Remove MAX_PATH restrictions from RTS, I/O manager and various utilities

Summary:
This shims out fopen and sopen so that they use modern APIs under the hood
along with namespaced paths.

This lifts the MAX_PATH restrictions from Haskell programs and makes the new
limit ~32k.

There are only some slight caveats that have been documented.

Some utilities have not been upgraded such as lndir, since all these things are
different cabal packages I have been forced to copy the source in different places
which is less than ideal. But it's the only way to keep sdist working.

Test Plan: ./validate

Reviewers: hvr, bgamari, erikd, simonmar

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #10822

Differential Revision: https://phabricator.haskell.org/D4416

1 files changed, 46 insertions, 2 deletions

diff --git a/docs/users_guide/8.6.1-notes.rst b/docs/users_guide/8.6.1-notes.rst
index 548702159c..6300a3f3e4 100644
--- a/docs/users_guide/8.6.1-notes.rst
+++ b/docs/users_guide/8.6.1-notes.rst
@@ -82,8 +82,10 @@ Runtime system
   else. This fixes ``iuuc`` on Windows given the proper search directories (e.g
   ``-L/mingw64/lib``).
 
-	- The GHC runtime linker now uses ``LIBRARY_PATH`` and the runtime loader now also
-	  searches ``LD_LIBRARY_PATH``.
+- The GHC runtime linker now uses ``LIBRARY_PATH`` and the runtime loader now also
+  searches ``LD_LIBRARY_PATH``.
+
+- The GHC runtime on Windows is no longer constrained by MAX_PATH.
 
 Template Haskell
 ~~~~~~~~~~~~~~~~
@@ -107,6 +109,48 @@ Template Haskell
 Build system
 ~~~~~~~~~~~~
 
+Windows Paths
+~~~~~~~~~~~~~
+
+Windows paths are not all the same. The different kinds of paths each have
+different meanings. The MAX_PATH limitation is not a limitation of the Operating
+System nor the File System. It is a limitation of the default namespace enforced
+by the Win32 API for backwards compatibility.
+
+The NT Kernel however allows you ways to opt out of this path preprocessing by
+the Win32 APIs. This is done by explicitly using the desired namespace in the
+PATH.
+
+The namespaces are:
+
+ - file namespace: \\?\
+ - device namespace: \\.\
+ - nt namespace: \
+
+Each of these turn off Path processing completely by the Win32 API and the paths
+are passed untouched to the filesystem.
+
+Paths with a drive letter are `legacy` paths. The drive letters are actually
+meaningless to the kernel. Just like Unix operating systems, drive letters are
+just a mount point. You can view your mount points by using the `mountvol`
+command.
+
+The Haskell I/O manager will now automatically promote paths in the legacy
+format to Win32 file namespace. By default the I/O manager will do two things to
+your paths:
+
+  - replace / with \\
+  - expand relative paths to absolute paths
+
+If you want to opt out of all preprocessing just expliticly use namespaces in
+your paths. Due to this change, if you need to open raw devices (e.g. COM ports)
+you need to use the device namespace explicitly. (e.g. `\\.\COM1`). GHC and
+Haskell programs in general no longer support opening devices in the `legacy`
+format.
+
+See https://msdn.microsoft.com/en-us/library/windows/desktop/aa365247.aspx for
+more details.
+
 
 Included libraries
 ------------------