Cannot resolve or detect symlinks on Windows

[nlowe]

jruby 1.7.21 (1.9.3p551) 2015-07-07 a741a82 on Java HotSpot(TM) 64-Bit Server VM 1.8.0_51-b16 +jit [Windows 8.1-amd64]

While doing some testing for running gollum under windows I discovered that JRuby under windows appears to lack support for resolving symlinks. From gollum/gollum#1044:

---

It doesn't seem that Pathname can detect windows symlinks. For this test, I have created the following:

• C:\SymlinkTest\a.txt
• C:\SymlinkTest\b.txt => softlink to C:\SymlinkTest\a.txt (mklink C:\SymlinkTest\b.txt C:\SymlinkTest\a.txt)
• C:\SymlinkTest\c.txt => hardlink to C:\SymlinkTest\a.txt (fsutil hardlink create C:\SymlinkTest\b.txt C:\SymlinkTest\a.txt)

irb(main):001:0> b = Pathname.new('C:\\SymlinkTest\\b.txt')
=> <Pathname:C:\SymlinkTest\b.txt>
irb(main):002:0> p.symlink?
=> false
irb(main):003:0> c = Pathname.new('C:\\SymlinkTest\\c.txt')
=> <Pathname:C:\SymlinkTest\c.txt>
irb(main):004:0> p.symlink?
=> false
irb(main):005:0> b.realpath
=> <Pathname:C:\SymlinkTest\b.txt>
irb(main):006:0> c.realpath
=> <Pathname:C:\SymlinkTest\c.txt>
irb(main):007:0> File.readlink('C:\\SymlinkTests\\b.txt')
NotImplementedError: readlink unsupported or native support failed to load
        from org/jruby/RubyFile.java:1049:in `readlink'
        from (irb):1:in `evaluate'
        from org/jruby/RubyKernel.java:1079:in `eval'
        from org/jruby/RubyKernel.java:1479:in `loop'
        from org/jruby/RubyKernel.java:1242:in `catch'
        from org/jruby/RubyKernel.java:1242:in `catch'
        from C:/jruby-1.7.21/bin/jirb:13:in `(root)'
irb(main):008:0> File.readlink('C:\\SymlinkTests\\c.txt')
NotImplementedError: readlink unsupported or native support failed to load
        from org/jruby/RubyFile.java:1049:in `readlink'
        from (irb):2:in `evaluate'
        from org/jruby/RubyKernel.java:1079:in `eval'
        from org/jruby/RubyKernel.java:1479:in `loop'
        from org/jruby/RubyKernel.java:1242:in `catch'
        from org/jruby/RubyKernel.java:1242:in `catch'
        from C:/jruby-1.7.21/bin/jirb:13:in `(root)'

[enebo]

Article on links for windows as a note when we try and figure this out: http://www.sevenforums.com/tutorials/278262-mklink-create-use-links-windows.html. This will get fixed upstream in jnr/jnr-posix to get fixed in jruby proper.

[headius]

Does this work in MRI? I thought I might find that they had shimmed out a win32 version of readlink, but it appears they simply raise NotImplementedError when readlink is not present on the given system. We could do that too; perhaps gollum checks if it's supported before trying to use it?

[headius]

I went ahead and "fixed" this the same way MRI does, by marking File.readlink as not implemented on Windows. I do feel like we could both provide readlink using Win32 API calls, but our behavior now aligns with MRI.

@techwiz24 Tonight's nightly builds of 1.7.23 and 9.0.2.0 will have this fix...perhaps you can check whether it's working? I suspect gollum checks for readlink or perhaps it should. If you believe we should actually support readlink, we'll need to have that conversation with ruby-core too.

[nlowe]

@headius Thank you for looking into this. For the specific test that was failing, this is what gollum looks for:

# Return the file path to this file on disk, if available.
#
# Returns nil if the file isn't available on disk. This can occur if the
# repo is bare, if the commit isn't the HEAD, or if there are problems
# resolving symbolic links.
def get_disk_reference(name, commit)
  return false if @wiki.repo.bare
  return false if commit.sha != @wiki.repo.head.commit.sha

  # This will try to resolve symbolic links, as well
  pathname = Pathname.new(::File.expand_path(::File.join(@wiki.repo.path, '..', name)))
  if pathname.symlink?
    source   = ::File.readlink(pathname.to_path)
    realpath = ::File.join(::File.dirname(pathname.to_path), source)
    return false unless realpath && ::File.exist?(realpath)
    @on_disk_path = realpath.to_s
  else
    @on_disk_path = pathname.to_path
  end
  true
end

I believe what you're saying is that we'll have to wrap pathname.symlink? to catch a NotImplemented under windows, correct?

[bartkamphorst]

I do feel like we could both provide readlink using Win32 API calls, but our behavior now aligns with MRI.

Thank you @headius , appreciate your time and effort! Another way to go might be to rely on java.nio.file, which, as @techwiz24 confirmed, can be used to read symlinks on Windows. Unsure if that would be a viable option though. And yes, there is of course the alignment with MRI issue.

[Wadeck]

Necroposting apart, it could good for reader coming from Google search.

rely on java.nio.file

I imagine you meant: File#toPath().toRealPath()
Just a short notice on that workaround, it works iff you check a file/folder that exist. If you have a symlink inside the hierarchy of a file that is not yet written, the method will throw an exception.

Concerning the support in "native" getCanonicalPath, it was requested in OpenJDK in 2012 and still not done.

[bartkamphorst]

Thanks @Wadeck for sharing this info!