schemeshard: remove unreasonable iteration over entire ShardInfos

PathShardsLimit() performs pedantic validation of the path's current shards
count by recalculating it through iteration over ShardInfos of the entire
database and matching them to the path (!). See CollectAllShards().
That is unreasonable and too slow for the hot spot that is PersistSingleStats().
(Though appropriate for slow paths like TSubOperation::Propose() etc.)

Replacing PathShardsLimit() also eliminates aditional TPath construction
and hashmap lookup.

Author: ijon

ydb/core/tx/schemeshard/schemeshard__table_stats.cpp

@@ -475,18 +475,32 @@ bool TTxStoreTableStats::PersistSingleStats(const TPathId& pathId,
         return true;
     }
 
-    auto path = TPath::Init(pathId, Self);
-    auto checks = path.Check();
-    constexpr ui64 deltaShards = 2;
-    checks
-        .PathShardsLimit(deltaShards)
-        .ShardsLimit(deltaShards);
-    if (!checks) {
-        LOG_NOTICE_S(ctx, NKikimrServices::FLAT_TX_SCHEMESHARD,
-                        "Do not request full stats from datashard"
-                            << ", datashard: " << datashardId
-                            << ", reason: " << checks.GetError());
-        return true;
+    //NOTE: intentionally avoid using TPath.Check().{PathShardsLimit,ShardsLimit}() here.
+    // PathShardsLimit() performs pedantic validation by recalculating shard count through
+    // iteration over entire ShardInfos, which is too slow for this hot spot. It also performs
+    // additional lookups we want to avoid.
+    {
+        constexpr ui64 deltaShards = 2;
+        if ((pathElement->GetShardsInside() + deltaShards) > subDomainInfo->GetSchemeLimits().MaxShardsInPath) {
+            LOG_NOTICE_S(ctx, NKikimrServices::FLAT_TX_SCHEMESHARD, "Do not request full stats from datashard " << datashardId
+                << ", reason: shards count limit exceeded (in path)"
+                << ", limit: " << subDomainInfo->GetSchemeLimits().MaxShardsInPath
+                << ", current: " << pathElement->GetShardsInside()
+                << ", delta: " << deltaShards
+            );
+            return true;
+        }
+        const auto currentShards = (subDomainInfo->GetShardsInside() - subDomainInfo->GetBackupShards());
+        if ((currentShards + deltaShards) > subDomainInfo->GetSchemeLimits().MaxShards) {
+            LOG_NOTICE_S(ctx, NKikimrServices::FLAT_TX_SCHEMESHARD, "Do not request full stats from datashard " << datashardId
+                << ", datashard: " << datashardId
+                << ", reason: shards count limit exceeded (in subdomain)"
+                << ", limit: " << subDomainInfo->GetSchemeLimits().MaxShards
+                << ", current: " << currentShards
+                << ", delta: " << deltaShards
+            );
+            return true;
+        }
     }
 
     if (newStats.HasBorrowedData) {
@@ -496,9 +510,11 @@ bool TTxStoreTableStats::PersistSingleStats(const TPathId& pathId,
         return true;
     }
 
-    if (path.IsLocked()) {
+    // path.IsLocked() and path.LockedBy() equivalent
+    if (const auto& found = Self->LockedPaths.find(pathId); found != Self->LockedPaths.end()) {
+        const auto txId = found->second;
         LOG_DEBUG_S(ctx, NKikimrServices::FLAT_TX_SCHEMESHARD,
-            "Postpone split tablet " << datashardId << " because it is locked by " << path.LockedBy());
+            "Postpone split tablet " << datashardId << " because it is locked by " << txId);
         return true;
     }