connection_pool.rb

require 'thread'
require 'concurrent/map'
require 'monitor'

module ActiveRecord
  # Raised when a connection could not be obtained within the connection
  # acquisition timeout period: because max connections in pool
  # are in use.
  class ConnectionTimeoutError < ConnectionNotEstablished
  end

  # Raised when a pool was unable to get ahold of all its connections
  # to perform a "group" action such as
  # {ActiveRecord::Base.connection_pool.disconnect!}[rdoc-ref:ConnectionAdapters::ConnectionPool#disconnect!]
  # or {ActiveRecord::Base.clear_reloadable_connections!}[rdoc-ref:ConnectionAdapters::ConnectionHandler#clear_reloadable_connections!].
  class ExclusiveConnectionTimeoutError < ConnectionTimeoutError
  end

  module ConnectionAdapters
    # Connection pool base class for managing Active Record database
    # connections.
    #
    # == Introduction
    #
    # A connection pool synchronizes thread access to a limited number of
    # database connections. The basic idea is that each thread checks out a
    # database connection from the pool, uses that connection, and checks the
    # connection back in. ConnectionPool is completely thread-safe, and will
    # ensure that a connection cannot be used by two threads at the same time,
    # as long as ConnectionPool's contract is correctly followed. It will also
    # handle cases in which there are more threads than connections: if all
    # connections have been checked out, and a thread tries to checkout a
    # connection anyway, then ConnectionPool will wait until some other thread
    # has checked in a connection.
    #
    # == Obtaining (checking out) a connection
    #
    # Connections can be obtained and used from a connection pool in several
    # ways:
    #
    # 1. Simply use {ActiveRecord::Base.connection}[rdoc-ref:ConnectionHandling.connection]
    #    as with Active Record 2.1 and
    #    earlier (pre-connection-pooling). Eventually, when you're done with
    #    the connection(s) and wish it to be returned to the pool, you call
    #    {ActiveRecord::Base.clear_active_connections!}[rdoc-ref:ConnectionAdapters::ConnectionHandler#clear_active_connections!].
    #    This will be the default behavior for Active Record when used in conjunction with
    #    Action Pack's request handling cycle.
    # 2. Manually check out a connection from the pool with
    #    {ActiveRecord::Base.connection_pool.checkout}[rdoc-ref:#checkout]. You are responsible for
    #    returning this connection to the pool when finished by calling
    #    {ActiveRecord::Base.connection_pool.checkin(connection)}[rdoc-ref:#checkin].
    # 3. Use {ActiveRecord::Base.connection_pool.with_connection(&block)}[rdoc-ref:#with_connection], which
    #    obtains a connection, yields it as the sole argument to the block,
    #    and returns it to the pool after the block completes.
    #
    # Connections in the pool are actually AbstractAdapter objects (or objects
    # compatible with AbstractAdapter's interface).
    #
    # == Options
    #
    # There are several connection-pooling-related options that you can add to
    # your database connection configuration:
    #
    # * +pool+: number indicating size of connection pool (default 5)
    # * +checkout_timeout+: number of seconds to block and wait for a connection
    #   before giving up and raising a timeout error (default 5 seconds).
    # * +reaping_frequency+: frequency in seconds to periodically run the
    #   Reaper, which attempts to find and recover connections from dead
    #   threads, which can occur if a programmer forgets to close a
    #   connection at the end of a thread or a thread dies unexpectedly.
    #   Regardless of this setting, the Reaper will be invoked before every
    #   blocking wait. (Default nil, which means don't schedule the Reaper).
    #
    #--
    # Synchronization policy:
    # * all public methods can be called outside +synchronize+
    # * access to these i-vars needs to be in +synchronize+:
    #   * @connections
    #   * @now_connecting
    # * private methods that require being called in a +synchronize+ blocks
    #   are now explicitly documented
    class ConnectionPool
      # Threadsafe, fair, FIFO queue.  Meant to be used by ConnectionPool
      # with which it shares a Monitor.  But could be a generic Queue.
      #
      # The Queue in stdlib's 'thread' could replace this class except
      # stdlib's doesn't support waiting with a timeout.
      class Queue
        def initialize(lock = Monitor.new)
          @lock = lock
          @cond = @lock.new_cond
          @num_waiting = 0
          @queue = []
        end

        # Test if any threads are currently waiting on the queue.
        def any_waiting?
          synchronize do
            @num_waiting > 0
          end
        end

        # Returns the number of threads currently waiting on this
        # queue.
        def num_waiting
          synchronize do
            @num_waiting
          end
        end

        # Add +element+ to the queue.  Never blocks.
        def add(element)
          synchronize do
            @queue.push element
            @cond.signal
          end
        end

        # If +element+ is in the queue, remove and return it, or nil.
        def delete(element)
          synchronize do
            @queue.delete(element)
          end
        end

        # Remove all elements from the queue.
        def clear
          synchronize do
            @queue.clear
          end
        end

        # Remove the head of the queue.
        #
        # If +timeout+ is not given, remove and return the head the
        # queue if the number of available elements is strictly
        # greater than the number of threads currently waiting (that
        # is, don't jump ahead in line).  Otherwise, return nil.
        #
        # If +timeout+ is given, block if there is no element
        # available, waiting up to +timeout+ seconds for an element to
        # become available.
        #
        # Raises:
        # - ActiveRecord::ConnectionTimeoutError if +timeout+ is given and no element
        # becomes available within +timeout+ seconds,
        def poll(timeout = nil)
          synchronize { internal_poll(timeout) }
        end

        private

        def internal_poll(timeout)
          no_wait_poll || (timeout && wait_poll(timeout))
        end

        def synchronize(&block)
          @lock.synchronize(&block)
        end

        # Test if the queue currently contains any elements.
        def any?
          !@queue.empty?
        end

        # A thread can remove an element from the queue without
        # waiting if and only if the number of currently available
        # connections is strictly greater than the number of waiting
        # threads.
        def can_remove_no_wait?
          @queue.size > @num_waiting
        end

        # Removes and returns the head of the queue if possible, or nil.
        def remove
          @queue.shift
        end

        # Remove and return the head the queue if the number of
        # available elements is strictly greater than the number of
        # threads currently waiting.  Otherwise, return nil.
        def no_wait_poll
          remove if can_remove_no_wait?
        end

        # Waits on the queue up to +timeout+ seconds, then removes and
        # returns the head of the queue.
        def wait_poll(timeout)
          @num_waiting += 1

          t0 = Time.now
          elapsed = 0
          loop do
            @cond.wait(timeout - elapsed)

            return remove if any?

            elapsed = Time.now - t0
            if elapsed >= timeout
              msg = 'could not obtain a connection from the pool within %0.3f seconds (waited %0.3f seconds); all pooled connections were in use' %
                [timeout, elapsed]
              raise ConnectionTimeoutError, msg
            end
          end
        ensure
          @num_waiting -= 1
        end
      end

      # Adds the ability to turn a basic fair FIFO queue into one
      # biased to some thread.
      module BiasableQueue # :nodoc:
        class BiasedConditionVariable # :nodoc:
          # semantics of condition variables guarantee that +broadcast+, +broadcast_on_biased+,
          # +signal+ and +wait+ methods are only called while holding a lock
          def initialize(lock, other_cond, preferred_thread)
            @real_cond = lock.new_cond
            @other_cond = other_cond
            @preferred_thread = preferred_thread
            @num_waiting_on_real_cond = 0
          end

          def broadcast
            broadcast_on_biased
            @other_cond.broadcast
          end

          def broadcast_on_biased
            @num_waiting_on_real_cond = 0
            @real_cond.broadcast
          end

          def signal
            if @num_waiting_on_real_cond > 0
              @num_waiting_on_real_cond -= 1
              @real_cond
            else
              @other_cond
            end.signal
          end

          def wait(timeout)
            if Thread.current == @preferred_thread
              @num_waiting_on_real_cond += 1
              @real_cond
            else
              @other_cond
            end.wait(timeout)
          end
        end

        def with_a_bias_for(thread)
          previous_cond = nil
          new_cond      = nil
          synchronize do
            previous_cond = @cond
            @cond = new_cond = BiasedConditionVariable.new(@lock, @cond, thread)
          end
          yield
        ensure
          synchronize do
            @cond = previous_cond if previous_cond
            new_cond.broadcast_on_biased if new_cond # wake up any remaining sleepers
          end
        end
      end

      # Connections must be leased while holding the main pool mutex. This is
      # an internal subclass that also +.leases+ returned connections while
      # still in queue's critical section (queue synchronizes with the same
      # +@lock+ as the main pool) so that a returned connection is already
      # leased and there is no need to re-enter synchronized block.
      class ConnectionLeasingQueue < Queue # :nodoc:
        include BiasableQueue

        private
        def internal_poll(timeout)
          conn = super
          conn.lease if conn
          conn
        end
      end

      # Every +frequency+ seconds, the reaper will call +reap+ on +pool+.
      # A reaper instantiated with a nil frequency will never reap the
      # connection pool.
      #
      # Configure the frequency by setting "reaping_frequency" in your
      # database yaml file.
      class Reaper
        attr_reader :pool, :frequency

        def initialize(pool, frequency)
          @pool      = pool
          @frequency = frequency
        end

        def run
          return unless frequency
          Thread.new(frequency, pool) { |t, p|
            loop do
              sleep t
              p.reap
            end
          }
        end
      end

      include MonitorMixin

      attr_accessor :automatic_reconnect, :checkout_timeout, :schema_cache
      attr_reader :spec, :connections, :size, :reaper

      # Creates a new ConnectionPool object. +spec+ is a ConnectionSpecification
      # object which describes database connection information (e.g. adapter,
      # host name, username, password, etc), as well as the maximum size for
      # this ConnectionPool.
      #
      # The default ConnectionPool maximum size is 5.
      def initialize(spec)
        super()

        @spec = spec

        @checkout_timeout = (spec.config[:checkout_timeout] && spec.config[:checkout_timeout].to_f) || 5
        @reaper = Reaper.new(self, (spec.config[:reaping_frequency] && spec.config[:reaping_frequency].to_f))
        @reaper.run

        # default max pool size to 5
        @size = (spec.config[:pool] && spec.config[:pool].to_i) || 5

        # The cache of threads mapped to reserved connections, the sole purpose
        # of the cache is to speed-up +connection+ method, it is not the authoritative
        # registry of which thread owns which connection, that is tracked by
        # +connection.owner+ attr on each +connection+ instance.
        # The invariant works like this: if there is mapping of <tt>thread => conn</tt>,
        # then that +thread+ does indeed own that +conn+, however an absence of a such
        # mapping does not mean that the +thread+ doesn't own the said connection, in
        # that case +conn.owner+ attr should be consulted.
        # Access and modification of +@thread_cached_conns+ does not require
        # synchronization.
        @thread_cached_conns = Concurrent::Map.new(:initial_capacity => @size)

        @connections         = []
        @automatic_reconnect = true

        # Connection pool allows for concurrent (outside the main +synchronize+ section)
        # establishment of new connections. This variable tracks the number of threads
        # currently in the process of independently establishing connections to the DB.
        @now_connecting = 0

        # A boolean toggle that allows/disallows new connections.
        @new_cons_enabled = true

        @available = ConnectionLeasingQueue.new self
      end

      # Retrieve the connection associated with the current thread, or call
      # #checkout to obtain one if necessary.
      #
      # #connection can be called any number of times; the connection is
      # held in a cache keyed by a thread.
      def connection
        @thread_cached_conns[connection_cache_key(Thread.current)] ||= checkout
      end

      # Is there an open connection that is being used for the current thread?
      #
      # This method only works for connections that have been obtained through
      # #connection or #with_connection methods, connections obtained through
      # #checkout will not be detected by #active_connection?
      def active_connection?
        @thread_cached_conns[connection_cache_key(Thread.current)]
      end

      # Signal that the thread is finished with the current connection.
      # #release_connection releases the connection-thread association
      # and returns the connection to the pool.
      #
      # This method only works for connections that have been obtained through
      # #connection or #with_connection methods, connections obtained through
      # #checkout will not be automatically released.
      def release_connection(owner_thread = Thread.current)
        if conn = @thread_cached_conns.delete(connection_cache_key(owner_thread))
          checkin conn
        end
      end

      # If a connection obtained through #connection or #with_connection methods
      # already exists yield it to the block. If no such connection
      # exists checkout a connection, yield it to the block, and checkin the
      # connection when finished.
      def with_connection
        unless conn = @thread_cached_conns[connection_cache_key(Thread.current)]
          conn = connection
          fresh_connection = true
        end
        yield conn
      ensure
        release_connection if fresh_connection
      end

      # Returns true if a connection has already been opened.
      def connected?
        synchronize { @connections.any? }
      end

      # Disconnects all connections in the pool, and clears the pool.
      #
      # Raises:
      # - ActiveRecord::ExclusiveConnectionTimeoutError if unable to gain ownership of all
      #   connections in the pool within a timeout interval (default duration is
      #   <tt>spec.config[:checkout_timeout] * 2</tt> seconds).
      def disconnect(raise_on_acquisition_timeout = true)
        with_exclusively_acquired_all_connections(raise_on_acquisition_timeout) do
          synchronize do
            @connections.each do |conn|
              checkin conn
              conn.disconnect!
            end
            @connections = []
            @available.clear
          end
        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
      # do so within a timeout interval (default duration is
      # <tt>spec.config[:checkout_timeout] * 2</tt> seconds), the pool is forcefully
      # disconnected without any regard for other connection owning threads.
      def disconnect!
        disconnect(false)
      end

      # Clears the cache which maps classes and re-connects connections that
      # require reloading.
      #
      # Raises:
      # - ActiveRecord::ExclusiveConnectionTimeoutError if unable to gain ownership of all
      #   connections in the pool within a timeout interval (default duration is
      #   <tt>spec.config[:checkout_timeout] * 2</tt> seconds).
      def clear_reloadable_connections(raise_on_acquisition_timeout = true)
        num_new_conns_required = 0

        with_exclusively_acquired_all_connections(raise_on_acquisition_timeout) do
          synchronize do
            @connections.each do |conn|
              checkin conn
              conn.disconnect! if conn.requires_reloading?
            end
            @connections.delete_if(&:requires_reloading?)

            @available.clear

            if @connections.size < @size
              # because of the pruning done by this method, we might be running
              # low on connections, while threads stuck in queue are helpless
              # (not being able to establish new connections for themselves),
              # see also more detailed explanation in +remove+
              num_new_conns_required = num_waiting_in_queue - @connections.size
            end

            @connections.each do |conn|
              @available.add conn
            end
          end
        end

        bulk_make_new_connections(num_new_conns_required) if num_new_conns_required > 0
      end

      # Clears the cache which maps classes and re-connects connections that
      # require reloading.
      #
      # The pool first tries to gain ownership of all connections, if unable to
      # do so within a timeout interval (default duration is
      # <tt>spec.config[:checkout_timeout] * 2</tt> seconds), the pool forcefully
      # clears the cache and reloads connections without any regard for other
      # connection owning threads.
      def clear_reloadable_connections!
        clear_reloadable_connections(false)
      end

      # Check-out a database connection from the pool, indicating that you want
      # to use it. You should call #checkin when you no longer need this.
      #
      # This is done by either returning and leasing existing connection, or by
      # creating a new connection and leasing it.
      #
      # If all connections are leased and the pool is at capacity (meaning the
      # number of currently leased connections is greater than or equal to the
      # size limit set), an ActiveRecord::ConnectionTimeoutError exception will be raised.
      #
      # Returns: an AbstractAdapter object.
      #
      # Raises:
      # - ActiveRecord::ConnectionTimeoutError no connection can be obtained from the pool.
      def checkout(checkout_timeout = @checkout_timeout)
        checkout_and_verify(acquire_connection(checkout_timeout))
      end

      # Check-in a database connection back into the pool, indicating that you
      # no longer need this connection.
      #
      # +conn+: an AbstractAdapter object, which was obtained by earlier by
      # calling #checkout on this pool.
      def checkin(conn)
        synchronize do
          remove_connection_from_thread_cache conn

          conn._run_checkin_callbacks do
            conn.expire
          end

          @available.add conn
        end
      end

      # Remove a connection from the connection pool. The connection will
      # remain open and active but will no longer be managed by this pool.
      def remove(conn)
        needs_new_connection = false

        synchronize do
          remove_connection_from_thread_cache conn

          @connections.delete conn
          @available.delete conn

          # @available.any_waiting? => true means that prior to removing this
          # conn, the pool was at its max size (@connections.size == @size)
          # this would mean that any threads stuck waiting in the queue wouldn't
          # know they could checkout_new_connection, so let's do it for them.
          # Because condition-wait loop is encapsulated in the Queue class
          # (that in turn is oblivious to ConnectionPool implementation), threads
          # that are "stuck" there are helpless, they have no way of creating
          # new connections and are completely reliant on us feeding available
          # connections into the Queue.
          needs_new_connection = @available.any_waiting?
        end

        # This is intentionally done outside of the synchronized section as we
        # would like not to hold the main mutex while checking out new connections,
        # thus there is some chance that needs_new_connection information is now
        # stale, we can live with that (bulk_make_new_connections will make
        # sure not to exceed the pool's @size limit).
        bulk_make_new_connections(1) if needs_new_connection
      end

      # Recover lost connections for the pool. A lost connection can occur if
      # a programmer forgets to checkin a connection at the end of a thread
      # or a thread dies unexpectedly.
      def reap
        stale_connections = synchronize do
          @connections.select do |conn|
            conn.in_use? && !conn.owner.alive?
          end
        end

        stale_connections.each do |conn|
          synchronize do
            if conn.active?
              conn.reset!
              checkin conn
            else
              remove conn
            end
          end
        end
      end

      def num_waiting_in_queue # :nodoc:
        @available.num_waiting
      end

      private
      #--
      # this is unfortunately not concurrent
      def bulk_make_new_connections(num_new_conns_needed)
        num_new_conns_needed.times do
          # try_to_checkout_new_connection will not exceed pool's @size limit
          if new_conn = try_to_checkout_new_connection
            # make the new_conn available to the starving threads stuck @available Queue
            checkin(new_conn)
          end
        end
      end

      #--
      # From the discussion on GitHub:
      #  https://github.com/rails/rails/pull/14938#commitcomment-6601951
      # This hook-in method allows for easier monkey-patching fixes needed by
      # JRuby users that use Fibers.
      def connection_cache_key(thread)
        thread
      end

      # Take control of all existing connections so a "group" action such as
      # reload/disconnect can be performed safely. It is no longer enough to
      # wrap it in +synchronize+ because some pool's actions are allowed
      # to be performed outside of the main +synchronize+ block.
      def with_exclusively_acquired_all_connections(raise_on_acquisition_timeout = true)
        with_new_connections_blocked do
          attempt_to_checkout_all_existing_connections(raise_on_acquisition_timeout)
          yield
        end
      end

      def attempt_to_checkout_all_existing_connections(raise_on_acquisition_timeout = true)
        collected_conns = synchronize do
          # account for our own connections
          @connections.select {|conn| conn.owner == Thread.current}
        end

        newly_checked_out = []
        timeout_time      = Time.now + (@checkout_timeout * 2)

        @available.with_a_bias_for(Thread.current) do
          loop do
            synchronize do
              return if collected_conns.size == @connections.size && @now_connecting == 0
              remaining_timeout = timeout_time - Time.now
              remaining_timeout = 0 if remaining_timeout < 0
              conn = checkout_for_exclusive_access(remaining_timeout)
              collected_conns   << conn
              newly_checked_out << conn
            end
          end
        end
      rescue ExclusiveConnectionTimeoutError
        # <tt>raise_on_acquisition_timeout == false</tt> means we are directed to ignore any
        # timeouts and are expected to just give up: we've obtained as many connections
        # as possible, note that in a case like that we don't return any of the
        # +newly_checked_out+ connections.

        if raise_on_acquisition_timeout
          release_newly_checked_out = true
          raise
        end
      rescue Exception # if something else went wrong
        # this can't be a "naked" rescue, because we have should return conns
        # even for non-StandardErrors
        release_newly_checked_out = true
        raise
      ensure
        if release_newly_checked_out && newly_checked_out
          # releasing only those conns that were checked out in this method, conns
          # checked outside this method (before it was called) are not for us to release
          newly_checked_out.each {|conn| checkin(conn)}
        end
      end

      #--
      # Must be called in a synchronize block.
      def checkout_for_exclusive_access(checkout_timeout)
        checkout(checkout_timeout)
      rescue ConnectionTimeoutError
        # this block can't be easily moved into attempt_to_checkout_all_existing_connections's
        # rescue block, because doing so would put it outside of synchronize section, without
        # being in a critical section thread_report might become inaccurate
        msg = "could not obtain ownership of all database connections in #{checkout_timeout} seconds"

        thread_report = []
        @connections.each do |conn|
          unless conn.owner == Thread.current
            thread_report << "#{conn} is owned by #{conn.owner}"
          end
        end

        msg << " (#{thread_report.join(', ')})" if thread_report.any?

        raise ExclusiveConnectionTimeoutError, msg
      end

      def with_new_connections_blocked
        previous_value = nil
        synchronize do
          previous_value, @new_cons_enabled = @new_cons_enabled, false
        end
        yield
      ensure
        synchronize { @new_cons_enabled = previous_value }
      end

      # Acquire a connection by one of 1) immediately removing one
      # from the queue of available connections, 2) creating a new
      # connection if the pool is not at capacity, 3) waiting on the
      # queue for a connection to become available.
      #
      # Raises:
      # - ActiveRecord::ConnectionTimeoutError if a connection could not be acquired
      #
      #--
      # Implementation detail: the connection returned by +acquire_connection+
      # will already be "+connection.lease+ -ed" to the current thread.
      def acquire_connection(checkout_timeout)
        # NOTE: we rely on +@available.poll+ and +try_to_checkout_new_connection+ to
        # +conn.lease+ the returned connection (and to do this in a +synchronized+
        # section), this is not the cleanest implementation, as ideally we would
        # <tt>synchronize { conn.lease }</tt> in this method, but by leaving it to +@available.poll+
        # and +try_to_checkout_new_connection+ we can piggyback on +synchronize+ sections
        # of the said methods and avoid an additional +synchronize+ overhead.
        if conn = @available.poll || try_to_checkout_new_connection
          conn
        else
          reap
          @available.poll(checkout_timeout)
        end
      end

      #--
      # if owner_thread param is omitted, this must be called in synchronize block
      def remove_connection_from_thread_cache(conn, owner_thread = conn.owner)
        @thread_cached_conns.delete_pair(connection_cache_key(owner_thread), conn)
      end
      alias_method :release, :remove_connection_from_thread_cache

      def new_connection
        Base.send(spec.adapter_method, spec.config).tap do |conn|
          conn.schema_cache = schema_cache.dup if schema_cache
        end
      end

      # If the pool is not at a +@size+ limit, establish new connection. Connecting
      # to the DB is done outside main synchronized section.
      #--
      # Implementation constraint: a newly established connection returned by this
      # method must be in the +.leased+ state.
      def try_to_checkout_new_connection
        # first in synchronized section check if establishing new conns is allowed
        # and increment @now_connecting, to prevent overstepping this pool's @size
        # constraint
        do_checkout = synchronize do
          if @new_cons_enabled && (@connections.size + @now_connecting) < @size
            @now_connecting += 1
          end
        end
        if do_checkout
          begin
            # if successfully incremented @now_connecting establish new connection
            # outside of synchronized section
            conn = checkout_new_connection
          ensure
            synchronize do
              if conn
                adopt_connection(conn)
                # returned conn needs to be already leased
                conn.lease
              end
              @now_connecting -= 1
            end
          end
        end
      end

      def adopt_connection(conn)
        conn.pool = self
        @connections << conn
      end

      def checkout_new_connection
        raise ConnectionNotEstablished unless @automatic_reconnect
        new_connection
      end

      def checkout_and_verify(c)
        c._run_checkout_callbacks do
          c.verify!
        end
        c
      rescue
        remove c
        c.disconnect!
        raise
      end
    end

    # ConnectionHandler is a collection of ConnectionPool objects. It is used
    # for keeping separate connection pools that connect to different databases.
    #
    # For example, suppose that you have 5 models, with the following hierarchy:
    #
    #   class Author < ActiveRecord::Base
    #   end
    #
    #   class BankAccount < ActiveRecord::Base
    #   end
    #
    #   class Book < ActiveRecord::Base
    #     establish_connection "library_db"
    #   end
    #
    #   class ScaryBook < Book
    #   end
    #
    #   class GoodBook < Book
    #   end
    #
    # And a database.yml that looked like this:
    #
    #   development:
    #     database: my_application
    #     host: localhost
    #
    #   library_db:
    #     database: library
    #     host: some.library.org
    #
    # Your primary database in the development environment is "my_application"
    # but the Book model connects to a separate database called "library_db"
    # (this can even be a database on a different machine).
    #
    # Book, ScaryBook and GoodBook will all use the same connection pool to
    # "library_db" while Author, BankAccount, and any other models you create
    # will use the default connection pool to "my_application".
    #
    # The various connection pools are managed by a single instance of
    # ConnectionHandler accessible via ActiveRecord::Base.connection_handler.
    # All Active Record models use this handler to determine the connection pool that they
    # should use.
    #
    # The ConnectionHandler class is not coupled with the Active models, as it has no knowlodge
    # about the model. The model, needs to pass a specification name to the handler,
    # in order to lookup the correct connection pool.
    class ConnectionHandler
      def initialize
        # These caches are keyed by spec.name (ConnectionSpecification#name).
        @owner_to_pool = Concurrent::Map.new(:initial_capacity => 2) do |h,k|
          h[k] = Concurrent::Map.new(:initial_capacity => 2)
        end
      end

      def connection_pool_list
        owner_to_pool.values.compact
      end
      alias :connection_pools :connection_pool_list

      def establish_connection(config)
        resolver = ConnectionSpecification::Resolver.new(Base.configurations)
        spec = resolver.spec(config)

        remove_connection(spec.name)
        owner_to_pool[spec.name] = ConnectionAdapters::ConnectionPool.new(spec)
      end

      # Returns true if there are any active connections among the connection
      # pools that the ConnectionHandler is managing.
      def active_connections?
        connection_pool_list.any?(&:active_connection?)
      end

      # Returns any connections in use by the current thread back to the pool,
      # and also returns connections to the pool cached by threads that are no
      # longer alive.
      def clear_active_connections!
        connection_pool_list.each(&:release_connection)
      end

      # Clears the cache which maps classes.
      #
      # See ConnectionPool#clear_reloadable_connections! for details.
      def clear_reloadable_connections!
        connection_pool_list.each(&:clear_reloadable_connections!)
      end

      def clear_all_connections!
        connection_pool_list.each(&:disconnect!)
      end

      # Locate the connection of the nearest super class. This can be an
      # active or defined connection: if it is the latter, it will be
      # opened and set as the active connection for the class it was defined
      # for (not necessarily the current class).
      def retrieve_connection(spec_name) #:nodoc:
        pool = retrieve_connection_pool(spec_name)
        raise ConnectionNotEstablished, "No connection pool with id '#{spec_name}' found." unless pool
        conn = pool.connection
        raise ConnectionNotEstablished, "No connection for '#{spec_name}' in connection pool" unless conn
        conn
      end

      # Returns true if a connection that's accessible to this class has
      # already been opened.
      def connected?(spec_name)
        conn = retrieve_connection_pool(spec_name)
        conn && conn.connected?
      end

      # Remove the connection for this class. This will close the active
      # connection and the defined connection (if they exist). The result
      # can be used as an argument for establish_connection, for easily
      # re-establishing the connection.
      def remove_connection(spec_name)
        if pool = owner_to_pool.delete(spec_name)
          pool.automatic_reconnect = false
          pool.disconnect!
          pool.spec.config
        end
      end

      # Retrieving the connection pool happens a lot, so we cache it in @owner_to_pool.
      # This makes retrieving the connection pool O(1) once the process is warm.
      # When a connection is established or removed, we invalidate the cache.
      def retrieve_connection_pool(spec_name)
        owner_to_pool.fetch(spec_name) do
          # Check if a connection was previously established in an ancestor process,
          # which may have been forked.
          if ancestor_pool = pool_from_any_process_for(spec_name)
            # A connection was established in an ancestor process that must have
            # subsequently forked. We can't reuse the connection, but we can copy
            # the specification and establish a new connection with it.
            establish_connection(ancestor_pool.spec.to_hash).tap do |pool|
              pool.schema_cache = ancestor_pool.schema_cache if ancestor_pool.schema_cache
            end
          else
            owner_to_pool[spec_name] = nil
          end
        end
      end

      private

      def owner_to_pool
        @owner_to_pool[Process.pid]
      end

      def pool_from_any_process_for(spec_name)
        owner_to_pool = @owner_to_pool.values.find { |v| v[spec_name] }
        owner_to_pool && owner_to_pool[spec_name]
      end
    end
  end
end