18004: Fixes a couple of race condition bugs related to caching remote users.

[arvados.git] / services / api / app / models / arvados_model.rb

# Copyright (C) The Arvados Authors. All rights reserved.
#
# SPDX-License-Identifier: AGPL-3.0

require 'arvados_model_updates'
require 'has_uuid'
require 'record_filters'
require 'serializers'
require 'request_error'

class ArvadosModel < ApplicationRecord
  self.abstract_class = true

  include ArvadosModelUpdates
  include CurrentApiClient      # current_user, current_api_client, etc.
  include DbCurrentTime
  extend RecordFilters

  after_find :schedule_restoring_changes
  after_initialize :log_start_state
  before_save :ensure_permission_to_save
  before_save :ensure_owner_uuid_is_permitted
  before_save :ensure_ownership_path_leads_to_user
  before_destroy :ensure_owner_uuid_is_permitted
  before_destroy :ensure_permission_to_destroy
  before_create :update_modified_by_fields
  before_update :maybe_update_modified_by_fields
  after_create :log_create
  after_update :log_update
  after_destroy :log_destroy
  before_validation :normalize_collection_uuids
  before_validation :set_default_owner
  validate :ensure_valid_uuids

  # Note: This only returns permission links. It does not account for
  # permissions obtained via user.is_admin or
  # user.uuid==object.owner_uuid.
  has_many(:permissions,
           ->{where(link_class: 'permission')},
           foreign_key: :head_uuid,
           class_name: 'Link',
           primary_key: :uuid)

  # If async is true at create or update, permission graph
  # update is deferred allowing making multiple calls without the performance
  # penalty.
  attr_accessor :async_permissions_update

  # Ignore listed attributes on mass assignments
  def self.protected_attributes
    []
  end

  class PermissionDeniedError < RequestError
    def http_status
      403
    end
  end

  class AlreadyLockedError < RequestError
    def http_status
      422
    end
  end

  class LockFailedError < RequestError
    def http_status
      422
    end
  end

  class InvalidStateTransitionError < RequestError
    def http_status
      422
    end
  end

  class UnauthorizedError < RequestError
    def http_status
      401
    end
  end

  class UnresolvableContainerError < RequestError
    def http_status
      422
    end
  end

  def self.kind_class(kind)
    kind.match(/^arvados\#(.+)$/)[1].classify.safe_constantize rescue nil
  end

  def href
    "#{current_api_base}/#{self.class.to_s.pluralize.underscore}/#{self.uuid}"
  end

  def self.permit_attribute_params raw_params
    # strong_parameters does not provide security: permissions are
    # implemented with before_save hooks.
    #
    # The following permit! is necessary even with
    # "ActionController::Parameters.permit_all_parameters = true",
    # because permit_all does not permit nested attributes.
    raw_params ||= {}

    if raw_params
      raw_params = raw_params.to_hash
      raw_params.delete_if { |k, _| self.protected_attributes.include? k }
      serialized_attributes.each do |colname, coder|
        param = raw_params[colname.to_sym]
        if param.nil?
          # ok
        elsif !param.is_a?(coder.object_class)
          raise ArgumentError.new("#{colname} parameter must be #{coder.object_class}, not #{param.class}")
        elsif has_nonstring_keys?(param)
          raise ArgumentError.new("#{colname} parameter cannot have non-string hash keys")
        end
      end
      # Check JSONB columns that aren't listed on serialized_attributes
      columns.select{|c| c.type == :jsonb}.collect{|j| j.name}.each do |colname|
        if serialized_attributes.include? colname || raw_params[colname.to_sym].nil?
          next
        end
        if has_nonstring_keys?(raw_params[colname.to_sym])
          raise ArgumentError.new("#{colname} parameter cannot have non-string hash keys")
        end
      end
    end
    ActionController::Parameters.new(raw_params).permit!
  end

  def initialize raw_params={}, *args
    super(self.class.permit_attribute_params(raw_params), *args)
  end

  # Reload "old attributes" for logging, too.
  def reload(*args)
    super
    log_start_state
    self
  end

  def self.create raw_params={}, *args
    super(permit_attribute_params(raw_params), *args)
  end

  def update_attributes raw_params={}, *args
    super(self.class.permit_attribute_params(raw_params), *args)
  end

  def self.selectable_attributes(template=:user)
    # Return an array of attribute name strings that can be selected
    # in the given template.
    api_accessible_attributes(template).map { |attr_spec| attr_spec.first.to_s }
  end

  def self.searchable_columns operator
    textonly_operator = !operator.match(/[<=>]/)
    self.columns.select do |col|
      case col.type
      when :string, :text
        true
      when :datetime, :integer, :boolean
        !textonly_operator
      else
        false
      end
    end.map(&:name)
  end

  def self.attribute_column attr
    self.columns.select { |col| col.name == attr.to_s }.first
  end

  def self.attributes_required_columns
    # This method returns a hash.  Each key is the name of an API attribute,
    # and it's mapped to a list of database columns that must be fetched
    # to generate that attribute.
    # This implementation generates a simple map of attributes to
    # matching column names.  Subclasses can override this method
    # to specify that method-backed API attributes need to fetch
    # specific columns from the database.
    all_columns = columns.map(&:name)
    api_column_map = Hash.new { |hash, key| hash[key] = [] }
    methods.grep(/^api_accessible_\w+$/).each do |method_name|
      next if method_name == :api_accessible_attributes
      send(method_name).each_pair do |api_attr_name, col_name|
        col_name = col_name.to_s
        if all_columns.include?(col_name)
          api_column_map[api_attr_name.to_s] |= [col_name]
        end
      end
    end
    api_column_map
  end

  def self.ignored_select_attributes
    ["href", "kind", "etag"]
  end

  def self.columns_for_attributes(select_attributes)
    if select_attributes.empty?
      raise ArgumentError.new("Attribute selection list cannot be empty")
    end
    api_column_map = attributes_required_columns
    invalid_attrs = []
    select_attributes.each do |s|
      next if ignored_select_attributes.include? s
      if not s.is_a? String or not api_column_map.include? s
        invalid_attrs << s
      end
    end
    if not invalid_attrs.empty?
      raise ArgumentError.new("Invalid attribute(s): #{invalid_attrs.inspect}")
    end
    # Given an array of attribute names to select, return an array of column
    # names that must be fetched from the database to satisfy the request.
    select_attributes.flat_map { |attr| api_column_map[attr] }.uniq
  end

  def self.default_orders
    ["#{table_name}.modified_at desc", "#{table_name}.uuid"]
  end

  def self.unique_columns
    ["id", "uuid"]
  end

  def self.limit_index_columns_read
    # This method returns a list of column names.
    # If an index request reads that column from the database,
    # APIs that return lists will only fetch objects until reaching
    # max_index_database_read bytes of data from those columns.
    []
  end

  # If current user can manage the object, return an array of uuids of
  # users and groups that have permission to write the object. The
  # first two elements are always [self.owner_uuid, current user's
  # uuid].
  #
  # If current user can write but not manage the object, return
  # [self.owner_uuid, current user's uuid].
  #
  # If current user cannot write this object, just return
  # [self.owner_uuid].
  def writable_by
    return [owner_uuid] if not current_user
    unless (owner_uuid == current_user.uuid or
            current_user.is_admin or
            (current_user.groups_i_can(:manage) & [uuid, owner_uuid]).any?)
      if ((current_user.groups_i_can(:write) + [current_user.uuid]) &
          [uuid, owner_uuid]).any?
        return [owner_uuid, current_user.uuid]
      else
        return [owner_uuid]
      end
    end
    [owner_uuid, current_user.uuid] + permissions.collect do |p|
      if ['can_write', 'can_manage'].index p.name
        p.tail_uuid
      end
    end.compact.uniq
  end

  # Return a query with read permissions restricted to the union of the
  # permissions of the members of users_list, i.e. if something is readable by
  # any user in users_list, it will be readable in the query returned by this
  # function.
  def self.readable_by(*users_list)
    # Get rid of troublesome nils
    users_list.compact!

    # Load optional keyword arguments, if they exist.
    if users_list.last.is_a? Hash
      kwargs = users_list.pop
    else
      kwargs = {}
    end

    # Collect the UUIDs of the authorized users.
    sql_table = kwargs.fetch(:table_name, table_name)
    include_trash = kwargs.fetch(:include_trash, false)
    include_old_versions = kwargs.fetch(:include_old_versions, false)

    sql_conds = nil
    user_uuids = users_list.map { |u| u.uuid }
    all_user_uuids = []

    # For details on how the trashed_groups table is constructed, see
    # see db/migrate/20200501150153_permission_table.rb

    exclude_trashed_records = ""
    if !include_trash and (sql_table == "groups" or sql_table == "collections") then
      # Only include records that are not trashed
      exclude_trashed_records = "AND (#{sql_table}.trash_at is NULL or #{sql_table}.trash_at > statement_timestamp())"
    end

    trashed_check = ""
    if !include_trash && sql_table != "api_client_authorizations"
      trashed_check = "#{sql_table}.owner_uuid NOT IN (SELECT group_uuid FROM #{TRASHED_GROUPS} " +
                      "where trash_at <= statement_timestamp()) #{exclude_trashed_records}"
    end

    if users_list.select { |u| u.is_admin }.any?
      # Admin skips most permission checks, but still want to filter on trashed items.
      if !include_trash && sql_table != "api_client_authorizations"
        # Only include records where the owner is not trashed
        sql_conds = trashed_check
      end
    else
      # The core of the permission check is a join against the
      # materialized_permissions table to determine if the user has at
      # least read permission to either the object itself or its
      # direct owner (if traverse_owned is true).  See
      # db/migrate/20200501150153_permission_table.rb for details on
      # how the permissions are computed.

      # A user can have can_manage access to another user, this grants
      # full access to all that user's stuff.  To implement that we
      # need to include those other users in the permission query.

      # This was previously implemented by embedding the subquery
      # directly into the query, but it was discovered later that this
      # causes the Postgres query planner to do silly things because
      # the query heuristics assumed the subquery would have a lot
      # more rows that it does, and choose a bad merge strategy.  By
      # doing the query here and embedding the result as a constant,
      # Postgres also knows exactly how many items there are and can
      # choose the right query strategy.
      #
      # (note: you could also do this with a temporary table, but that
      # would require all every request be wrapped in a transaction,
      # which is not currently the case).

      all_user_uuids = ActiveRecord::Base.connection.exec_query %{
#{USER_UUIDS_SUBQUERY_TEMPLATE % {user: "'#{user_uuids.join "', '"}'", perm_level: 1}}
},
                                             'readable_by.user_uuids'

      user_uuids_subquery = ":user_uuids"

      # Note: it is possible to combine the direct_check and
      # owner_check into a single IN (SELECT) clause, however it turns
      # out query optimizer doesn't like it and forces a sequential
      # table scan.  Constructing the query with separate IN (SELECT)
      # clauses enables it to use the index.
      #
      # see issue 13208 for details.

      # Match a direct read permission link from the user to the record uuid
      direct_check = "#{sql_table}.uuid IN (SELECT target_uuid FROM #{PERMISSION_VIEW} "+
                     "WHERE user_uuid IN (#{user_uuids_subquery}) AND perm_level >= 1)"

      # Match a read permission for the user to the record's
      # owner_uuid.  This is so we can have a permissions table that
      # mostly consists of users and groups (projects are a type of
      # group) and not have to compute and list user permission to
      # every single object in the system.
      #
      # Don't do this for API keys (special behavior) or groups
      # (already covered by direct_check).
      #
      # The traverse_owned flag indicates whether the permission to
      # read an object also implies transitive permission to read
      # things the object owns.  The situation where this is important
      # are determining if we can read an object owned by another
      # user.  This makes it possible to have permission to read the
      # user record without granting permission to read things the
      # other user owns.
      owner_check = ""
      if sql_table != "api_client_authorizations" and sql_table != "groups" then
        owner_check = "#{sql_table}.owner_uuid IN (SELECT target_uuid FROM #{PERMISSION_VIEW} "+
                      "WHERE user_uuid IN (#{user_uuids_subquery}) AND perm_level >= 1 AND traverse_owned) "

        # We want to do owner_check before direct_check in the OR
        # clause.  The order of the OR clause isn't supposed to
        # matter, but in practice, it does -- apparently in the
        # absence of other hints, it uses the ordering from the query.
        # For certain types of queries (like filtering on owner_uuid),
        # every item will match the owner_check clause, so then
        # Postgres will optimize out the direct_check entirely.
        direct_check = " OR " + direct_check
      end

      links_cond = ""
      if sql_table == "links"
        # Match any permission link that gives one of the authorized
        # users some permission _or_ gives anyone else permission to
        # view one of the authorized users.
        links_cond = "OR (#{sql_table}.link_class IN (:permission_link_classes) AND "+
                       "(#{sql_table}.head_uuid IN (#{user_uuids_subquery}) OR #{sql_table}.tail_uuid IN (#{user_uuids_subquery})))"
      end

      sql_conds = "(#{owner_check} #{direct_check} #{links_cond}) #{trashed_check.empty? ? "" : "AND"} #{trashed_check}"

    end

    if !include_old_versions && sql_table == "collections"
      exclude_old_versions = "#{sql_table}.uuid = #{sql_table}.current_version_uuid"
      if sql_conds.nil?
        sql_conds = exclude_old_versions
      else
        sql_conds += " AND #{exclude_old_versions}"
      end
    end

    self.where(sql_conds,
               user_uuids: all_user_uuids.collect{|c| c["target_uuid"]},
               permission_link_classes: ['permission', 'resources'])
  end

  def save_with_unique_name!
    uuid_was = uuid
    name_was = name
    max_retries = 2
    transaction do
      conn = ActiveRecord::Base.connection
      conn.exec_query 'SAVEPOINT save_with_unique_name'
      begin
        save!
      rescue ActiveRecord::RecordNotUnique => rn
        raise if max_retries == 0
        max_retries -= 1

        conn.exec_query 'ROLLBACK TO SAVEPOINT save_with_unique_name'

        # Dig into the error to determine if it is specifically calling out a
        # (owner_uuid, name) uniqueness violation.  In this specific case, and
        # the client requested a unique name with ensure_unique_name==true,
        # update the name field and try to save again.  Loop as necessary to
        # discover a unique name.  It is necessary to handle name choosing at
        # this level (as opposed to the client) to ensure that record creation
        # never fails due to a race condition.
        err = rn.cause
        raise unless err.is_a?(PG::UniqueViolation)

        # Unfortunately ActiveRecord doesn't abstract out any of the
        # necessary information to figure out if this the error is actually
        # the specific case where we want to apply the ensure_unique_name
        # behavior, so the following code is specialized to Postgres.
        detail = err.result.error_field(PG::Result::PG_DIAG_MESSAGE_DETAIL)
        raise unless /^Key \(owner_uuid, name\)=\([a-z0-9]{5}-[a-z0-9]{5}-[a-z0-9]{15}, .*?\) already exists\./.match detail

        new_name = "#{name_was} (#{db_current_time.utc.iso8601(3)})"
        if new_name == name
          # If the database is fast enough to do two attempts in the
          # same millisecond, we need to wait to ensure we try a
          # different timestamp on each attempt.
          sleep 0.002
          new_name = "#{name_was} (#{db_current_time.utc.iso8601(3)})"
        end

        self[:name] = new_name
        if uuid_was.nil? && !uuid.nil?
          self[:uuid] = nil
          if self.is_a? Collection
            # Reset so that is assigned to the new UUID
            self[:current_version_uuid] = nil
          end
        end
        conn.exec_query 'SAVEPOINT save_with_unique_name'
        retry
      ensure
        conn.exec_query 'RELEASE SAVEPOINT save_with_unique_name'
      end
    end
  end

  def user_owner_uuid
    if self.owner_uuid.nil?
      return current_user.uuid
    end
    owner_class = ArvadosModel.resource_class_for_uuid(self.owner_uuid)
    if owner_class == User
      self.owner_uuid
    else
      owner_class.find_by_uuid(self.owner_uuid).user_owner_uuid
    end
  end

  def logged_attributes
    attributes.except(*Rails.configuration.AuditLogs.UnloggedAttributes.stringify_keys.keys)
  end

  def self.full_text_searchable_columns
    self.columns.select do |col|
      [:string, :text, :jsonb].include?(col.type)
    end.map(&:name)
  end

  def self.full_text_coalesce
    full_text_searchable_columns.collect do |column|
      is_jsonb = self.columns.select{|x|x.name == column}[0].type == :jsonb
      cast = (is_jsonb || serialized_attributes[column]) ? '::text' : ''
      "coalesce(#{column}#{cast},'')"
    end
  end

  def self.full_text_trgm
    "(#{full_text_coalesce.join(" || ' ' || ")})"
  end

  def self.full_text_tsvector
    parts = full_text_searchable_columns.collect do |column|
      is_jsonb = self.columns.select{|x|x.name == column}[0].type == :jsonb
      cast = (is_jsonb || serialized_attributes[column]) ? '::text' : ''
      "coalesce(#{column}#{cast},'')"
    end
    "to_tsvector('english', substr(#{parts.join(" || ' ' || ")}, 0, 8000))"
  end

  def self.apply_filters query, filters
    ft = record_filters filters, self
    if not ft[:cond_out].any?
      return query
    end
    ft[:joins].each do |t|
      query = query.joins(t)
    end
    query.where('(' + ft[:cond_out].join(') AND (') + ')',
                          *ft[:param_out])
  end

  protected

  def self.deep_sort_hash(x)
    if x.is_a? Hash
      x.sort.collect do |k, v|
        [k, deep_sort_hash(v)]
      end.to_h
    elsif x.is_a? Array
      x.collect { |v| deep_sort_hash(v) }
    else
      x
    end
  end

  def ensure_ownership_path_leads_to_user
    if new_record? or owner_uuid_changed?
      uuid_in_path = {owner_uuid => true, uuid => true}
      x = owner_uuid
      while (owner_class = ArvadosModel::resource_class_for_uuid(x)) != User
        begin
          if x == uuid
            # Test for cycles with the new version, not the DB contents
            x = owner_uuid
          elsif !owner_class.respond_to? :find_by_uuid
            raise ActiveRecord::RecordNotFound.new
          else
            x = owner_class.find_by_uuid(x).owner_uuid
          end
        rescue ActiveRecord::RecordNotFound => e
          errors.add :owner_uuid, "is not owned by any user: #{e}"
          throw(:abort)
        end
        if uuid_in_path[x]
          if x == owner_uuid
            errors.add :owner_uuid, "would create an ownership cycle"
          else
            errors.add :owner_uuid, "has an ownership cycle"
          end
          throw(:abort)
        end
        uuid_in_path[x] = true
      end
    end
    true
  end

  def set_default_owner
    if new_record? and current_user and respond_to? :owner_uuid=
      self.owner_uuid ||= current_user.uuid
    end
  end

  def ensure_owner_uuid_is_permitted
    raise PermissionDeniedError if !current_user

    if self.owner_uuid.nil?
      errors.add :owner_uuid, "cannot be nil"
      raise PermissionDeniedError
    end

    rsc_class = ArvadosModel::resource_class_for_uuid owner_uuid
    unless rsc_class == User or rsc_class == Group
      errors.add :owner_uuid, "must be set to User or Group"
      raise PermissionDeniedError
    end

    if new_record? || owner_uuid_changed?
      # Permission on owner_uuid_was is needed to move an existing
      # object away from its previous owner (which implies permission
      # to modify this object itself, so we don't need to check that
      # separately). Permission on the new owner_uuid is also needed.
      [['old', owner_uuid_was],
       ['new', owner_uuid]
      ].each do |which, check_uuid|
        if check_uuid.nil?
          # old_owner_uuid is nil? New record, no need to check.
        elsif !current_user.can?(write: check_uuid)
          logger.warn "User #{current_user.uuid} tried to set ownership of #{self.class.to_s} #{self.uuid} but does not have permission to write #{which} owner_uuid #{check_uuid}"
          errors.add :owner_uuid, "cannot be set or changed without write permission on #{which} owner"
          raise PermissionDeniedError
        elsif rsc_class == Group && Group.find_by_uuid(owner_uuid).group_class != "project"
          errors.add :owner_uuid, "must be a project"
          raise PermissionDeniedError
        end
      end
    else
      # If the object already existed and we're not changing
      # owner_uuid, we only need write permission on the object
      # itself.
      if !current_user.can?(write: self.uuid)
        logger.warn "User #{current_user.uuid} tried to modify #{self.class.to_s} #{self.uuid} without write permission"
        errors.add :uuid, " #{uuid} is not writable by #{current_user.uuid}"
        raise PermissionDeniedError
      end
    end

    true
  end

  def ensure_permission_to_save
    unless (new_record? ? permission_to_create : permission_to_update)
      raise PermissionDeniedError
    end
  end

  def permission_to_create
    current_user.andand.is_active
  end

  def permission_to_update
    if !current_user
      logger.warn "Anonymous user tried to update #{self.class.to_s} #{self.uuid_was}"
      return false
    end
    if !current_user.is_active
      logger.warn "Inactive user #{current_user.uuid} tried to update #{self.class.to_s} #{self.uuid_was}"
      return false
    end
    return true if current_user.is_admin
    if self.uuid_changed?
      logger.warn "User #{current_user.uuid} tried to change uuid of #{self.class.to_s} #{self.uuid_was} to #{self.uuid}"
      return false
    end
    return true
  end

  def ensure_permission_to_destroy
    raise PermissionDeniedError unless permission_to_destroy
  end

  def permission_to_destroy
    if [system_user_uuid, system_group_uuid, anonymous_group_uuid,
        anonymous_user_uuid, public_project_uuid].include? uuid
      false
    else
      permission_to_update
    end
  end

  def maybe_update_modified_by_fields
    update_modified_by_fields if self.changed? or self.new_record?
    true
  end

  def update_modified_by_fields
    current_time = db_current_time
    self.created_at ||= created_at_was || current_time
    self.updated_at = current_time
    self.owner_uuid ||= current_default_owner if self.respond_to? :owner_uuid=
    if !anonymous_updater
      self.modified_by_user_uuid = current_user ? current_user.uuid : nil
    end
    if !timeless_updater
      self.modified_at = current_time
    end
    self.modified_by_client_uuid = current_api_client ? current_api_client.uuid : nil
    true
  end

  def self.has_nonstring_keys? x
    if x.is_a? Hash
      x.each do |k,v|
        return true if !(k.is_a?(String) || k.is_a?(Symbol)) || has_nonstring_keys?(v)
      end
    elsif x.is_a? Array
      x.each do |v|
        return true if has_nonstring_keys?(v)
      end
    end
    false
  end

  def self.where_serialized(colname, value, md5: false)
    colsql = colname.to_s
    if md5
      colsql = "md5(#{colsql})"
    end
    if value.empty?
      # rails4 stores as null, rails3 stored as serialized [] or {}
      sql = "#{colsql} is null or #{colsql} IN (?)"
      sorted = value
    else
      sql = "#{colsql} IN (?)"
      sorted = deep_sort_hash(value)
    end
    params = [sorted.to_yaml, SafeJSON.dump(sorted)]
    if md5
      params = params.map { |x| Digest::MD5.hexdigest(x) }
    end
    where(sql, params)
  end

  Serializer = {
    Hash => HashSerializer,
    Array => ArraySerializer,
  }

  def self.serialize(colname, type)
    coder = Serializer[type]
    @serialized_attributes ||= {}
    @serialized_attributes[colname.to_s] = coder
    super(colname, coder)
  end

  def self.serialized_attributes
    @serialized_attributes ||= {}
  end

  def serialized_attributes
    self.class.serialized_attributes
  end

  def foreign_key_attributes
    attributes.keys.select { |a| a.match(/_uuid$/) }
  end

  def skip_uuid_read_permission_check
    %w(modified_by_client_uuid)
  end

  def skip_uuid_existence_check
    []
  end

  def normalize_collection_uuids
    foreign_key_attributes.each do |attr|
      attr_value = send attr
      if attr_value.is_a? String and
          attr_value.match(/^[0-9a-f]{32,}(\+[@\w]+)*$/)
        begin
          send "#{attr}=", Collection.normalize_uuid(attr_value)
        rescue
          # TODO: abort instead of silently accepting unnormalizable value?
        end
      end
    end
  end

  @@prefixes_hash = nil
  def self.uuid_prefixes
    unless @@prefixes_hash
      @@prefixes_hash = {}
      Rails.application.eager_load!
      ActiveRecord::Base.descendants.reject(&:abstract_class?).each do |k|
        if k.respond_to?(:uuid_prefix)
          @@prefixes_hash[k.uuid_prefix] = k
        end
      end
    end
    @@prefixes_hash
  end

  def self.uuid_like_pattern
    "#{Rails.configuration.ClusterID}-#{uuid_prefix}-_______________"
  end

  def self.uuid_regex
    %r/[a-z0-9]{5}-#{uuid_prefix}-[a-z0-9]{15}/
  end

  def check_readable_uuid attr, attr_value
    return if attr_value.nil?
    if (r = ArvadosModel::resource_class_for_uuid attr_value)
      unless skip_uuid_read_permission_check.include? attr
        r = r.readable_by(current_user)
      end
      if r.where(uuid: attr_value).count == 0
        errors.add(attr, "'#{attr_value}' not found")
      end
    else
      # Not a valid uuid or PDH, but that (currently) is not an error.
    end
  end

  def ensure_valid_uuids
    specials = [system_user_uuid]

    foreign_key_attributes.each do |attr|
      if new_record? or send (attr + "_changed?")
        next if skip_uuid_existence_check.include? attr
        attr_value = send attr
        next if specials.include? attr_value
        check_readable_uuid attr, attr_value
      end
    end
  end

  def ensure_filesystem_compatible_name
    if name == "." || name == ".."
      errors.add(:name, "cannot be '.' or '..'")
    elsif Rails.configuration.Collections.ForwardSlashNameSubstitution == "" && !name.nil? && name.index('/')
      errors.add(:name, "cannot contain a '/' character")
    end
  end

  class Email
    def self.kind
      "email"
    end

    def kind
      self.class.kind
    end

    def self.readable_by (*u)
      self
    end

    def self.where (u)
      [{:uuid => u[:uuid]}]
    end
  end

  def self.resource_class_for_uuid(uuid)
    if uuid.is_a? ArvadosModel
      return uuid.class
    end
    unless uuid.is_a? String
      return nil
    end

    uuid.match HasUuid::UUID_REGEX do |re|
      return uuid_prefixes[re[1]] if uuid_prefixes[re[1]]
    end

    if uuid.match(/.+@.+/)
      return Email
    end

    nil
  end

  # Fill in implied zero/false values in database records that were
  # created before #17014 made them explicit, and reset the Rails
  # "changed" state so the record doesn't appear to have been modified
  # after loading.
  #
  # Invoked by Container and ContainerRequest models as an after_find
  # hook.
  def fill_container_defaults_after_find
    fill_container_defaults
    set_attribute_was('runtime_constraints', runtime_constraints)
    set_attribute_was('scheduling_parameters', scheduling_parameters)
    clear_changes_information
  end

  # Fill in implied zero/false values. Invoked by ContainerRequest as
  # a before_validation hook in order to (a) ensure every key has a
  # value in the updated database record and (b) ensure the attribute
  # whitelist doesn't reject a change from an explicit zero/false
  # value in the database to an implicit zero/false value in an update
  # request.
  def fill_container_defaults
    self.runtime_constraints = {
      'API' => false,
      'keep_cache_ram' => 0,
      'ram' => 0,
      'vcpus' => 0,
    }.merge(attributes['runtime_constraints'] || {})
    self.scheduling_parameters = {
      'max_run_time' => 0,
      'partitions' => [],
      'preemptible' => false,
    }.merge(attributes['scheduling_parameters'] || {})
  end

  # ArvadosModel.find_by_uuid needs extra magic to allow it to return
  # an object in any class.
  def self.find_by_uuid uuid
    if self == ArvadosModel
      # If called directly as ArvadosModel.find_by_uuid rather than via subclass,
      # delegate to the appropriate subclass based on the given uuid.
      self.resource_class_for_uuid(uuid).find_by_uuid(uuid)
    else
      super
    end
  end

  def is_audit_logging_enabled?
    return !(Rails.configuration.AuditLogs.MaxAge.to_i == 0 &&
             Rails.configuration.AuditLogs.MaxDeleteBatch.to_i > 0)
  end

  def schedule_restoring_changes
    # This will be checked at log_start_state, to reset any (virtual) changes
    # produced by the act of reading a serialized attribute.
    @fresh_from_database = true
  end

  def log_start_state
    if is_audit_logging_enabled?
      @old_attributes = Marshal.load(Marshal.dump(attributes))
      @old_logged_attributes = Marshal.load(Marshal.dump(logged_attributes))
      if @fresh_from_database
        # This instance was created from reading a database record. Attributes
        # haven't been changed, but those serialized attributes will be reported
        # as unpersisted, so we restore them to avoid issues with lock!() and
        # with_lock().
        restore_attributes
        @fresh_from_database = nil
      end
    end
  end

  def log_change(event_type)
    if is_audit_logging_enabled?
      log = Log.new(event_type: event_type).fill_object(self)
      yield log
      log.save!
      log_start_state
    end
  end

  def log_create
    if is_audit_logging_enabled?
      log_change('create') do |log|
        log.fill_properties('old', nil, nil)
        log.update_to self
      end
    end
  end

  def log_update
    if is_audit_logging_enabled?
      log_change('update') do |log|
        log.fill_properties('old', etag(@old_attributes), @old_logged_attributes)
        log.update_to self
      end
    end
  end

  def log_destroy
    if is_audit_logging_enabled?
      log_change('delete') do |log|
        log.fill_properties('old', etag(@old_attributes), @old_logged_attributes)
        log.update_to nil
      end
    end
  end
end