[GHC] #10822: GHC inconsistently handles \\?\ for long paths on Windows

[GHC]

#10822: GHC inconsistently handles \\?\ for long paths on Windows
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              Reporter:  snoyberg    |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.10.2
              Keywords:              |  Operating System:  Windows
          Architecture:              |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
             Test Case:              |        Blocked By:
              Blocking:              |   Related Tickets:
Differential Revisions:              |
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 To demonstrate what I mean first:

 {{{
 >ghc-pkg init \\?\c:\Users\michael\Desktop\some-pkg-db
 >dir \\?\c:\Users\Michael\Desktop\some-pkg-db
  Volume in drive \\?\c: is Windows8_OS
  Volume Serial Number is CEF7-874D

  Directory of \\?\c:\Users\Michael\Desktop\some-pkg-db

 09/01/2015  09:38 AM    <DIR>          .
 09/01/2015  09:38 AM    <DIR>          ..
 09/01/2015  09:38 AM                40 package.cache
                1 File(s)             40 bytes
                2 Dir(s)   5,907,509,248 bytes free
 >ghc -package-db \\?\c:\Users\Michael\Desktop\some-pkg-db
 ghc: can't find a package database at \\?\c:\Users\Michael\Desktop\some-
 pkg-db
 }}}

 The problem: on Windows by default, paths are limited to 260 characters,
 which can become a real problem in practice when dealing with the standard
 library path hierarchy the Cabal library uses and any package with a
 decent name length. This was a real problem in practice, see:

 https://github.com/haskell/cabal/issues/2502

 Windows has a workaround for this: you can prefix an absolute path with
 \\?\, see:

 https://msdn.microsoft.com/en-
 us/library/windows/desktop/aa365247%28v=vs.85%29.aspx#maxpath

 It would be very useful if such paths worked with GHC, but currently they
 do not. I don't yet know how deep the problem is.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10822>
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