Rake task gitlab:db:lock_writes should include table partitions

What does this MR do and why?

Addressing issue: #388308 (closed)

Background:

• Since we introduced the support for multiple databases which is the current setup on GitLab.com (production) and (staging), we have added a rake task to lock the writes on the gitlab_main tables on the database ci, and gitlab_ci tables on the main database. With this MR, we are also including the attached partitions of the tables to into the locking mechanism. With this we also make sure that partitions are also not written to directly from the application. Because that's not supposed to happen anyway.

Some other small issues to address in this issue.

• Renaming let blocks and changing include_examples to it_behaves_like. Because using the same let with parameterized variables causes a caching problem in the let. With this it returns the wrong variable. See the 2nd warning here.
• Not using the same instance object as the default one for tests. Because TablesLocker calls lock_writes and unlock_writes in each run. But we need to test against a particular table only. Either lock or unlock.

How to set up and validate locally

Steps to validate locally

1. Make sure you are running in decomposed database mode

2. Create test partitions on both main and ci

export PSQL_STATEMENT="CREATE TABLE gitlab_partitions_dynamic.test_partition PARTITION OF security_findings FOR VALUES IN (0)"
export MAIN_DATABASE=$(cat config/database.yml|yq .development.main.database)
export CI_DATABASE=$(cat config/database.yml|yq .development.ci.database)

gdk psql -d $MAIN_DATABASE -c $PSQL_STATEMENT
gdk psql -d $CI_DATABASE -c $PSQL_STATEMENT

3. Run this rake task, to lock the writes on the gitlab_ci tables on the main, and gitlab_main on the ci database

VERBOSE=1 ./bin/rake gitlab:db:lock_writes

4. Try to write/delete from the partitions on both main and ci

PSQL_STATEMENT="DELETE FROM gitlab_partitions_dynamic.test_partition"

It should not fail on main

gdk psql -d $MAIN_DATABASE -c $PSQL_STATEMENT

But it should fail on ci

gdk psql -d $CI_DATABASE -c $PSQL_STATEMENT

5. Clean up

• Unlock the writes on all the tables

VERBOSE=1 ./bin/rake gitlab:db:unlock_writes

• Drop the partitions

PSQL_STATEMENT="DROP TABLE gitlab_partitions_dynamic.test_partition"
gdk psql -d $MAIN_DATABASE -c $PSQL_STATEMENT
gdk psql -d $CI_DATABASE -c $PSQL_STATEMENT

Related Followup

In this related follow up issue: #391638 (closed) we are going to include tests in a similar fashion for the detached partitions

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #388308 (closed)