Make SQLite3Adapter fork-safe.

Sqlite itself is not fork-safe, as documented in
https://www.sqlite.org/howtocorrupt.html section 2.6. Any databases
that are inherited from the parent process are unsafe to properly
close and may impact either the parent or the child and potentially
lead to database corruption.

In this commit, a pre-fork callback `prepare_to_fork!` is introduced
for the connection pools and adapters to take any necessary actions
before forking. The callback is only implemented by the SQLite3Adapter
which now invokes `disconnect!` to close all open prepared statements
and database connections.

As a side effect, the parent process will end up having to re-open
database connections if it continues to do work, which may be a small
performance overhead for some use cases, but is necessary in order to
prevent database corruption.

See rails/solid_queue#324 for examples of
the type of corruption that can occur.

activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb

@@ -442,6 +442,16 @@ def disconnect(raise_on_acquisition_timeout = true)
         end
       end
 
+      # Notify connections that a fork is imminent, to allow them to take necessary measures to
+      # ensure fork-safety.
+      #
+      # See AbstractAdapter#prepare_to_fork!
+      def prepare_to_fork! # :nodoc:
+        @connections.each do |conn|
+          conn.prepare_to_fork!
+        end
+      end
+
       # Disconnects all connections in the pool, and clears the pool.
       #
       # The pool first tries to gain ownership of all connections. If unable to

activerecord/lib/active_record/connection_adapters/abstract_adapter.rb

@@ -696,6 +696,13 @@ def reconnect!(restore_transactions: false)
         end
       end
 
+      # Prepare the connection for an imminent fork. Some database adapters are not fork-safe
+      # (specifically sqlite3), and so this gives the adapter an opportunity to close or finalize
+      # any objects that the child process should not inherit.
+      def prepare_to_fork!
+        # This should be overridden by concrete adapters.
+      end
+
       # Disconnects from the database if already connected. Otherwise, this
       # method does nothing.
       def disconnect!

activerecord/lib/active_record/connection_adapters/pool_config.rb

@@ -17,6 +17,10 @@ def schema_reflection
       private_constant :INSTANCES
 
       class << self
+        def prepare_to_fork!
+          INSTANCES.each_key(&:prepare_to_fork!)
+        end
+
         def discard_pools!
           INSTANCES.each_key(&:discard_pool!)
         end
@@ -66,6 +70,14 @@ def pool
         @pool || synchronize { @pool ||= ConnectionAdapters::ConnectionPool.new(self) }
       end
 
+      def prepare_to_fork!
+        synchronize do
+          return unless @pool
+
+          @pool.prepare_to_fork!
+        end
+      end
+
       def discard_pool!
         return unless @pool
 
@@ -80,4 +92,5 @@ def discard_pool!
   end
 end
 
+ActiveSupport::ForkTracker.before_fork { ActiveRecord::ConnectionAdapters::PoolConfig.prepare_to_fork! }
 ActiveSupport::ForkTracker.after_fork { ActiveRecord::ConnectionAdapters::PoolConfig.discard_pools! }

activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb

@@ -497,6 +497,19 @@ def initialize_type_map(m)
       TYPE_MAP = Type::TypeMap.new.tap { |m| initialize_type_map(m) }
       EXTENDED_TYPE_MAPS = Concurrent::Map.new
 
+      def prepare_to_fork!
+        # It's not safe to fork with an open sqlite connection, because when the object is GCed in
+        # the child process, it can interfere with existing or subsequent connections to the
+        # database in the child process, which can lead to corruption.
+        #
+        # Note that closing the database will raise a BusyException unless all related statements
+        # are also closed, which the AbstractAdapter's disconnect! method handles for us.
+        #
+        # More on sqlite fork-safety: https://www.sqlite.org/howtocorrupt.html section 2.6
+        # More on Rails db corruption: https://github.com/rails/solid_queue/issues/324
+        disconnect!
+      end
+
       private
         # See https://www.sqlite.org/limits.html,
         # the default value is 999 when not configured.