[ruby-core:115104] [Ruby master Feature#19965] Make the name resolution interruptible

Issue #19965 has been reported by mame (Yusuke Endoh).

----------------------------------------
Feature #19965: Make the name resolution interruptible
https://bugs.ruby-lang.org/issues/19965

* Author: mame (Yusuke Endoh)
* Status: Open
* Priority: Normal
----------------------------------------
## Problem

Currently, Ruby name resolution is not interruptible.

```
$ cat /etc/resolv.conf
nameserver 198.51.100.1

$ ./local/bin/ruby -rsocket -e 'Addrinfo.getaddrinfo("www.ruby-lang.org", 80)'
^C^C^C^C
```

If you set a non-responsive IP as the nameserver, you cannot stop `Addrinfo.getaddrinfo` by pressing Ctrl+C. Note that `Timeout.timeout` does not work either.

This is because there is no way to cancel `getaddrinfo(3)`.

## Proposal

I wrote a patch to make `getaddrinfo(3)` work in a separate pthread.

https://github.com/ruby/ruby/pull/8695

Whenever it needs name resolution, it creates a worker pthread, and executes `getaddrinfo(3)` in it.
The caller thread waits for the worker to complete.
When an interrupt occurs, the caller thread leaves stop waiting and leaves the worker pthread.
The detached worker pthread will exit after `getaddrinfo(3)` completes (or name resolution times out).

## Evaluation

By applying this patch, name resolution is now interruptible.

```
$ ./local/bin/ruby -rsocket -e 'pp Addrinfo.getaddrinfo("www.ruby-lang.org", 80)'
^C-e:1:in `getaddrinfo': Interrupt
        from -e:1:in `<main>'
```

As a drawback, name resolution performance will be degraded.

```
10000.times { Addrinfo.getaddrinfo("www.ruby-lang.org", 80) }
# Before patch: 2.3 sec.
# After ptach: 3.0 sec.
```

However, I think that name resolution is typically short enough for the application's runtime. For example, the difference is small for the performance of `URI.open`.

```
100.times { URI.open("https://www.ruby-lang.org").read }
# Before patch: 3.36 sec.
# After ptach: 3.40 sec.
```

## Alternative approaches

I proposed using c-ares to resolve this issue (#19430). However, there was an opinion that it would be a problem that c-ares does not respect the platform-dependent own name resolution.

## Room for improvement

* Currently, this patch works only when pthread is available.
* It might be possible to force to stop the worker threads by using `pthread_cancel`. However, `pthread_cancel` with `getaddrinfo(3)` seems still premature; there seems to be a bug in glibc until recently: https://bugzilla.redhat.com/show_bug.cgi?id=1405071 https://sourceware.org/bugzilla/show_bug.cgi?id=20975
* It would be more efficient to pool worker pthreads instead of creating them each time.




-- 
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