RFC: Rewrite homedir based on uv_os_homedir

[ararslan]

Fixes #11331

Rather than relying on environment variables to determine the user's home directory, this calls uv_os_homedir from libuv, per @stevengj's suggestion.

The "magic number" of 1024 in there is what we're setting as the PATH_MAX if none is found in the system (see src/support/dirpath.h), so it seemed to me like a decent choice for the number of bytes to allocate for the output path. Let me know if that isn't reasonable or if there's a better choice.

For reference, here is the upstream libuv documentation on uv_os_homedir: http:https://docs.libuv.org/en/v1.x/misc.html#c.uv_os_homedir

[yuyichao]

And a better implementation is

julia> function homedir2()
           sz = Ref{Csize_t}(1024 + 1)
           buf = Vector{UInt8}(sz[] - 1)
           while true
               rc = ccall(:uv_os_homedir, Cint, (Ptr{UInt8}, Ptr{Csize_t}), buf, sz)
               if rc == 0
                   resize!(buf, sz[])
                   return String(buf)
               elseif rc == Base.UV_ENOBUFS # No Base. in base
                   resize!(buf, sz[] - 1)
               else
                   error("Unable to retrieve home directory.")
               end
           end
       end

[yuyichao]

Hard to imagine it will happen but the correct way is to use sz[] - 1 to resize the array.

[yuyichao]

Should be sizeof(buf) + 1

[yuyichao]

This is wrong and inefficient.

[ararslan]

Thanks for the feedback, @yuyichao!

String(buf) causes an error because Cchar is Int8, not UInt8, so String can't convert the array. That's why I had initially used unsafe_string with pointer. Would it be better to convert to UInt8, then call the String constructor?

[yuyichao]

That s why I used UInt8

[ararslan]

Oh sorry! Missed that part.

[ararslan]

Looking better, @yuyichao?

[asampal]

With this change, is there any way to override the home directory Julia uses? I'd like my Julia homedir to be the home directory under my MSYS2/MinGW installation instead of the standard user dir Windows defines.

[stevengj]

@asampal, yes. From the libuv documentation:

Gets the current user’s home directory. On Windows, uv_os_homedir() first checks the USERPROFILE environment variable using GetEnvironmentVariableW(). If USERPROFILE is not set, GetUserProfileDirectoryW() is called. On all other operating systems, uv_os_homedir() first checks the HOME environment variable using getenv(3). If HOME is not set, getpwuid_r(3) is called. The user’s home directory is stored in buffer. When uv_os_homedir() is called, size indicates the maximum size of buffer. On success size is set to the string length of buffer. On UV_ENOBUFS failure size is set to the required length for buffer, including the null byte.

So, setting USERPROFILE seems like it should do it, which according to Microsoft refers to:

The user's profile folder. A typical path is C:\Documents and Settings\username. Applications should not create files or folders at this level; they should put their data under the locations referred to by CSIDL_APPDATA or CSIDL_LOCAL_APPDATA.

[asampal]

@stevengj if I redefine USERPROFILE in juliarc.jl I can get homedir() as I need it, but this will be overwritten by a new install. I can't use .juliarc.jl, to set the home dir, obviously, and this isn't an env variable that should be changed globally (actually marked as read-only). So without running Julia using a batch file, there's no other option?