From 37b221eac28946fe9cb523ba2c0f517e844e0e92 Mon Sep 17 00:00:00 2001
From: murphy-4o <murphy.wang@clickhouse.com>
Date: Wed, 18 Mar 2026 12:54:36 +0000
Subject: [PATCH] Merge pull request #99838 from
 Desel72/fix-iceberg-modify-column-comment-crash

Reject unsupported MODIFY COLUMN on Iceberg tables to prevent crash
---
 .../DataLakes/Iceberg/IcebergMetadata.cpp     | 10 +++++++
 ...r_modify_column_comment_segfault.reference |  1 +
 ...rg_alter_modify_column_comment_segfault.sh | 28 +++++++++++++++++++
 3 files changed, 39 insertions(+)
 create mode 100644 tests/queries/0_stateless/04042_iceberg_alter_modify_column_comment_segfault.reference
 create mode 100755 tests/queries/0_stateless/04042_iceberg_alter_modify_column_comment_segfault.sh

diff --git a/src/Storages/ObjectStorage/DataLakes/Iceberg/IcebergMetadata.cpp b/src/Storages/ObjectStorage/DataLakes/Iceberg/IcebergMetadata.cpp
index 58f2926cc07e..ebe5c291c3c2 100644
--- a/src/Storages/ObjectStorage/DataLakes/Iceberg/IcebergMetadata.cpp
+++ b/src/Storages/ObjectStorage/DataLakes/Iceberg/IcebergMetadata.cpp
@@ -545,6 +545,16 @@ void IcebergMetadata::checkAlterIsPossible(const AlterCommands & commands)
         if (command.type != AlterCommand::Type::ADD_COLUMN && command.type != AlterCommand::Type::DROP_COLUMN
             && command.type != AlterCommand::Type::MODIFY_COLUMN)
             throw Exception(ErrorCodes::NOT_IMPLEMENTED, "Alter of type '{}' is not supported by Iceberg storage", command.type);
+
+        if (command.type == AlterCommand::Type::MODIFY_COLUMN && command.to_remove != AlterCommand::RemoveProperty::NO_PROPERTY)
+            throw Exception(
+                ErrorCodes::NOT_IMPLEMENTED,
+                "Removing column property '{}' from column '{}' is not supported by Iceberg storage", command.to_remove, command.column_name);
+
+        if (command.type == AlterCommand::Type::MODIFY_COLUMN && !command.data_type)
+            throw Exception(
+                ErrorCodes::NOT_IMPLEMENTED,
+                "Modifying column '{}' without changing its type is not supported by Iceberg storage", command.column_name);
     }
 }
 
diff --git a/tests/queries/0_stateless/04042_iceberg_alter_modify_column_comment_segfault.reference b/tests/queries/0_stateless/04042_iceberg_alter_modify_column_comment_segfault.reference
new file mode 100644
index 000000000000..23fb9dac9555
--- /dev/null
+++ b/tests/queries/0_stateless/04042_iceberg_alter_modify_column_comment_segfault.reference
@@ -0,0 +1 @@
+NOT_IMPLEMENTED
diff --git a/tests/queries/0_stateless/04042_iceberg_alter_modify_column_comment_segfault.sh b/tests/queries/0_stateless/04042_iceberg_alter_modify_column_comment_segfault.sh
new file mode 100755
index 000000000000..69b07a2db708
--- /dev/null
+++ b/tests/queries/0_stateless/04042_iceberg_alter_modify_column_comment_segfault.sh
@@ -0,0 +1,28 @@
+#!/usr/bin/env bash
+# Tags: no-fasttest
+
+# Regression test for https://github.com/ClickHouse/ClickHouse/issues/99523
+# ALTER TABLE ... MODIFY COLUMN ... COMMENT on an Iceberg table
+# should return an error instead of causing a segfault.
+
+CURDIR=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
+# shellcheck source=../shell_config.sh
+. "$CURDIR"/../shell_config.sh
+
+TABLE="t_${CLICKHOUSE_DATABASE}_${RANDOM}"
+TABLE_PATH="${USER_FILES_PATH}/${TABLE}/"
+
+${CLICKHOUSE_CLIENT} --query "DROP TABLE IF EXISTS ${TABLE}"
+${CLICKHOUSE_CLIENT} --query "
+    CREATE TABLE ${TABLE} (c0 Int)
+    ENGINE = IcebergLocal('${TABLE_PATH}')
+"
+# To have at least one real snapshot. Otherwise alter can be noop.
+${CLICKHOUSE_CLIENT} --allow_insert_into_iceberg=1 --query "INSERT INTO ${TABLE} VALUES (1)"
+
+${CLICKHOUSE_CLIENT} --query "
+    ALTER TABLE ${TABLE} MODIFY COLUMN c0 COMMENT 'hello'
+" 2>&1 | grep -o -m1 "NOT_IMPLEMENTED"
+
+${CLICKHOUSE_CLIENT} --query "DROP TABLE IF EXISTS ${TABLE}"
+rm -rf "${TABLE_PATH}"