Added a test (but it doesn't reproduce the issue #6746)

dbms/tests/queries/0_stateless/01002_alter_nullable_adaptive_granularity.sh

+#!/usr/bin/env bash
+
+CURDIR=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
+. $CURDIR/../shell_config.sh
+
+set -e
+
+$CLICKHOUSE_CLIENT --query "DROP TABLE IF EXISTS test";
+$CLICKHOUSE_CLIENT --query "CREATE TABLE test (x UInt8, s String MATERIALIZED toString(rand64())) ENGINE = MergeTree ORDER BY s";
+
+function thread1()
+{
+    while true; do 
+        $CLICKHOUSE_CLIENT --query "INSERT INTO test SELECT rand() FROM numbers(1000)";
+    done
+}
+
+function thread2()
+{
+    while true; do
+        $CLICKHOUSE_CLIENT -n --query "ALTER TABLE test MODIFY COLUMN x Nullable(UInt8);";
+        sleep 0.0$RANDOM
+        $CLICKHOUSE_CLIENT -n --query "ALTER TABLE test MODIFY COLUMN x UInt8;";
+        sleep 0.0$RANDOM
+    done
+}
+
+function thread3()
+{
+    while true; do
+        $CLICKHOUSE_CLIENT -n --query "SELECT count() FROM test FORMAT Null";
+    done
+}
+
+function thread4()
+{
+    while true; do
+        $CLICKHOUSE_CLIENT -n --query "OPTIMIZE TABLE test FINAL";
+        sleep 0.1$RANDOM
+    done
+}
+
+# https://stackoverflow.com/questions/9954794/execute-a-shell-function-with-timeout
+export -f thread1;
+export -f thread2;
+export -f thread3;
+export -f thread4;
+
+TIMEOUT=10
+
+timeout $TIMEOUT bash -c thread1 2> /dev/null &
+timeout $TIMEOUT bash -c thread2 2> /dev/null &
+timeout $TIMEOUT bash -c thread3 2> /dev/null &
+timeout $TIMEOUT bash -c thread4 2> /dev/null &
+
+wait
+
+$CLICKHOUSE_CLIENT -q "DROP TABLE test"