[ruby-core:112476] [Ruby master Bug#19041] Weakref is still alive after major garbage collection

Issue #19041 has been updated by parker (Parker Finch).

File manifest_weakref_issue.rb added

Hi @tenderlovemaking! I'm having difficulty interpreting the results of the `ObjectSpace` dump and I'm hoping you can help.

I've adjusted the script to print out the address of the underlying object, and then (when the issue manifests) print all lines from `ObjectSpace.dump_all` that match that address. The code is attached, here's some example output:
```
Ruby version: 3.3.0
Iteration: 0
Object address: 0x1051cd788
Inner iterations: 1
Iteration: 1
Object address: 0x105205ae8
Inner iterations: 1
Inner iterations: 2
Inner iterations: 3
{"address":"0x105205ae8", "type":"OBJECT", "shape_id":5, "slot_size":40, "class":"0x1029bfe80", "embedded":true, "ivars":0, "memsize":40, "flags":{"wb_protected":true, "old":true, "uncollectible":true, "marked":true}}
{"address":"0x10520da90", "type":"STRING", "shape_id":0, "slot_size":40, "class":"0x1029beda0", "embedded":true, "bytesize":11, "value":"0x105205ae8", "encoding":"UTF-8", "coderange":"7bit", "memsize":40, "flags":{"wb_protected":true, "old":true, "uncollectible":true, "marked":true}}
```

In that example, the underlying object was at `0x105205ae8`. But as far as I can tell, there's nothing else that points at it. (The other object there is the String used to hold that address.) I would have expected that, if nothing was referencing it, it would be collected by GC.

One interesting tidbit is that just calling `ObjectSpace.dump_all` prevents the issue from manifesting. Is it possible that something _was_ referencing the object address, then running `dump_all` caused that reference to be removed?

----------------------------------------
Bug #19041: Weakref is still alive after major garbage collection
https://bugs.ruby-lang.org/issues/19041#change-101919

* Author: parker (Parker Finch)
* Status: Closed
* 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
```

---Files--------------------------------
manifest_weakref_issue.rb (1.52 KB)


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