[GHC] #13373: Handle long file paths on Windows

[GHC]

#13373: Handle long file paths on Windows
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        Reporter:  ezyang            |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  libraries/base    |              Version:  8.1
      Resolution:                    |             Keywords:
Operating System:  Windows           |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Rufflewind):

 Turns out, despite what [https://msdn.microsoft.com/en-
 us/library/windows/desktop/aa365530.aspx MSDN] says,
 `SetCurrentDirectoryW` does not seem to support long paths through the
 `\\?\` prefix.  I saw an [https://news.ycombinator.com/item?id=13093529
 anecdote] that it ''does'' work if one opts-in through the Win10-only
 feature (through manifest and/or (?) registry tweak), as long as you
 append a backslash.  Since it still did not work with the `\\?\` prefix
 even after appending a backslash, so I can only assume that `\\?\` is
 broken for `SetCurrentDirectoryW`, but the registry/manifest tweak likely
 does work.  My guess is that because in the past `SetCurrentDirectoryW`
 never supported `\\?\` to begin with, when they added the Win10 feature,
 whoever updated the documentation probably copy-pasted that section
 throughout and forgot that `\\?\` was ''specifically not enabled'' for
 `SetCurrentDirectoryW` unlike many other functions, likely because it
 would make the counterpart `GetCurrentDirectoryW` risky for improperly
 written programs.

 TL;DR: It does not seem possible to set the current directory to a long
 path except through the Win-10 only mechanism.

 Also, a lot of Windows functions will choke on relative paths if the
 resulting path becomes too long, so users will have to use exclusively
 `\\?\`-style absolute paths.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13373#comment:6>
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