[ruby-core:101816] [Ruby master Bug#17488] Regression in Ruby 3: Hash#key? is non-deterministic when argument uses DelegateClass

Issue #17488 has been updated by marcandre (Marc-Andre Lafortune).


shyouhei (Shyouhei Urabe) wrote in #note-4:
> Also I want to know your `clang --version`.

```
$ clang --version
Apple clang version 11.0.0 (clang-1100.0.33.17)
Target: x86_64-apple-darwin18.7.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin
```

----------------------------------------
Bug #17488: Regression in Ruby 3: Hash#key? is non-deterministic when argument uses DelegateClass
https://bugs.ruby-lang.org/issues/17488#change-89643

* Author: myronmarston (Myron Marston)
* Status: Feedback
* Priority: Normal
* ruby -v: ruby 3.0.0p0 (2020-12-25 revision 95aff21468) [x86_64-darwin19]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN
----------------------------------------
Upon upgrading a library to run on Ruby 3.0, I have observed that `Hash#key?` has non-deterministic behavior when the argument uses `DelegateClass`. This non-deterministic behavior was not present in Ruby 2.7.

Reproducing this is slightly difficult; the behavior appears to be deterministic (but not necessarily correct) within a single ruby process. To reproduce the non-determinism, you need to start ruby many times to observe different results.  My script below does this.

## Reproduction script

``` ruby
puts "Running on Ruby: #{RUBY_DESCRIPTION}"

program = <<~EOS
  require "delegate"
  TypeName = DelegateClass(String)

  hash = {
    "Int" => true,
    "Float" => true,
    "String" => true,
    "Boolean" => true,
    "WidgetFilter" => true,
    "WidgetAggregation" => true,
    "WidgetEdge" => true,
    "WidgetSortOrder" => true,
    "WidgetGrouping" => true,
  }

  puts hash.key?(TypeName.new("WidgetAggregation"))
EOS

iterations = 20
results = iterations.times.map { `ruby -e '#{program}'`.chomp }.tally

puts "Results of checking `Hash#key?` #{iterations} times: #{results.inspect}"
```

Put this in a file like `ruby3_hash_bug.rb`, and run it using either Ruby 2.7 (to see `Hash#key?` consistently return `true`) or Ruby 3.0 (to see `Hash#key?` produce non-deterministic behavior).

## Ruby 2.7 results

```
Running on Ruby: ruby 2.7.1p83 (2020-03-31 revision a0c7c23c9c) [x86_64-darwin19]
Results of checking `Hash#key?` 20 times: {"true"=>20}
```

## Ruby 3.0 results

```
Running on Ruby: ruby 3.0.0p0 (2020-12-25 revision 95aff21468) [x86_64-darwin19]
Results of checking `Hash#key?` 20 times: {"true"=>12, "false"=>8}
```

Note that the ratio of `true` to `false` is non-deterministic; here are a couple other runs on Ruby 3.0 with different results:

```
Running on Ruby: ruby 3.0.0p0 (2020-12-25 revision 95aff21468) [x86_64-darwin19]
Results of checking `Hash#key?` 20 times: {"false"=>7, "true"=>13}
```

```
Running on Ruby: ruby 3.0.0p0 (2020-12-25 revision 95aff21468) [x86_64-darwin19]
Results of checking `Hash#key?` 20 times: {"true"=>11, "false"=>9}
```




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