args.has_value()) {
-      return;
-    }
-
-    LOG(INFO) << "Replicating " << *replicaof_args;
-
-    // If NO ONE was supplied, just stop the current replica (if it exists)
-    if (replicaof_args->IsReplicaOfNoOne()) {
-      if (!ss->is_master) {
-        CHECK(replica_);
-
-        SetMasterFlagOnAllThreads(true);  // Flip flag before clearing replica
-        last_master_data_ = replica_->Stop();
-        replica_.reset();
+  cmd_cntx.rb->SendOk();
+}
 
-        StopAllClusterReplicas();
-      }
+void ServerFamily::ReplicaOfNoOne(SinkReplyBuilder* builder) {
+  util::fb2::LockGuard lk(replicaof_mu_);
+  ServerState* ss = ServerState::tlocal();
 
-      // May not switch to ACTIVE if the process is, for example, shutting down at the same time.
-      service_.SwitchState(GlobalState::LOADING, GlobalState::ACTIVE);
+  if (!ss->is_master) {
+    CHECK(replica_);
+    // flip flag before clearing replica_
+    SetMasterFlagOnAllThreads(true);
+    // TODO we should not allow partial sync after NO-ONE. Only after Takeover.
+    last_master_data_ = replica_->Stop();
+    // TODO set thread locals to nullptr
+    replica_.reset();
+    StopAllClusterReplicas();
+    service_.proactor_pool().AwaitFiberOnAll([](auto index, auto* cntx) { tl_replica = nullptr; });
+  }
 
-      return builder->SendOk();
-    }
+  // May not switch to ACTIVE if the process is, for example, shutting down at the same time.
+  service_.SwitchState(GlobalState::LOADING, GlobalState::ACTIVE);
 
-    // If any replication is in progress, stop it, cancellation should kick in immediately
+  return builder->SendOk();
+}
 
-    if (replica_)
-      last_master_data = replica_->Stop();
-    StopAllClusterReplicas();
+bool ServerFamily::IsDragonflyLoadingAtomic() {
+  util::fb2::LockGuard lk(replicaof_mu_);
+  ServerState* ss = ServerState::tlocal();
 
-    const GlobalState gstate = ServerState::tlocal()->gstate();
-    if (gstate == GlobalState::TAKEN_OVER) {
-      service_.SwitchState(GlobalState::TAKEN_OVER, GlobalState::LOADING);
-    } else if (auto new_state = service_.SwitchState(GlobalState::ACTIVE, GlobalState::LOADING);
-               new_state != GlobalState::LOADING) {
-      LOG(WARNING) << new_state << " in progress, ignored";
-      builder->SendError("Invalid state");
-      return;
-    }
+  return ss->is_master && ss->gstate() == GlobalState::LOADING;
+}
 
-    // Create a new replica and assign it
-    new_replica = make_shared<Replica>(replicaof_args->host, replicaof_args->port, &service_,
-                                       master_replid(), replicaof_args->slot_range);
+void ServerFamily::ReplicaOfInternal(CmdArgList args, Transaction* tx, SinkReplyBuilder* builder,
+                                     ActionOnConnectionFail on_err) {
+  std::optional<Replica::LastMasterSyncData> last_master_data;
 
-    replica_ = new_replica;
+  auto replicaof_args = ReplicaOfArgs::FromCmdArgs(args, builder);
+  if (!replicaof_args.has_value()) {
+    return;
+  }
 
-    // TODO: disconnect pending blocked clients (pubsub, blocking commands)
-    SetMasterFlagOnAllThreads(false);  // Flip flag after assiging replica
+  LOG(INFO) << "Initiate replication with: " << *replicaof_args;
+  // This is a "weak" check. For example, if the node is already a replica,
+  // it could be the case that one of the flows disconnects. The MainReplicationFiber
+  // will then loop and if it can't partial sync it will enter LOADING state because of
+  // full sync. Note that the fiber is not aware of the replicaof_mu_ so even
+  // if that mutex is locked below before any state check we can't really enforce
+  // that the old replication fiber won't try to full sync and update the state to LOADING.
+  // What is more here is that we always call `replica->Stop()`. So even if we end up in the
+  // scenario described, the semantics are well defined. First, cancel the old replica and
+  // move on with the new one. Cancelation will be slower and ReplicaOf() will
+  // induce higher latency -- but that's ok because it's an highly improbable flow with
+  // well defined semantics.
+  if (IsDragonflyLoadingAtomic()) {
+    builder->SendError(kLoadingErr);
+    return;
+  }
 
-  }  // release the lock, lk.unlock()
-  // We proceed connecting below without the lock to allow interrupting the replica immediately.
-  // From this point and onward, it should be highly responsive.
+  // replicaof no one
+  if (replicaof_args->IsReplicaOfNoOne()) {
+    return ReplicaOfNoOne(builder);
+  }
 
+  auto new_replica = make_shared<Replica>(replicaof_args->host, replicaof_args->port, &service_,
+                                          master_replid(), replicaof_args->slot_range);
   GenericError ec{};
   switch (on_err) {
     case ActionOnConnectionFail::kReturnOnError:
@@ -3507,30 +3519,31 @@ void ServerFamily::ReplicaOfInternal(CmdArgList args, Transaction* tx, SinkReply
       break;
   };
 
-  // If the replication attempt failed, clean up global state. The replica should have stopped
-  // internally.
-  util::fb2::LockGuard lk(replicaof_mu_);  // Only one REPLICAOF command can run at a time
-
-  // If there was an error above during Start we must not start the main replication fiber.
-  // However, it could be the case that Start() above connected succefully and by the time
-  // we acquire the lock, the context got cancelled because another ReplicaOf command
-  // executed and acquired the replicaof_mu_ before us.
-  const bool cancelled = new_replica->IsContextCancelled();
-  if (ec || cancelled) {
-    if (replica_ == new_replica) {
-      service_.SwitchState(GlobalState::LOADING, GlobalState::ACTIVE);
-      SetMasterFlagOnAllThreads(true);
-      replica_.reset();
-    }
-    builder->SendError(ec ? ec.Format() : "replication cancelled");
-    return;
+  if (ec) {
+    return builder->SendError(ec.Format());
   }
-  // Successfully connected now we flush
-  // If we are called by "Replicate", tx will be null but we do not need
-  // to flush anything.
+
+  util::fb2::LockGuard lk(replicaof_mu_);
+  if (replica_)
+    last_master_data = replica_->Stop();
+
+  StopAllClusterReplicas();
+
+  if (ServerState::tlocal()->gstate() == GlobalState::TAKEN_OVER)
+    service_.SwitchState(GlobalState::TAKEN_OVER, GlobalState::LOADING);
+
+  // Update thread locals. That way INFO never blocks
+  replica_ = new_replica;
+  service_.proactor_pool().AwaitFiberOnAll([new_replica](auto index, auto* context) {
+    tl_replica = new_replica;
+    tl_cluster_replicas.clear();
+  });
+  SetMasterFlagOnAllThreads(false);
+
   if (on_err == ActionOnConnectionFail::kReturnOnError) {
-    new_replica->StartMainReplicationFiber(last_master_data);
+    replica_->StartMainReplicationFiber(last_master_data);
   }
+
   builder->SendOk();
 }
 
@@ -3608,6 +3621,9 @@ void ServerFamily::ReplTakeOver(CmdArgList args, const CommandContext& cmd_cntx)
   SetMasterFlagOnAllThreads(true);
   last_master_data_ = replica_->Stop();
   replica_.reset();
+
+  service_.proactor_pool().AwaitFiberOnAll([](auto index, auto* context) { tl_replica = nullptr; });
+
   return builder->SendOk();
 }
 

src/server/server_family.h

@@ -412,6 +412,9 @@ class ServerFamily {
   // Set accepting_connections_ and update listners according to it
   void ChangeConnectionAccept(bool accept);
 
+  void ReplicaOfNoOne(SinkReplyBuilder* builder);
+  bool IsDragonflyLoadingAtomic();
+
   util::fb2::Fiber snapshot_schedule_fb_;
   util::fb2::Fiber load_fiber_;
 
@@ -424,7 +427,7 @@ class ServerFamily {
 
   mutable util::fb2::Mutex replicaof_mu_, save_mu_;
   std::shared_ptr<Replica> replica_ ABSL_GUARDED_BY(replicaof_mu_);
-  std::vector<std::unique_ptr<Replica>> cluster_replicas_
+  std::vector<std::shared_ptr<Replica>> cluster_replicas_
       ABSL_GUARDED_BY(replicaof_mu_);  // used to replicating multiple nodes to single dragonfly
 
   std::unique_ptr<ScriptMgr> script_mgr_;