chore: clean ups around command squasher (#5011)

1. Eliminate replies reverse call - will allow to unite replies and cmd vectors into one.
2. Introduce SwitchTxCmd function to avoid code duplication.

Signed-off-by: Roman Gershman <roman@dragonflydb.io>

Author: romange

src/server/conn_context.cc

@@ -157,6 +157,11 @@ void ConnectionContext::ChangeMonitor(bool start) {
   EnableMonitoring(start);
 }
 
+void ConnectionContext::SwitchTxCmd(const CommandId* cid) {
+  transaction->MultiSwitchCmd(cid);
+  this->cid = cid;
+}
+
 void ConnectionContext::ChangeSubscription(bool to_add, bool to_reply, CmdArgList args,
                                            facade::RedisReplyBuilder* rb) {
   vector<unsigned> result = ChangeSubscriptions(args, false, to_add, to_reply);

src/server/conn_context.h

@@ -301,6 +301,7 @@ class ConnectionContext : public facade::ConnectionContext {
   void UnsubscribeAll(bool to_reply, facade::RedisReplyBuilder* rb);
   void PUnsubscribeAll(bool to_reply, facade::RedisReplyBuilder* rb);
   void ChangeMonitor(bool start);  // either start or stop monitor on a given connection
+  void SwitchTxCmd(const CommandId* cid);
 
   size_t UsedMemory() const override;
 

src/server/debugcmd.cc

@@ -1309,9 +1309,7 @@ void DebugCmd::DoPopulateBatch(const PopulateOptions& options, const PopulateBat
         args_view.push_back(arg);
       }
       auto args_span = absl::MakeSpan(args_view);
-
-      stub_tx->MultiSwitchCmd(cid);
-      local_cntx.cid = cid;
+      local_cntx.SwitchTxCmd(cid);
       crb.SetReplyMode(ReplyMode::NONE);
       stub_tx->InitByArgs(cntx_->ns, local_cntx.conn_state.db_index, args_span);
 
@@ -1332,8 +1330,7 @@ void DebugCmd::DoPopulateBatch(const PopulateOptions& options, const PopulateBat
         args_view.push_back(arg);
       }
       auto args_span = absl::MakeSpan(args_view);
-      stub_tx->MultiSwitchCmd(cid);
-      local_cntx.cid = cid;
+      local_cntx.SwitchTxCmd(cid);
       crb.SetReplyMode(ReplyMode::NONE);
       stub_tx->InitByArgs(cntx_->ns, local_cntx.conn_state.db_index, args_span);
       sf_.service().InvokeCmd(cid, args_span, &crb, &local_cntx);

src/server/main_service.cc

@@ -2220,8 +2220,7 @@ void Service::Exec(CmdArgList args, const CommandContext& cmd_cntx) {
       for (auto& scmd : exec_info.body) {
         VLOG(2) << "TX CMD " << scmd.Cid()->name() << " " << scmd.NumArgs();
 
-        cmd_cntx.tx->MultiSwitchCmd(scmd.Cid());
-        cntx->cid = scmd.Cid();
+        cntx->SwitchTxCmd(scmd.Cid());
 
         arg_vec.resize(scmd.NumArgs());
         scmd.Fill(&arg_vec);

src/server/multi_command_squasher.cc

@@ -151,8 +151,7 @@ bool MultiCommandSquasher::ExecuteStandalone(facade::RedisReplyBuilder* rb, Stor
   }
 
   auto* tx = cntx_->transaction;
-  tx->MultiSwitchCmd(cmd->Cid());
-  cntx_->cid = cmd->Cid();
+  cntx_->SwitchTxCmd(cmd->Cid());
 
   if (cmd->Cid()->IsTransactional())
     tx->InitByArgs(cntx_->ns, cntx_->conn_state.db_index, args);
@@ -189,8 +188,7 @@ OpStatus MultiCommandSquasher::SquashedHopCb(EngineShard* es, RespVersion resp_v
       }
     }
 
-    local_tx->MultiSwitchCmd(cmd->Cid());
-    local_cntx.cid = cmd->Cid();
+    local_cntx.SwitchTxCmd(cmd->Cid());
     crb.SetReplyMode(cmd->ReplyMode());
 
     local_tx->InitByArgs(cntx_->ns, local_cntx.conn_state.db_index, args);
@@ -205,7 +203,6 @@ OpStatus MultiCommandSquasher::SquashedHopCb(EngineShard* es, RespVersion resp_v
     CheckConnStateClean(local_state);
   }
 
-  reverse(sinfo.replies.begin(), sinfo.replies.end());
   return OpStatus::OK;
 }
 
@@ -255,24 +252,27 @@ bool MultiCommandSquasher::ExecuteSquashed(facade::RedisReplyBuilder* rb) {
   bool aborted = false;
 
   for (auto idx : order_) {
-    auto& replies = sharded_[idx].replies;
-    CHECK(!replies.empty());
+    auto& sinfo = sharded_[idx];
+    auto& replies = sinfo.replies;
+    DCHECK_LT(sinfo.reply_id, replies.size());
 
-    aborted |= opts_.error_abort && CapturingReplyBuilder::TryExtractError(replies.back());
-
-    current_reply_size_.fetch_sub(Size(replies.back()), std::memory_order_relaxed);
-    CapturingReplyBuilder::Apply(std::move(replies.back()), rb);
-    replies.pop_back();
+    auto& reply = replies[sinfo.reply_id++];
+    aborted |= opts_.error_abort && CapturingReplyBuilder::TryExtractError(reply);
 
+    current_reply_size_.fetch_sub(Size(reply), std::memory_order_relaxed);
+    CapturingReplyBuilder::Apply(std::move(reply), rb);
     if (aborted)
       break;
   }
   uint64_t after_reply = proactor->GetMonotonicTimeNs();
   ServerState::SafeTLocal()->stats.multi_squash_exec_hop_usec += (after_hop - start) / 1000;
   ServerState::SafeTLocal()->stats.multi_squash_exec_reply_usec += (after_reply - after_hop) / 1000;
 
-  for (auto& sinfo : sharded_)
+  for (auto& sinfo : sharded_) {
     sinfo.cmds.clear();
+    sinfo.replies.clear();
+    sinfo.reply_id = 0;
+  }
 
   order_.clear();
   return !aborted;

src/server/multi_command_squasher.h

@@ -45,6 +45,7 @@ class MultiCommandSquasher {
 
     std::vector<StoredCmd*> cmds;  // accumulated commands
     std::vector<facade::CapturingReplyBuilder::Payload> replies;
+    unsigned reply_id = 0;
     boost::intrusive_ptr<Transaction> local_tx;  // stub-mode tx for use inside shard
   };