Use custom class for pending migrations connection

In bensheldon/good_job#1103 it was reported that a connection not
established is being seen when there are multiple configs for an
environment (a multi-db app).

While I think that something is happening where Good Job is accessing
the connection while we're checking for pending migrations, this could
happen in other gems if we're clobbering the connection on
`ActiveRecord::Base` that is needed.

Here we are making a special class to establish connections to the
database. Because the class does not inherit from `Base` and establishes
it's own connection, it won't clobber any existing connections on
`Base`, leaving them in-tact while the app boots.

The reason this wasn't seen in 7.0 is because pending migrations didn't
check all the available configs. Once that was fixed this error
appeared.

I didn't add a test for this because it's really hard to reproduce
without good job and actually booting a server and running a request.

Fixes: rails#49689

Author: eileencodes

activerecord/lib/active_record/migration.rb

@@ -7,6 +7,7 @@
 require "active_support/core_ext/enumerable"
 require "active_support/core_ext/module/attribute_accessors"
 require "active_support/actionable_error"
+require "active_record/migration/pending_migration_connection"
 
 module ActiveRecord
   class MigrationError < ActiveRecordError # :nodoc:
@@ -768,9 +769,11 @@ def db_configs_in_current_env
         def pending_migrations
           pending_migrations = []
 
-          ActiveRecord::Tasks::DatabaseTasks.with_temporary_connection_for_each(env: env) do |connection|
-            if pending = connection.migration_context.open.pending_migrations
-              pending_migrations << pending
+          ActiveRecord::Base.configurations.configs_for(env_name: env).each do |db_config|
+            ActiveRecord::PendingMigrationConnection.establish_temporary_connection(db_config) do |conn|
+              if pending = conn.migration_context.open.pending_migrations
+                pending_migrations << pending
+              end
             end
           end
 

activerecord/lib/active_record/migration/pending_migration_connection.rb

@@ -0,0 +1,21 @@
+# frozen_string_literal: true
+
+module ActiveRecord
+  class PendingMigrationConnection # :nodoc:
+    def self.establish_temporary_connection(db_config, &block)
+      pool = ActiveRecord::Base.connection_handler.establish_connection(db_config, owner_name: self)
+
+      yield pool.connection
+    ensure
+      ActiveRecord::Base.connection_handler.remove_connection_pool(self.name)
+    end
+
+    def self.primary_class?
+      false
+    end
+
+    def self.current_preventing_writes
+      false
+    end
+  end
+end