Merge pull request ClickHouse#88716 from ClickHouse/backport/25.8/88605

Backport ClickHouse#88605 to 25.8: Fix potential crash caused by concurrent mutation of underlying const PREWHERE columns

Author: clickhouse-gh[bot]

src/Storages/MergeTree/MergeTreeReadTask.cpp

@@ -255,7 +255,12 @@ MergeTreeReadTask::BlockAndProgress MergeTreeReadTask::read()
     if (read_result.num_rows != 0)
     {
         for (const auto & column : read_result.columns)
-            column->assumeMutableRef().shrinkToFit();
+        {
+            /// We may have columns that has other references, usually it is a constant column that has been created during analysis
+            /// (that will not be const here anymore, i.e. after materialize()), and we do not need to shrink it anyway.
+            if (column->use_count() == 1)
+                column->assumeMutableRef().shrinkToFit();
+        }
         block = sample_block.cloneWithColumns(read_result.columns);
     }
 

tests/queries/0_stateless/03680_mergetree_shrink_const_from_prewhere.reference

@@ -0,0 +1,3 @@
+1
+2
+3

tests/queries/0_stateless/03680_mergetree_shrink_const_from_prewhere.sql

@@ -0,0 +1,9 @@
+DROP TABLE IF EXISTS const_node;
+CREATE TABLE const_node (`v` Nullable(UInt8)) ENGINE = MergeTree ORDER BY tuple();
+SYSTEM STOP MERGES const_node;
+INSERT INTO const_node VALUES (1);
+INSERT INTO const_node VALUES (2);
+INSERT INTO const_node VALUES (3);
+-- Here we have condition with a constant "materialize(255)", for which convertToFullColumnIfConst() will return underlying column w/o copying,
+-- and later shrinkToFit() will be called from multiple threads on this column, and leads to UB
+SELECT v FROM const_node PREWHERE and(materialize(255), *) ORDER BY v;