From: "byroot (Jean Boussier)" <noreply@...>
Date: 2022-10-06T19:32:16+00:00
Subject: [ruby-core:110219] [Ruby master Bug#19041] Weakref is still alive after major garbage collection

Issue #19041 has been updated by byroot (Jean Boussier).


I don't think this is a bug per say. The Ruby GC is conservative. That means it goes over the whole stack in search for potential references to objects, and mark them.

As a result, it can happen that an object ref stays in an unused saved register and prevent an object from being merged.

----------------------------------------
Bug #19041: Weakref is still alive after major garbage collection
https://bugs.ruby-lang.org/issues/19041#change-99503

* Author: parker (Parker Finch)
* Status: Open
* Priority: Normal
* ruby -v: ruby 3.1.2p20 (2022-04-12 revision 4491bb740a) [x86_64-darwin21]
* Backport: 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN
----------------------------------------
I am able to get into an infinite loop waiting for garbage collection to take a WeakRef.

### Reproduction Process
The following script prints a "0", then a "1", and then hangs forever. I expect it to keep printing numbers.
```
require "weakref"

iterations = 0

loop do
  print "\r#{iterations}"
  obj = WeakRef.new(Object.new)
  GC.start while obj.weakref_alive?
  iterations += 1
end
```

### Ruby Version
I have tested this on Ruby 3.1.2, 3.1.0, 3.0.4, 3.0.0, 2.7.6, and 2.7.0 on macOS. All exhibit this behavior.

### Further Investigation
#### Sleeping
Sleeping before the garbage collection allows the loop to continue. The below exhibits the expected behavior:
```
require "weakref"

iterations = 0

loop do
  print "\r#{iterations}"
  obj = WeakRef.new(Object.new)
  (sleep(0.5); GC.start) while obj.weakref_alive?
  iterations += 1
end
```

However, sleeping _after_ the garbage collection still shows the buggy behavior (loop hangs):
```
require "weakref"

iterations = 0

loop do
  print "\r#{iterations}"
  obj = WeakRef.new(Object.new)
  (GC.start; sleep(0.5)) while obj.weakref_alive?
  iterations += 1
end
```

#### Running Garbage Collection Multiple Times
Explicitly running garbage collection multiple times allows the loop to continue. This has the expected behavior, more numbers continue to be printed:
```
require "weakref"

iterations = 0

loop do
  print "\r#{iterations}"
  obj = WeakRef.new(Object.new)
  while obj.weakref_alive?
    GC.start
    GC.start
    GC.start
  end
  iterations += 1
end
```

However, with certain rubies, running those garbage collection calls in a `times` block prevents even a single iteration from completing. The following prints only "0" with ruby 3.0.4 on macOS, ruby 2.7.6 on macOS, and ruby 3.1.2 on linux (`ruby 3.1.2p20 (2022-04-12 revision 4491bb740a) [x86_64-linux]` on a virtual machine). It shows the expected behavior on ruby 3.1.2 on macOS.
```
require "weakref"

iterations = 0

loop do
  print "\r#{iterations}"
  obj = WeakRef.new(Object.new)
  3.times { GC.start } while obj.weakref_alive?
  iterations += 1
end
```



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