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# An InternalId is a strictly monotone sequence of integers
# generated for a given scope and usage.
#
# For example, issues use their project to scope internal ids:
# In that sense, scope is "project" and usage is "issues".
# Generated internal ids for an issue are unique per project.
#
# See InternalId#usage enum for available usages.
#
# In order to leverage InternalId for other usages, the idea is to
# * Add `usage` value to enum
# * (Optionally) add columns to `internal_ids` if needed for scope.
class InternalId < ActiveRecord::Base
belongs_to :project
enum usage: { issues: 0 }
validates :usage, presence: true
REQUIRED_SCHEMA_VERSION = 20180305095250
# Increments #last_value and saves the record
#
# The operation locks the record and gathers a `ROW SHARE` lock (in PostgreSQL).
# As such, the increment is atomic and safe to be called concurrently.
def increment_and_save!
lock!
self.last_value = (last_value || 0) + 1
save!
last_value
end
class << self
def generate_next(subject, scope, usage, init)
# Shortcut if `internal_ids` table is not available (yet)
# This can be the case in other (unrelated) migration specs
return (init.call(subject) || 0) + 1 unless available?
InternalIdGenerator.new(subject, scope, usage, init).generate
end
def available?
@available_flag ||= ActiveRecord::Migrator.current_version >= REQUIRED_SCHEMA_VERSION # rubocop:disable Gitlab/PredicateMemoization
end
# Flushes cached information about schema
def reset_column_information
@available_flag = nil
super
end
end
class InternalIdGenerator
# Generate next internal id for a given scope and usage.
#
# For currently supported usages, see #usage enum.
#
# The method implements a locking scheme that has the following properties:
# 1) Generated sequence of internal ids is unique per (scope and usage)
# 2) The method is thread-safe and may be used in concurrent threads/processes.
# 3) The generated sequence is gapless.
# 4) In the absence of a record in the internal_ids table, one will be created
# and last_value will be calculated on the fly.
#
# subject: The instance we're generating an internal id for. Gets passed to init if called.
# scope: Attributes that define the scope for id generation.
# usage: Symbol to define the usage of the internal id, see InternalId.usages
# init: Block that gets called to initialize InternalId record if not present
# Make sure to not throw exceptions in the absence of records (if this is expected).
attr_reader :subject, :scope, :init, :scope_attrs, :usage
def initialize(subject, scope, usage, init)
@subject = subject
@scope = scope
@init = init
@usage = usage
raise ArgumentError, 'Scope is not well-defined, need at least one column for scope (given: 0)' if scope.empty?
unless InternalId.usages.has_key?(usage.to_s)
raise ArgumentError, "Usage '#{usage}' is unknown. Supported values are #{InternalId.usages.keys} from InternalId.usages"
end
end
# Generates next internal id and returns it
def generate
subject.transaction do
# Create a record in internal_ids if one does not yet exist
# and increment its last value
#
# Note this will acquire a ROW SHARE lock on the InternalId record
(lookup || create_record).increment_and_save!
end
end
private
# Retrieve InternalId record for (project, usage) combination, if it exists
def lookup
InternalId.find_by(**scope, usage: usage_value)
end
def usage_value
@usage_value ||= InternalId.usages[usage.to_s]
end
# Create InternalId record for (scope, usage) combination, if it doesn't exist
#
# We blindly insert without synchronization. If another process
# was faster in doing this, we'll realize once we hit the unique key constraint
# violation. We can safely roll-back the nested transaction and perform
# a lookup instead to retrieve the record.
def create_record
subject.transaction(requires_new: true) do
InternalId.create!(
**scope,
usage: usage_value,
last_value: init.call(subject) || 0
)
end
rescue ActiveRecord::RecordNotUnique
lookup
end
end
end
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