add foreign key as part of set statement when reserved connection is needed

[harshit-gangal]

Description

This PR fixes the issue with DDL statements which do not acknowledge the foreign_key_checks setting.

Related Issue(s)

• Fixes Bug Report: Cannot import mysqldump due to order of tables and foreign keys #14699

Checklist

• "Backport to:" labels have been added if this change should be back-ported - Only on main
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on the CI
• Documentation was added or is not required

[vitess-bot]

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[GuptaManan100]

Does this change affect only DDL queries or will it also cause DML queries to execute a set statement too?

[harshit-gangal]

Does this change affect only DDL queries or will it also cause DML queries to execute a set statement too?

It is session-based. If a session receives a DDL and there was a foreign_key_checks off set on that session. The session will be marked for reserved connection. This means that any query sent down after will send set foreign_key_checks = off to the vttablet along with the query. This set statement is used to select the right connection which has those system settings on it.

[GuptaManan100]

The new test is failing in CI -

2023-12-06T09:51:42.8063742Z === RUN   TestDDLFk
2023-12-06T09:51:42.8063872Z     fk_test.go:1224: 
2023-12-06T09:51:42.8064346Z         	Error Trace:	/home/runner/work/vitess/vitess/go/test/endtoend/utils/cmp.go:195
2023-12-06T09:51:42.8065180Z         	            				/home/runner/work/vitess/vitess/go/test/endtoend/vtgate/foreignkey/fk_test.go:1224
2023-12-06T09:51:42.8065402Z         	Error:      	Received unexpected error:
2023-12-06T09:51:42.8069112Z         	            	target: uks.0.primary: vttablet: rpc error: code = Unknown desc = Failed to open the referenced table 'temp2' (errno 1824) (sqlstate HY000) (CallerID: userData1): Sql: "create table temp1 (\n\tid bigint auto_increment primary key,\n\tcol varchar(20) not null,\n\tforeign key (col) references temp2 (col)\n)", BindVars: {} (errno 1824) (sqlstate HY000) during query: create table temp1(id bigint auto_increment primary key, col varchar(20) not null,
2023-12-06T09:51:42.8069489Z         	            	foreign key (col) references temp2(col))
2023-12-06T09:51:42.8069637Z         	Test:       	TestDDLFk
2023-12-06T09:51:42.8069856Z         	Messages:   	[Vitess Error] for query: 
2023-12-06T09:51:42.8070519Z         	            	create table temp1(id bigint auto_increment primary key, col varchar(20) not null,
2023-12-06T09:51:42.8070892Z         	            	foreign key (col) references temp2(col))
2023-12-06T09:51:42.8071028Z --- FAIL: TestDDLFk (0.28s)

[GuptaManan100]

Couple more things to fix. Approving because they are just refactors, the code changes look good to me