Killing a Thread in Crystal lang - multithreading

Question

I wrote a program a crystal program to calculate prime numbers upto a range with Sieve.
Code
#!/usr/bin/env crystal
def sieve(max)
t = Thread.new do
dot, ary, colours = ".", ["\xE2\xA0\x81", "\xE2\xA0\x88", "\xE2\xA0\xA0", "\xE2\xA0\x84"] * 2, [154, 184, 208, 203, 198, 164, 129, 92]
print "\e[?25l"
loop do
ary.size.times do |x|
print("\e[2K#{ary[x]} \e[38;5;#{colours[x]}mPlease Wait#{dot * x}\e[0m\r")
sleep(0.1)
end
end
end
s = [nil, nil] + (2..max).to_a
s.each do |x|
next unless x
break if (sq = x ** 2) > max
(sq..max).step(x) { |y| s[y] = nil }
end
puts "\e[?25h"
s.tap { |x| x.compact! }
end
p sieve(2_000_000).size
The way I want to display it is
Issue
The problem is the thread isn't killed when puts is writing the sieve. the method sieve(n) just returns an array. The array size then is calculated, and printed. You can see that the animation freezes for a time, and then continues until it's printed and exited. If I use spawn do...end the print in spawn pauses until the sieve is calculated.
Not killing threads causes issues like this
In ruby I used to do
t = Thread.new { loop while ... }
<some other time consuming stuff here>
t.kill
return calculated_stuffs
Crystal Details
Crystal 0.31.1 (2019-10-21)
LLVM: 9.0.0
Default target: x86_64-pc-linux-gnu
How to kill a thread in crystal?

Answer 1

Thread is part of Crystal's internal API, and is not meant to be used directly.
The good news is Crystal natively supports a concurrency model called CSP, where Fibers (light-weight threads) send each others messages over thread-safe Channels in order to coordinate. So, rather than communicating by sharing state, Fibers share state by communicating - as they say in golang.
For your use case, you could run 3 Fibers:
A sieve, generating numbers and sending updates through a channel
A monitor, receiving on the sieve's channel, updating the UI and sending a completion message once the sieve is done
The main Fiber, waiting for the monitor to notify completion and able to decide what to do with the sieve's result
Here is what your code could look like
record Result, primes : Array(Int32)
record Tick
alias SieveUpdate = Result | Tick
def monitor(updates : Channel(SieveUpdate)) : Channel(Result)
Channel(Result).new.tap { |done|
spawn do
dot, ary, colours = ".", ["\xE2\xA0\x81", "\xE2\xA0\x88", "\xE2\xA0\xA0", "\xE2\xA0\x84"] * 2, [154, 184, 208, 203, 198, 164, 129, 92]
ary_idx = 0
update_n = 0
print "\e[?25l"
loop do
case value = updates.receive
when Tick
next unless (update_n+=1) % 50 == 0 # lower refresh rate
print("\e[2K#{ary[ary_idx]} \e[38;5;#{colours[ary_idx]}mPlease Wait#{dot * ary_idx}\e[0m\r")
ary_idx = (ary_idx + 1) % ary.size
when Result
puts "\e[?25h"
done.send value
break
end
end
end
}
end
def sieve(max) : Channel(SieveUpdate)
Channel(SieveUpdate).new.tap { |updates|
spawn do
s = [nil, nil] + (2..max).to_a
s.each do |x|
updates.send(Tick.new)
next unless x
break if (sq = x ** 2) > max
(sq..max).step(x) { |y| s[y] = nil }
end
updates.send Result.new(s.compact.as(Array(Int32)))
end
}
end
updates = sieve(2_000_000)
done = monitor(updates)
print done.receive.primes.size