[ruby-core:91656] [Ruby trunk Bug#15633] Pathological slowdown in File.expand_path on UNC paths on Windows

Issue #15633 has been reported by djberg96 (Daniel Berger).

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Bug #15633: Pathological slowdown in File.expand_path on UNC paths on Windows
https://bugs.ruby-lang.org/issues/15633

* Author: djberg96 (Daniel Berger)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: ruby 2.5.1p57 (2018-03-29 revision 63029) [x64-mingw32]
* Backport: 2.4: UNKNOWN, 2.5: UNKNOWN, 2.6: UNKNOWN
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For a while now, including Ruby 2.5, calling File.expand_path on an absolute UNC path seems to be strangely slow. I've put together some benchmarks here:

```
require 'benchmark'

MAX = 200000

Benchmark.bm(30) do |x|
  x.report("expand_path('foo/bar')") do
    str = "foo/bar"
    MAX.times{ File.expand_path(str) }
  end

  x.report("expand_path('C:/foo/bar')") do
    str = "C:/foo/bar"
    MAX.times{ File.expand_path(str) }
  end

  x.report("expand_path('//foo/bar')") do
    str = "//foo/bar"
    MAX.times{ File.expand_path(str) }
  end

  x.report("expand_path('foo//bar///')") do
    str = "foo//bar///"
    MAX.times{ File.expand_path(str) }
  end

  x.report("expand_path('~')") do
    str = "~"
    MAX.times{ File.expand_path(str) }
  end

  x.report("expand_path('')") do
    str = ""
    MAX.times{ File.expand_path(str) }
  end

  x.report("expand_path('', '~')") do
    MAX.times{ File.expand_path('', '~') }
  end
end
```
What I see is that most of these take only a second or two, while the UNC path takes 30+ seconds.



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