[ruby-core:114681] [Ruby master Bug#19873] Thread#native_thread_id is incorrectly cached across fork on Linux

Issue #19873 has been reported by kjtsanaktsidis (KJ Tsanaktsidis).

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Bug #19873: Thread#native_thread_id is incorrectly cached across fork on Linux
https://bugs.ruby-lang.org/issues/19873

* Author: kjtsanaktsidis (KJ Tsanaktsidis)
* Status: Open
* Priority: Normal
* Backport: 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN
----------------------------------------
On Linux, when a process forks, the native thread ID of the (only) thread in the forked child should change.

```
>> libc = Fiddle.dlopen(nil)
=> #<Fiddle::Handle:0x0000ffffb0d8e900>
>> gettid = Fiddle::Function.new(libc['gettid'], [], Fiddle::TYPE_INT)
=>
#<Fiddle::Function:0x0000ffffb15d1130
...
>> gettid.call
=> 33
>> fork { puts gettid.call }; nil;
=> nil
36
```

However, the value of `Thread#native_thread_id` is (incorrectly) still set to the old value.

```
>> Thread.current.native_thread_id
=> 33
>> fork { puts Thread.current.native_thread_id }; nil;
=> nil
33
```

I think `#native_thread_id` should be re-fetched from the operating system via `gettid(2)` after fork.



-- 
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