Question

I need to value an if condition, but I need to be careful about possible modification of a variable by other threads.

My solution is

array.each do |anything|
    if (mutex.synchronize do shared_var <= my_var end)
        next
    end
end

It seems to work. But what is exactly the if statement checking? And how about this other code?

array.each do |anything|
    mutex.synchronize do
        if shared_var <= my_var
            next
        end
    end
end

Are they equivalent? Does the inner next do its job even if it is included in a mutex? And why?

Just look at the structure, I showed only the part of the program relevant for my question.

Answer 1

But what is exactly the if statement checking?

synchronize returns whatever the block passed to it returns, so the if statement is ultimately checking shared_var <= my_var.

And how about this other code?

I think it’s self-explanatory what the if statement here is checking, and it is the same as above.

Are they equivalent?

No.

Does the inner next do its job even if it is included in a mutex?

No, and this is why they are not equivalent. When next is called inside the block of the synchronize call, it is nexting in that block (not the foreach), which is equivalent to return here since there’s no “next”. When it’s outside that block, it’s nexting over the actual outer block (foreach) you expect.

Thus, you likely want the first code block.