chore: Add debug logs to help tracking transactional deadlocks (#4669)

* chore: reproduce a bug related to #4663

Add various debug logs to help tracking the deadlock.

Add more assertions in helio and provide state time for fibers
during stacktrace printings.
---------

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

.pre-commit-config.yaml

@@ -1,4 +1,4 @@
-default_stages: [commit]
+default_stages: [pre-commit]
 exclude: |
     (?x)(
       src/redis/.* |

src/server/db_slice.cc

@@ -898,6 +898,8 @@ void DbSlice::FlushDbIndexes(const std::vector<DbIndex>& indexes) {
 }
 
 void DbSlice::FlushDb(DbIndex db_ind) {
+  DVLOG(1) << "Flushing db " << db_ind;
+
   // clear client tracking map.
   client_tracking_map_.clear();
 

src/server/debugcmd.cc

@@ -1066,12 +1066,7 @@ void DebugCmd::TxAnalysis(facade::SinkReplyBuilder* builder) {
   string result;
   for (unsigned i = 0; i < shard_set->size(); ++i) {
     const auto& info = shard_info[i];
-    StrAppend(&result, "shard", i, ":\n", "  tx armed ", info.tx_armed, ", total: ", info.tx_total,
-              ",global:", info.tx_global, ",runnable:", info.tx_runnable, "\n");
-    StrAppend(&result, "  locks total:", info.total_locks, ",contended:", info.contended_locks,
-              "\n");
-    StrAppend(&result, "  max contention score: ", info.max_contention_score,
-              ",lock_name:", info.max_contention_lock, "\n");
+    StrAppend(&result, "shard", i, ":\n", info.Format(), "\n");
   }
   auto* rb = static_cast<RedisReplyBuilder*>(builder);
   rb->SendVerbatimString(result);
@@ -1114,7 +1109,14 @@ void DebugCmd::ObjHist(facade::SinkReplyBuilder* builder) {
 void DebugCmd::Stacktrace(facade::SinkReplyBuilder* builder) {
   fb2::Mutex m;
   shard_set->pool()->AwaitFiberOnAll([&m](unsigned index, ProactorBase* base) {
+    EngineShard* es = EngineShard::tlocal();
+    string txq;
+    if (es) {
+      EngineShard::TxQueueInfo txq_info = es->AnalyzeTxQueue();
+      txq = txq_info.Format();
+    }
     std::unique_lock lk(m);
+    LOG_IF(INFO, !txq.empty()) << "Shard" << index << ": " << txq;
     fb2::detail::FiberInterface::PrintAllFiberStackTraces();
   });
   base::FlushLogs();

src/server/engine_shard.cc

@@ -5,6 +5,7 @@
 #include "server/engine_shard.h"
 
 #include <absl/strings/match.h>
+#include <absl/strings/str_cat.h>
 
 #include "base/flags.h"
 #include "io/proc_reader.h"
@@ -276,6 +277,25 @@ ShardId Shard(string_view v, ShardId shard_num) {
   return hash % shard_num;
 }
 
+string EngineShard::TxQueueInfo::Format() const {
+  string res;
+
+  if (tx_total > 0) {
+    absl::StrAppend(&res, "tx armed ", tx_armed, ", total: ", tx_total, ",global:", tx_global,
+                    ",runnable:", tx_runnable, "\n");
+    absl::StrAppend(&res, ", head: ", head.debug_id_info, "\n");
+  }
+  if (total_locks > 0) {
+    absl::StrAppend(&res, "locks total:", total_locks, ",contended:", contended_locks, "\n");
+  }
+  if (max_contention_score > 0) {
+    absl::StrAppend(&res, "max contention score: ", max_contention_score,
+                    ", lock: ", max_contention_lock, "\n");
+  }
+
+  return res;
+}
+
 EngineShard::Stats& EngineShard::Stats::operator+=(const EngineShard::Stats& o) {
   static_assert(sizeof(Stats) == 64);
 
@@ -706,7 +726,7 @@ void EngineShard::RemoveContTx(Transaction* tx) {
 }
 
 void EngineShard::Heartbeat() {
-  DVLOG(2) << " Hearbeat";
+  DVLOG(3) << " Hearbeat";
   DCHECK(namespaces);
 
   CacheStats();

src/server/engine_shard.h

@@ -181,6 +181,8 @@ class EngineShard {
 
     // We can use a vector to hold debug info for all items in the txqueue
     TxQueueItem head;
+
+    std::string Format() const;
   };
 
   TxQueueInfo AnalyzeTxQueue() const;

src/server/protocol_client.cc

@@ -347,6 +347,7 @@ bool ProtocolClient::CheckRespFirstTypes(initializer_list<RespExpr::Type> types)
 
 error_code ProtocolClient::SendCommand(string_view command) {
   string formatted_command = RedisReplyBuilderBase::SerializeCommand(command);
+  DCHECK(sock_->proactor() == ProactorBase::me());
   auto ec = sock_->Write(io::Buffer(formatted_command));
   if (!ec)
     TouchIoTime();

src/server/replica.cc

@@ -87,7 +87,7 @@ Replica::~Replica() {
 static const char kConnErr[] = "could not connect to master: ";
 
 GenericError Replica::Start() {
-  VLOG(1) << "Starting replication";
+  VLOG(1) << "Starting replication " << this;
   ProactorBase* mythread = ProactorBase::me();
   CHECK(mythread);
 
@@ -138,7 +138,7 @@ void Replica::EnableReplication(facade::SinkReplyBuilder* builder) {
 }
 
 void Replica::Stop() {
-  VLOG(1) << "Stopping replication";
+  VLOG(1) << "Stopping replication " << this;
   // Stops the loop in MainReplicationFb.
 
   proactor_->Await([this] {
@@ -149,6 +149,7 @@ void Replica::Stop() {
   // Make sure the replica fully stopped and did all cleanup,
   // so we can freely release resources (connections).
   sync_fb_.JoinIfNeeded();
+  DVLOG(1) << "MainReplicationFb stopped " << this;
   acks_fb_.JoinIfNeeded();
   for (auto& flow : shard_flows_) {
     flow.reset();
@@ -183,7 +184,7 @@ std::error_code Replica::TakeOver(std::string_view timeout, bool save_flag) {
 }
 
 void Replica::MainReplicationFb() {
-  VLOG(1) << "Main replication fiber started";
+  VLOG(1) << "Main replication fiber started " << this;
   // Switch shard states to replication.
   SetShardStates(true);
 
@@ -546,11 +547,14 @@ error_code Replica::InitiateDflySync() {
         std::accumulate(is_full_sync.get(), is_full_sync.get() + num_df_flows, 0);
 
     if (num_full_flows == num_df_flows) {
+      DVLOG(1) << "Calling Flush on all slots " << this;
+
       if (slot_range_.has_value()) {
         JournalExecutor{&service_}.FlushSlots(slot_range_.value());
       } else {
         JournalExecutor{&service_}.FlushAll();
       }
+      DVLOG(1) << "Flush on all slots ended " << this;
     } else if (num_full_flows == 0) {
       sync_type = "partial";
     } else {

src/server/server_family.cc

@@ -2903,6 +2903,7 @@ void ServerFamily::ReplicaOfInternal(CmdArgList args, Transaction* tx, SinkReply
   // 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

src/server/transaction.cc

@@ -49,19 +49,14 @@ void AnalyzeTxQueue(const EngineShard* shard, const TxQueue* txq) {
     if (now >= last_log_time + 10) {
       last_log_time = now;
       EngineShard::TxQueueInfo info = shard->AnalyzeTxQueue();
-      string msg =
-          StrCat("TxQueue is too long. Tx count:", info.tx_total, ", armed:", info.tx_armed,
-                 ", runnable:", info.tx_runnable, ", total locks: ", info.total_locks,
-                 ", contended locks: ", info.contended_locks, "\n");
-      absl::StrAppend(&msg, "max contention score: ", info.max_contention_score,
-                      ", lock: ", info.max_contention_lock,
-                      ", poll_executions:", shard->stats().poll_execution_total);
+      string msg = StrCat("TxQueue is too long. ", info.Format());
+      absl::StrAppend(&msg, "poll_executions:", shard->stats().poll_execution_total);
+
       const Transaction* cont_tx = shard->GetContTx();
       if (cont_tx) {
         absl::StrAppend(&msg, " continuation_tx: ", cont_tx->DebugId(shard->shard_id()), " ",
                         cont_tx->DEBUG_IsArmedInShard(shard->shard_id()) ? " armed" : "");
       }
-      absl::StrAppend(&msg, "\nTxQueue head debug info ", info.head.debug_id_info);
 
       LOG(WARNING) << msg;
     }