Clear AR connections in tests before forking for parallelization

Fixes rails#41176 which seems to affect the mysql2 and postgresql adapters.

Occasionally large test suites see something like this:

```
/path/to/installs/ruby/3.4.1/lib/ruby/gems/3.4.0/gems/drb-2.2.1/lib/drb/drb.rb:1003:in 'IO#wait_readable': Bad file descriptor (Errno::EBADF)
	from /path/to/installs/ruby/3.4.1/lib/ruby/gems/3.4.0/gems/drb-2.2.1/lib/drb/drb.rb:1003:in 'DRb::DRbTCPSocket#alive?'
	from /path/to/installs/ruby/3.4.1/lib/ruby/gems/3.4.0/gems/drb-2.2.1/lib/drb/drb.rb:1335:in 'DRb::DRbConn#alive?'
	from /path/to/installs/ruby/3.4.1/lib/ruby/gems/3.4.0/gems/drb-2.2.1/lib/drb/drb.rb:1271:in 'block (3 levels) in DRb::DRbConn.make_pool'
	from /path/to/installs/ruby/3.4.1/lib/ruby/gems/3.4.0/gems/drb-2.2.1/lib/drb/drb.rb:1269:in 'Array#each'
	from /path/to/installs/ruby/3.4.1/lib/ruby/gems/3.4.0/gems/drb-2.2.1/lib/drb/drb.rb:1269:in 'block (2 levels) in DRb::DRbConn.make_pool'
	from /path/to/installs/ruby/3.4.1/lib/ruby/gems/3.4.0/gems/drb-2.2.1/lib/drb/drb.rb:1240:in 'block in DRb::ThreadObject#_execute'
	from /path/to/installs/ruby/3.4.1/lib/ruby/3.4.0/monitor.rb:201:in 'Monitor#synchronize'
	from /path/to/installs/ruby/3.4.1/lib/ruby/3.4.0/monitor.rb:201:in 'MonitorMixin#mon_synchronize'
	from /path/to/installs/ruby/3.4.1/lib/ruby/gems/3.4.0/gems/drb-2.2.1/lib/drb/drb.rb:1238:in 'DRb::ThreadObject#_execute'
	from /path/to/installs/ruby/3.4.1/lib/ruby/gems/3.4.0/gems/drb-2.2.1/lib/drb/drb.rb:1263:in 'block in DRb::DRbConn.make_pool'
```

Co-authored-by: Donal McBreen <[email protected]>

activerecord/CHANGELOG.md

@@ -1,3 +1,10 @@
+*   Introduce a before-fork hook in `ActiveSupport::Testing::Parallelization` to clear existing
+    connections, to avoid fork-safety issues with the mysql2 adapter.
+
+    Fixes #41776
+
+    *Mike Dalessio*, *Donal McBreen*
+
 *   Introduce versions formatter for the schema dumper.
 
     It is now possible to override how schema dumper formats versions information inside the

activerecord/lib/active_record/test_databases.rb

@@ -4,6 +4,11 @@
 
 module ActiveRecord
   module TestDatabases # :nodoc:
+    ActiveSupport::Testing::Parallelization.before_fork_hook do
+      puts "PREFORK: clearing connections"
+      ActiveRecord::Base.connection_handler.clear_all_connections!
+    end
+
     ActiveSupport::Testing::Parallelization.after_fork_hook do |i|
       create_and_load_schema(i, env_name: ActiveRecord::ConnectionHandling::DEFAULT_ENV.call)
     end

activesupport/CHANGELOG.md

@@ -1,3 +1,8 @@
+*   `ActiveSupport::Testing::Parallelization.before_fork_hook` allows declaration of callbacks that
+    are invoked immediately before forking test workers.
+
+    *Mike Dalessio*
+
 *   `nil.to_query("key")` now returns `key`.
 
     Previously it would return `key=`, preventing round tripping with `Rack::Utils.parse_nested_query`.

activesupport/lib/active_support/testing/parallelization.rb

@@ -9,6 +9,14 @@
 module ActiveSupport
   module Testing
     class Parallelization # :nodoc:
+      @@before_fork_hooks = []
+
+      def self.before_fork_hook(&blk)
+        @@before_fork_hooks << blk
+      end
+
+      cattr_reader :before_fork_hooks
+
       @@after_fork_hooks = []
 
       def self.after_fork_hook(&blk)
@@ -32,7 +40,14 @@ def initialize(worker_count)
         @url = DRb.start_service("drbunix:", @queue_server).uri
       end
 
+      def before_fork
+        Parallelization.before_fork_hooks.each do |cb|
+          cb.call
+        end
+      end
+
       def start
+        before_fork
         @worker_pool = @worker_count.times.map do |worker|
           Worker.new(worker, @url).start
         end