[ruby-core:119901] [Ruby master Bug#20891] Dir.foreach does not give a static list.

Issue #20891 has been reported by karasu (karasu k).

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Bug #20891: Dir.foreach does not give a static list.
https://bugs.ruby-lang.org/issues/20891

* Author: karasu (karasu k)
* Status: Open
* ruby -v: 2.7.0
* Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN
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I was using `Dir.foreach` with a block to batch change filenames.
The sample code is something like this:

```rb
look_up_table = File.open('look-up-table' ,'w')
Dir.foreach('.').each_with_index do |file, index|
  File.rename(file, index)
  look_up_table << "#{file}: #{index}\n"
end
```

As first I thought it's good, but I found that some indices might be missing.
After staring at the look-up table for hours, I realised some files were modified more than once, and this is strange for me. It means that in each iteration of `Dir.foreach`, it acutally **might** fetch the files to see if there are new files that are not in the list before?
What I mean **might** is that this situation does not occur anytime, I don't know how to or not to trigger it, but this is what I faced.

I am aware that in `Dir.foreach` it might have some `yield` so this behavior might be normal for you guys, but since the function is called `foreach`, I assumed that it should give a list that is not gonna change once it's established.
My later fix change to the sample code is:

```rb
look_up_table = File.open('look-up-table' ,'w')
Dir.foreach('.').to_a.each_with_index do |file, index|
  File.rename(file, index)
  look_up_table << "#{file}: #{index}\n"
end
```

You can see that I add a `to_a` to change the result from `Dir.foreach` into an array, and rename those files according to it, instead of the result itself.

I wonder if this is an old issue but I cannot find it anywhere else. Thanks in advance. Both English and Japanese are welcome!

(I know that `Dir.foreach` will give `.` and `..` as well, but let's just forget it for now.)



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