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FFI: break from within a callback block behaves differently than on MRI #3559

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paddor opened this issue Dec 21, 2015 · 2 comments
Closed

FFI: break from within a callback block behaves differently than on MRI #3559

paddor opened this issue Dec 21, 2015 · 2 comments
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Invalid or Duplicate

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@paddor
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paddor commented Dec 21, 2015

MRI correctly aborts the execution of of a C function when the FFI::Function passed to it breaks. On JRuby (and Rubinius, for that matter), this doesn't happen. If the C function has a loop that would return early in case the callback returned -1, it even keeps calling the callback.

Consider this C code, which simply calls a callback 10 times, unless the callback function returns -1:

#include <stdio.h>
typedef int (callback_fn) (void);

void
ten_times(callback_fn *callback)
{
    for (int i = 1; i <= 10; i++) {
        printf("libcallback: call #%i ...\n", i);
        int rc = (* callback)();
        if (rc == -1) {
            printf("libcallback: early return because rc == -1\n");
            return;
        }
    }
}

And here the Ruby script which attaches the function via FFI and verifies the bug with minitest:

require 'minitest/autorun'
require 'ffi'

module LibCallback
  extend ::FFI::Library

  lib_name = 'libcallback'
  ffi_lib "#{lib_name}.#{::FFI::Platform::LIBSUFFIX}"

  attach_function :ten_times, [:pointer], :void, blocking: true
end

def mk_callback
  FFI::Function.new(:void, [], blocking: true) do
    yield
  end
end

describe FFI::Function do
  def foo
    callback = mk_callback  do
      yield
    end
    LibCallback.ten_times(callback)
  end

  describe "with non-breaking block" do
    it "calls block 10 times" do
      called = 0
      foo { called += 1 }
      assert_equal 10, called
    end
  end

  describe "with breaking block" do

    it "doesn't call block anymore" do
      called = 0
      foo { called += 1; break }
      assert_equal 1, called
    end
  end
end

describe "pure Ruby block" do
  def foo
    10.times { yield }
  end

  describe "with non-breaking block" do
    it "calls block 10 times" do
      called = 0
      foo { called += 1 }
      assert_equal 10, called
    end
  end

  describe "with breaking block" do

    it "doesn't call block anymore" do
      called = 0
      foo { called += 1; break }
      assert_equal 1, called
    end
  end
end

Here what I expect, as when run on Ruby 2.2.4:

Run options: --seed 41083

# Running:

..libcallback: call #1 ...
libcallback: call #2 ...
libcallback: call #3 ...
libcallback: call #4 ...
libcallback: call #5 ...
libcallback: call #6 ...
libcallback: call #7 ...
libcallback: call #8 ...
libcallback: call #9 ...
libcallback: call #10 ...
.libcallback: call #1 ...
.

Finished in 0.002047s, 1954.3283 runs/s, 1954.3283 assertions/s.

4 runs, 4 assertions, 0 failures, 0 errors, 0 skips

Here the actual output on JRuby 9.0.4.0 (and also Rubinius 2.5.8):

Run options: --seed 3230

# Running:

.libcallback: call #1 ...
libcallback: call #2 ...
libcallback: call #3 ...
libcallback: call #4 ...
libcallback: call #5 ...
libcallback: call #6 ...
libcallback: call #7 ...
libcallback: call #8 ...
libcallback: call #9 ...
libcallback: call #10 ...
.libcallback: call #1 ...
libcallback: call #2 ...
libcallback: call #3 ...
libcallback: call #4 ...
libcallback: call #5 ...
libcallback: call #6 ...
libcallback: call #7 ...
libcallback: call #8 ...
libcallback: call #9 ...
libcallback: call #10 ...
F.

Finished in 0.023067s, 173.4047 runs/s, 173.4047 assertions/s.

  1) Failure:
FFI::Function::with breaking block#test_0001_doesn't call block anymore [test.rb:40]:
Expected: 1
  Actual: 10

4 runs, 4 assertions, 1 failures, 0 errors, 0 skips
ruby test.rb  6.48s user 0.29s system 330% cpu 2.048 total

The files needed to reproduce this bug, including a Makefile to build on OSX, are in this gist.
@paddor paddor changed the title FFI::Function callback doesn't return block value to C FFI: break from within a callback block behaves differently than on MRI Dec 21, 2015
@paddor paddor closed this as not planned Won't fix, can't repro, duplicate, stale Jul 16, 2024
@headius
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headius commented Jul 17, 2024

I see you closed this. Is it still a real issue though? We generally don't close old issues, but I understand you wanting to clean up your queue.

@paddor
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paddor commented Jul 17, 2024

@headius I don't know. I didn't check. Just wanted to clean up my queue. You can reopen if you like.

@enebo enebo added this to the Invalid or Duplicate milestone Nov 4, 2024
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@paddor @headius @enebo