[ruby-core:97602] [Ruby master Bug#15499] Breaking behavior on ruby 2.6: rb_thread_call_without_gvl doesn't invoke unblock_function when used on the main thread

Issue #15499 has been updated by nagachika (Tomoyuki Chikanaga).

Backport changed from 2.4: DONTNEED, 2.5: DONTNEED, 2.6: REQUIRED to 2.4: DONTNEED, 2.5: DONTNEED, 2.6: DONE

The ff98931dac is the fix for 23444302d9 and the 23444302d9 have not been backported into ruby_2_6 branch. 23444302d9 seems a kind of performance improvement.
I will restore Backport field to 2.6: DONE.

----------------------------------------
Bug #15499: Breaking behavior on ruby 2.6: rb_thread_call_without_gvl doesn't invoke unblock_function when used on the main thread 
https://bugs.ruby-lang.org/issues/15499#change-84788

* Author: apolcyn (alex polcyn)
* Status: Closed
* Priority: Normal
* Assignee: ko1 (Koichi Sasada)
* ruby -v: 2.6.0
* Backport: 2.4: DONTNEED, 2.5: DONTNEED, 2.6: DONE
----------------------------------------
This issue was noticed when trying to add ruby 2.6 support to the "grpc" ruby gem (this gem is a native C-extension), and was caught by a unit test.

There are several APIs on the grpc ruby gem (https://github.com/grpc/grpc/tree/master/src/ruby) that invoke "rb_thread_call_without_gvl" on the current thread, doing a blocking operation in the "without gvl" callback and cancel that blocking operation in the "unblocking function". These APIs work in ruby versions prior to ruby 2.6 (e.g. ruby 2.5), but have problems when used on ruby 2.6

Minimal repro:

My system:

```
> lsb_release -a
No LSB modules are available.
Distributor ID:	Debian
Description:	Debian GNU/Linux 9.6 (stretch)
Release:	9.6
Codename:	stretch

> ruby -v
ruby 2.6.0p0 (2018-12-25 revision 66547) [x86_64-linux

# I installed ruby 2.6.0 with rvm - https://rvm.io/rvm/install

> GRPC_CONFIG=dbg gem install grpc --platform ruby # build grpc gem from source with debug symbols
```

ruby script, "repro.rb" that looks like this:

```ruby
require 'grpc'

ch = GRPC::Core::Channel.new('localhost:1234', {}, :this_channel_is_insecure)
ch.watch_connectivity_state(ch.connectivity_state, Time.now + 360)
```

Run "ruby repro.rb" with an interactive shell, and it will hang there. At this point, ctrl^C the process, and it will not terminate.
What should happen is this unblocking func should be invoked: https://github.com/grpc/grpc/blob/master/src/ruby/ext/grpc/rb_channel.c#L354, but as seen with logging or debuggers, that unblocking func is never ran. Thus the blocking operation never completes and the main thread is stuck.

When the same repro.rb is ran on e.g. ruby 2.5.3 or ruby 2.4.1, the blocking operation is unblocked and the process terminates, as expected, when sending it a SIGINT.

Also note that if the blocking operation is put in a background thread, e.g. with this script:

```ruby
require 'grpc'

th = Thread.new do
  ch = GRPC::Core::Channel.new('localhost:1234', {}, :this_channel_is_insecure)
  ch.watch_connectivity_state(ch.connectivity_state, Time.now + 360)
end
th.join
```

then "unblocking" functions will in fact be invoked upon sending the process a SIGINT, so this looks like a problem specifically with rb_thread_call_without_gvl being used on the main thread.

Please let me know and I can provide more details or alternative repro cases.

Thanks in advance.



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