[filesystem] Management of junction on Windows is broken and api is inconsistent

From: Vince R. (forumer_at_[hidden])
Date: 2019-11-06 19:23:03

Hello,

I am bit disappointed to see that symlink and junctions are still not 
really properly handled on Windows with boost (and even with standard MS 
libs but that's another story.)
First my environment: Windows 10, VS2019 16.3.7, boost 1.71 (static, 
x64).
I would like to write a small c++ utility that does not use os dependent 
code to handle files so filesystem seems a good candidate.

Here is what I am doing on Windows:

cd "C:\Users\Vincent\AppData\Roaming\Apple Computer\MobileSync"
mklink /j Backup E:\Backup

So basically I am creating inside C:\..\MobileSync a junction folder 
that points to my second harddisk E:\Backup

Then I am checking that Windows recognize it by entering dir command:

01/11/2019  23:48    <DIR>          .
01/11/2019  23:48    <DIR>          ..
01/11/2019  23:29    <JUNCTION>     Backup [E:\Backup]
                0 fichier(s)                0 octets
                3 Rép(s)  59 393 630 208 octets libres

As you can see I have a junction targeting E:\Backup, so now let's see 
what happens with boost:

fs::path p(_T("C:\\Users\\Vincent\\AppData\\Roaming\\Apple 
Computer\\MobileSync\\Backup"));
sys::error_code ec;

if (fs::is_symlink(p))
{
      fs::path pathTarget = fs::read_symlink(p, ec);
}

When I display the content of pathTarget I have: L"\\Backup" ie Backup 
if I decode it into human string
I might not understand anything about junction but is it not supposed to 
return E:\Backup ??

When I look at source code I can see the following lines:

/********************** boost_1_71_0\libs\filesystem\src\operations.cpp 
**************************************/
symlink_path.assign(

static_cast<wchar_t*>(info.rdb.SymbolicLinkReparseBuffer.PathBuffer)
         + 
info.rdb.SymbolicLinkReparseBuffer.PrintNameOffset/sizeof(wchar_t),

static_cast<wchar_t*>(info.rdb.SymbolicLinkReparseBuffer.PathBuffer)
         + 
info.rdb.SymbolicLinkReparseBuffer.PrintNameOffset/sizeof(wchar_t)
         + 
info.rdb.SymbolicLinkReparseBuffer.PrintNameLength/sizeof(wchar_t));

This is wrong since we need to distinguish between a symlink and a 
junction folder so here is my contribution :

--- a/src/operations.cpp
+++ b/src/operations.cpp
@@ -201,6 +201,12 @@ typedef struct _REPARSE_DATA_BUFFER {
  # ifndef IO_REPARSE_TAG_SYMLINK
  #   define IO_REPARSE_TAG_SYMLINK (0xA000000CL)
  # endif
+# ifndef IO_REPARSE_TAG_MOUNT_POINT
+#   define IO_REPARSE_TAG_MOUNT_POINT (0xA0000003L)
+# endif
+# ifndef IO_REPARSE_TAG_DEDUP
+#   define IO_REPARSE_TAG_DEDUP (0x80000013L)
+# endif

  inline std::wstring wgetenv(const wchar_t* name)
  {
@@ -1661,12 +1667,24 @@ namespace detail
      if (!error(::DeviceIoControl(h.handle, FSCTL_GET_REPARSE_POINT,
            0, 0, info.buf, sizeof(info), &sz, 0) == 0 ? BOOST_ERRNO : 0, 
p, ec,
            "boost::filesystem::read_symlink" ))
+       if (info.rdb.ReparseTag == IO_REPARSE_TAG_SYMLINK)
+       {
        symlink_path.assign(

static_cast<wchar_t*>(info.rdb.SymbolicLinkReparseBuffer.PathBuffer)
          + 
info.rdb.SymbolicLinkReparseBuffer.PrintNameOffset/sizeof(wchar_t),

static_cast<wchar_t*>(info.rdb.SymbolicLinkReparseBuffer.PathBuffer)
          + 
info.rdb.SymbolicLinkReparseBuffer.PrintNameOffset/sizeof(wchar_t)
          + 
info.rdb.SymbolicLinkReparseBuffer.PrintNameLength/sizeof(wchar_t));
+       }
+       else if (info.rdb.ReparseTag == IO_REPARSE_TAG_MOUNT_POINT)
+       {
+          symlink_path.assign(
+ static_cast<wchar_t*>(info.rdb.MountPointReparseBuffer.PathBuffer)
+             + info.rdb.MountPointReparseBuffer.PrintNameOffset / 
sizeof(wchar_t),
+ static_cast<wchar_t*>(info.rdb.MountPointReparseBuffer.PathBuffer)
+             + info.rdb.MountPointReparseBuffer.PrintNameOffset / 
sizeof(wchar_t)
+             + info.rdb.MountPointReparseBuffer.PrintNameLength / 
sizeof(wchar_t));
+       }
  #     endif
      return symlink_path;
    }

I will add more suggestions in a next message because there is no 
consistency between boost and MSVC approach. I don't know the last c++ 
specifications but it shouldn't be difficult to have a simple approach;