Emit DDL event for ALTER TABLE ADD CONSTRAINT with non-unique constraint

[fudianchn]

AI 披露：本改动由 AI 编码代理辅助完成，我已逐行审改。

问题（What）

ALTER TABLE ... ADD CONSTRAINT 添加非 UNIQUE 约束（外键 / CHECK 等）时，canal 不产生任何 DDL 事件，下游感知不到这次表结构变更。

根因（Root cause）

DruidDdlParser 对 SQLAlterTableAddConstraint 分支只在约束为 SQLUnique 时才把 DdlResult 加入结果；其余情况（外键/CHECK 等）DdlResult 已构造、processName 已调用，却被丢弃——既没 add，也没设 EventType。

修复（Fix）

非 UNIQUE 约束按 EventType.ALTER 处理并加入结果（与既有 catch-all 分支一致），UNIQUE 仍为 CINDEX。仅给该分支补 else 并把 add 移出 if。

测试（Testing）

新增两个用例覆盖 SQLAlterTableAddConstraint 的两条路径：

• testAlterAddConstraintForeignKey：... add constraint fk1 foreign key (a) references p(id)，断言产生 1 条 DDL 事件、表名 retl_mark、EventType.ALTER。
• testAlterAddConstraintCheck：... add constraint ck1 check (a > 0)，断言同上。CHECK 走 catch-all else 分支，防止将来有人把 else 收窄成 instanceof SQLForeignKeyImpl 后 CHECK 再次被静默丢弃。

修复前两个用例均失败（结果列表为空，assertFalse 报错）；修复后通过。DruidDdlParserTest 全部 9 个用例通过。

复现（Reproduce）

List<DdlResult> r = DruidDdlParser.parse(
    "alter table retl_mark add constraint fk1 foreign key (a) references p(id)", "retl");
// 修复前：r 为空（无 DDL 事件）
// 修复后：r = [DdlResult [schemaName=retl , tableName=retl_mark , type=ALTER ;]]

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[wenshao]

Reviewed — no blockers. Suggestions are inline.

中文说明

已审查——无阻断问题。 建议见行内评论。

— qwen3.8-max-preview via Qwen Code /review

[wenshao]

[Suggestion] The PR description states it fixes non-UNIQUE constraints (foreign key AND CHECK), but the test only covers FOREIGN KEY. If a future refactor replaces the catch-all else with an explicit instanceof SQLForeignKeyImpl check, ALTER TABLE t ADD CONSTRAINT ck1 CHECK (col > 0) would be silently dropped again — the exact bug this PR fixes — and no test would fail. Consider adding a companion test:

@Test
public void testAlterAddConstraintCheck() {
    String queryString = "alter table retl_mark add constraint ck1 check (a > 0)";
    List<DdlResult> results = DruidDdlParser.parse(queryString, "retl");
    Assert.assertFalse("ADD CONSTRAINT CHECK should emit a DDL event", results.isEmpty());
    DdlResult result = results.get(0);
    Assert.assertEquals("retl", result.getSchemaName());
    Assert.assertEquals("retl_mark", result.getTableName());
    Assert.assertEquals(EventType.ALTER, result.getType());
}

中文说明

PR 描述中说明修复了非 UNIQUE 约束（外键和 CHECK），但测试仅覆盖了 FOREIGN KEY。如果将来有人将 catch-all else 重构为显式的 instanceof SQLForeignKeyImpl 判断，ALTER TABLE t ADD CONSTRAINT ck1 CHECK (col > 0) 将再次被静默丢弃——即本 PR 修复的 bug——且不会有测试失败。建议补充 CHECK 约束的测试用例。

— qwen3.8-max-preview via Qwen Code /review

[fudianchn]

Added a CHECK-constraint companion test testAlterAddConstraintCheck covering the catch-all else branch, so a future refactor replacing the else with an explicit instanceof SQLForeignKeyImpl would now be caught. Verified: Tests run: 9, Failures: 0, Errors: 0 on DruidDdlParserTest with BUILD SUCCESS.