Clear AR connections in tests before forking for parallelization

Fixes rails#41176 which seems to affect the mysql2 adapter on MacOS.

The underlying mysql problem was diagnosed in this comment by
@jgloudon:

rails#41176 (comment)

It seems like on MacOS the mysql8 client library is using kqueue which
creates a file descriptor that is not carried into the child process,
allowing for the FD to be re-used.

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

Author: flavorjones

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*
+
 *   PoolConfig no longer keeps a reference to the connection class.
 
     Keeping a reference to the class caused subtle issues when combined with reloading in

activerecord/lib/active_record/test_databases.rb

@@ -4,6 +4,10 @@
 
 module ActiveRecord
   module TestDatabases # :nodoc:
+    ActiveSupport::Testing::Parallelization.before_fork_hook do
+      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*
+
 *   Allow the `#freeze_time` testing helper to accept a date or time argument.
 
     ```ruby

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 +40,12 @@ def initialize(worker_count)
         @url = DRb.start_service("drbunix:", @queue_server).uri
       end
 
+      def before_fork
+        Parallelization.before_fork_hooks.each(&:cb)
+      end
+
       def start
+        before_fork
         @worker_pool = @worker_count.times.map do |worker|
           Worker.new(worker, @url).start
         end