Migrate to Rails 6.1 connection handling
What does this MR do?
Use new Rails 6.1 connection handling. This is in preparation for decomposition epic where we will make GitLab use multiple databases.
This should be a fairly safe setting to change as we don't currently use any multi-database methods from Rails, such as while_preventing_writes, preventing_writes, connected_to, connected_to_many, connecting_to, with_role_and_shard, connection_handlers, connection_handlers=, current_role.
However, this changes a behaviour inside Rails' establish_connection method (it switches from using LegacyPoolManager to PoolManager). This change should be safe and result in no-change but because Rails itself, and the GitLab database load balancer use this exact method on application initialization, we use an environment variable to control when this new connection handling gets introduced to production.
Related issue: #332451 (closed), #332288 (closed)
Screenshots (strongly suggested)
Does this MR meet the acceptance criteria?
Conformity
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I have included changelog trailers, or none are needed. (Does this MR need a changelog?) -
I have added/updated documentation, or it's not needed. (Is documentation required?) -
I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?) -
I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?) -
I have self-reviewed this MR per code review guidelines. -
This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines) -
I have followed the style guides. -
This change is backwards compatible across updates, or this does not apply.
Availability and Testing
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I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.) -
I have tested this MR in all supported browsers, or it's not needed. -
I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed. gitlab-com/gl-infra/production#4885 (closed)