Do not account memory for Buffer engine in max_memory_usage limit

Since background flush will not be accounted there, and it can be too
tricky to calclulate this limit, in case you have multiple materialized
views with Buffer engine.

v2: test adjustment

dbms/src/Storages/StorageBuffer.cpp

@@ -265,6 +265,8 @@ static void appendBlock(const Block & from, Block & to)
 
     size_t old_rows = to.rows();
 
+    auto temporarily_disable_memory_tracker = getCurrentMemoryTrackerActionLock();
+
     try
     {
         for (size_t column_no = 0, columns = to.columns(); column_no < columns; ++column_no)
@@ -282,9 +284,6 @@ static void appendBlock(const Block & from, Block & to)
         /// Rollback changes.
         try
         {
-            /// Avoid "memory limit exceeded" exceptions during rollback.
-            auto temporarily_disable_memory_tracker = getCurrentMemoryTrackerActionLock();
-
             for (size_t column_no = 0, columns = to.columns(); column_no < columns; ++column_no)
             {
                 ColumnPtr & col_to = to.getByPosition(column_no).column;
@@ -339,7 +338,7 @@ public:
             {
                 LOG_TRACE(storage.log, "Writing block with " << rows << " rows, " << bytes << " bytes directly.");
                 storage.writeBlockToDestination(block, destination);
-             }
+            }
             return;
         }
 
@@ -622,6 +621,8 @@ void StorageBuffer::writeBlockToDestination(const Block & block, StoragePtr tabl
         return;
     }
 
+    auto temporarily_disable_memory_tracker = getCurrentMemoryTrackerActionLock();
+
     auto insert = std::make_shared<ASTInsertQuery>();
 
     insert->database = destination_database;

dbms/tests/queries/0_stateless/01019_Buffer_and_max_memory_usage.sql

+DROP TABLE IF EXISTS null_;
+DROP TABLE IF EXISTS buffer_;
+
+CREATE TABLE null_ (key UInt64) Engine=Null();
+CREATE TABLE buffer_ (key UInt64) Engine=Buffer(currentDatabase(), null_,
+    1,    /* num_layers */
+    0,    /* min_time   */
+    86400,/* max_time   */
+    0,    /* min_rows   */
+    100e9,/* max_rows   */
+    0,    /* min_bytes  */
+    20e6  /* max_bytes  */
+);
+
+-- note that there is untracked_memory_limit (4MB) in MemoryTracker
+SET max_memory_usage=10e6;
+
+SET min_insert_block_size_bytes=9e6;
+INSERT INTO buffer_ SELECT toUInt64(number) FROM system.numbers LIMIT 10e6; -- { serverError 241 }
+
+OPTIMIZE TABLE buffer_; -- flush
+
+SET min_insert_block_size_bytes=1e6;
+INSERT INTO buffer_ SELECT toUInt64(number) FROM system.numbers LIMIT 10e6;