Exceeding the windows API MAX_PATH limit

[Angel Ezquerra]

Hi,

I've recently had some users that have shot themselves in the foot by
add some files with really long paths to a mercurial repo. The file
paths were longer than the windows API MAX_PATH, which is 260.

The problem occurred when importing some data from a clearcase
repository into mercurial. The ClearCase repository contained some
files with very long paths (more than 200 characters). However,
because ClearCase forces you (in our setup) to place all your
repositories on some folder right on your C:\ drive, the total length
of those file paths did not exceed 260 characters.

A user created a mercurial repository right on the clearcase
repository folder. Everything was added fine because in there all file
paths were below MAX_PATH. He then cloned this repo into our shared
server (into c:\mercurial\bad_repo). Again, all paths were below
MAX_PATH.

The problem came when another user tried to clone this repo into its
machine. This user places all his repositories in "My
Documents\repositories" (which I think is common for windows users).
The clone went fine until it tried to update to tip, which failed
because mercurial could not create some files whose paths would exceed
MAX_PATH. The user can clone the repo right into its C:\ root without
any issues.

I think this problem is particularly tricky because the user making a
commit cannot tell if it will fail in someone else's machine!

So far I've come up with 4 solutions, none of which is really satisfactory:

1. Rename the files: it may be feasible in this particular case but
there may be cases where this is just not possible (e.g. for source
code that you do not control)
2. Always clone into the c:\ root or at least have all your repos in
"c:\repos" or a similar path: This may be fine (and good advice) in
general, but may not be ok on multi-user machines and in some cases
you may not be able to chose where to clone into...
3. Use the subst command to create a temporary drive and work in
there: I've seen this suggested online, but I don't know if this will
cause problems since the files will live both on the "virtual drive"
and on the original deep folder.
4. Use folder junctions: same issues as 3?

Do you guys have any other solution?

BTW, I've looked for info on this MAX_PATH limitation and it seems
that this 260 MAX_PATH limit is a limitation of the windows API, not
of the underlying NTFS file system, which can support files of up to
32,767 characters (with a limit of 255 per individual 'path element').
Apparently the unicode version of the windows API _mostly_ supports
long paths. It seems that you can access the unicode APIs by
prepending your paths with "\\?\":

http://msdn.microsoft.com/en-us/library/aa365247%28VS.85%29.aspx#maxpath
http://blogs.msdn.com/b/bclteam/archive/2007/02/13/long-paths-in-net-part-1-of-3-kim-hamilton.aspx

The second link is particularly interesting (specially part 2).
Unfortunately a quick test shows that python's open function does not
support this.

Cheers,

Angel