[ruby-core:102025] [Ruby master Bug#17280] Dir.glob with FNM_DOTMATCH matches ".." and "." and results in duplicated entries

Issue #17280 has been updated by mame (Yusuke Endoh).


https://github.com/ruby/dev-meeting-log/blob/master/DevelopersMeeting20201210Japan.md#bug-17280-dirglob-with-fnm_dotmatch-matches--and--and-results-in-duplicated-entries-jeremyevans0

> matz: try to remove "." and ".." on 3.1 (early timing to check compatibility issues)

I think we can progress this change.

----------------------------------------
Bug #17280: Dir.glob with FNM_DOTMATCH matches ".." and "." and results in duplicated entries
https://bugs.ruby-lang.org/issues/17280#change-89877

* Author: Eregon (Benoit Daloze)
* Status: Open
* Priority: Normal
* ruby -v: ruby 2.6.6p146 (2020-03-31 revision 67876) [x86_64-linux]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN
----------------------------------------
```
% ruby -e 'p Dir.glob("**/*", File::FNM_DOTMATCH)'
[".", "bar", "bar/.", "bar/.baz", "bar/.baz/.", "bar/.baz/qux"]
% ruby -e 'p Dir.glob("**", File::FNM_DOTMATCH)' 
[".", "..", "bar"]
% ruby -e 'p Dir.glob("*", File::FNM_DOTMATCH)' 
[".", "..", "bar"]
```

I think `".."` was never intended by the user here, is it a bug?

Not sure about `"."`.

Note it also causes duplicated entries: `bar and bar/.baz` are twice in the Array!

I think `..` should always be ignored for glob purposes, it escapes the current directory.
And `.` seems useless and causing duplicates.

I think the intention of users of `File::FNM_DOTMATCH` is to match file/directories starting with a `.` like `.baz`.
Probably `Dir.glob("**/{*,.*}")` is a safer way to achieve that,
but still I think `FNM_DOTMATCH` should not produce such weird results.

From https://github.com/oracle/truffleruby/issues/2116
I could not figure out what was the intended semantics for FNM_DOTMATCH with regards to `.` and `..`.



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