Avoid possible redundant database initialization from multiple threads

[ikaronen-relex]

Based on discussion at #50 (comment)

[casperisfine]

Suggested change

	@db = nil
	def self.db
	@db || LOCK.synchronize { @db ||= new }
	def self.db
	@db ||= LOCK.synchronize { @db ||= new }

If you want to avoid the warning on older rubies. But both works I guess.

[ikaronen-relex]

Do you mean a different warning than the "instance variable not initialized" one? The @db = nil line above ought to avoid that one, or at least it did when I tested this on ruby 2.6.10p210.

(The double ||= would indeed work too, but I'd rather avoid that since it makes the already somewhat complicated idiom even more confusing for the unfamiliar reader.)

[casperisfine]

Yeah, I meant alternative solution to that same warning.

[SamSaffron]

I am ok with the implementation, its a bit of hair splitting going one way vs the other.