Crash in fiber switching with select

[kostya]

ch1 = Channel::Buffered(Int32).new(1)
ch2 = Channel::Buffered(Int32).new(1)
res = [] of Int32

spawn do
  loop do
    select
    when x = ch1.receive
      res << x
    when y = ch2.receive
      res << y
    end
  end
end

spawn do
  3.times do |i|
    select
    when ch1.send(i)
    when ch2.send(i)
    end
  end
end

Fiber.yield
p res

Invalid memory access (signal 11) at address 0x0
[4466037] *CallStack::print_backtrace:Int32 +117
[4444920] __crystal_sigfault_handler +56
[4758788] sigfault_handler +40
[140503573855408] ???
[0] ???

[bmmcginty]

Did some playing around with this today. Doesn't seem to happen if case statement is changed to if, and doesn't seem to happen if case statement is changed to if/elsif (with or without plain else statement).
Fails right after a stack switch, according to gdb. I logged every crystal function, and couldn't get any closer to the exact error location.
I'm trying to get a copy of the generated code to see what the end product of the case statements look like (it appears they're turned into if/elsif), but beyond that, I'm out of ideas.
I'm not sure how helpful any this is, but the issue was marked help-wanted, so figured I'd comment.

[kostya]

i tried to debug this, but its quite hard, something breaks in coroutines switches.

[firejox]

Because there is a same fiber in ch1 and ch2, that could push the same fiber into scheduler twice. If you add such logger, then you will see that.

ch1 = Channel(Int32).new(1)
ch2 = Channel(Int32).new(1)
res = [] of Int32

class Scheduler
  def self.log
    LibC.printf "#{@@runnables}\n"
  end
end

a = spawn do
  loop do
    select
    when x = ch1.receive
      res << x
    when y = ch2.receive
      res << y
    end
    LibC.printf "receive: "
    Scheduler.log
  end
end

b = spawn do
  3.times do |i|
    select
    when ch1.send(i)
    when ch2.send(i)
    end
    LibC.printf "send: "
    Scheduler.log
  end
end

p "receive fiber : #{a}"
p "send fiber : #{b}"

Fiber.yield
p res

"receive fiber : #<Fiber:0x1d37e00>"
"send fiber : #<Fiber:0x1d37d90>"
send: Deque{#<Fiber:0x1d37e00>}
send: Deque{#<Fiber:0x1d37e00>, #<Fiber:0x1d37e00>}
receive: Deque{#<Fiber:0x1d37e00>, #<Fiber:0x1d37d90>}
receive: Deque{#<Fiber:0x1d37e00>, #<Fiber:0x1d37d90>, #<Fiber:0x1d37d90>}
send: Deque{#<Fiber:0x1d37d90>, #<Fiber:0x1d37e00>}
Invalid memory access (signal 11) at address 0x0
[4413221] *CallStack::print_backtrace:Int32 +117
[4391592] __crystal_sigfault_handler +56
[4608565] sigfault_handler +40
[140189768220800] ???
[0] ???

After third sending, the fiber are going to the end and Scheduler will resume the fiber again. That's why it crashed.

[miketheman]

#triage

This issue appears to persist with 0.23.1 https://carc.in/#/r/2imm

An effort to rewrite the Channel implementation has been undertaken in #3912, and also appears to have not much feedback.

[jhass]

It doesn't segfault anymore, but still crashes at runtime: https://carc.in/#/r/7gtb

Somebody wants to try it against #8112? :)

[asterite]

I tried it with #8112 and it works fine 🎉

[asterite]

Closed by #8112

Please reopen if the issue remains.