Freeze results that come from the database

[tenderlove]

Freeze strings and row records so that we can more easily share results among Ractors

Silly example, but here's an example of using database results plus Ractors to make a parallel producer-consumer pattern:

require "sqlite3"
require "securerandom"

def fib n
  if n < 2
    n
  else
    fib(n-2) + fib(n-1)
  end
end

N = 10

# Make a Ractor safe queue that supports N workers
queue = Ractor.new do
  while (work = Ractor.receive)
    Ractor.yield work
  end
  # Shutdown the workers
  N.times { Ractor.yield nil }
end

# Spin up workers
workers = N.times.map do
  Ractor.new(queue) do |q|
    while (work = q.take)
      name, i = work
      p "Ractor: #{Ractor.current} #{name}: #{fib(i)}"
    end
  end
end

db = SQLite3::Database.new ":memory:"
db.execute 'CREATE TABLE "fibs" (name string, fib int)'
stmt = db.prepare "INSERT INTO fibs (name, fib) VALUES (?, ?)"
5000.times { stmt.execute SecureRandom.hex, rand(30) }
stmt.close

stmt = db.prepare 'SELECT name, fib FROM fibs'

# Send work from the database to the queue
stmt.each { |row| queue.send row }

stmt.close

# Tell the queue we've sent all the work
queue.send nil

# Wait for workers to finish
workers.map(&:take)

[tenderlove]

This change breaks one test in Rails, so I changed Rails: rails/rails#50859

[flavorjones]

Hmm, this seems to break Rails's schema cache tests (which use Marshal):

bin/test test/cases/connection_adapters/schema_cache_test.rb -n/test_gzip_dumps_identical/

Without this line, the test passes. With it, the test fails (in isolation).

I'm looking into it.

[flavorjones]

Context on that test: rails/rails#49166

[flavorjones]

See rails/rails#51581 for the exciting conclusion.