[newlib-cygwin] Cygwin: realpath: fix cygwin installation dir being access via junction

[Corinna Vinschen <corinna@sourceware.org>]

https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=6d898f43fc87a7bf3ab100d75538e381bd86f657

commit 6d898f43fc87a7bf3ab100d75538e381bd86f657
Author: Corinna Vinschen <corinna@vinschen.de>
Date:   Fri Feb 19 18:15:58 2021 +0100

    Cygwin: realpath: fix cygwin installation dir being access via junction
    
    Consider this case:
    
    - Cygwin installed in C:\cygwin64
    
    - mklink /j D:\cygwin64 C:\cygwin64
    
    - create testcase calling
    
        realpath("/", result);
        printf ("%s\n", result);
    
    - start cmd
    
        >C:\cygwin64\bin\bash -lc <path-to-testcase>
        /
        >D\cygwin64\bin\bash -lc <path-to-testcase>
        /cygdrive/c/cygwin64
    
    This scenario circumventing the mount point handling which is automated
    in terms of /, depending on the path returned from GetModuleFileNameW
    for the Cygwin DLL.  When calling D:\cygwin64\bin\bash the dir returned
    from GetModuleFileNameW is D:\cygwin64\bin, thus root is D:\cygwin64.
    
    However, junctions are treated as symlinks in Cygwin which explains why
    the path gets converted to a cygdrive path.
    
    Fix this by calling GetFinalPathNameByHandleW on the result from
    GetModuleFileNameW to get the correct root path, even if accessed via
    a junction point.
    
    Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

Diff:
---
 winsup/cygwin/cygheap.cc | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/winsup/cygwin/cygheap.cc b/winsup/cygwin/cygheap.cc
index 3a32c352d..01b49468e 100644
--- a/winsup/cygwin/cygheap.cc
+++ b/winsup/cygwin/cygheap.cc
@@ -144,6 +144,19 @@ init_cygheap::init_installation_root ()
     api_fatal ("Can't initialize Cygwin installation root dir.\n"
 	       "GetModuleFileNameW(%p, %p, %u), %E",
 	       cygwin_hmodule, installation_root_buf, PATH_MAX);
+
+  /* We don't care if fetching the final pathname fails, it's non-fatal and
+     the path returned by GetModuleFileNameW is still valid. */
+  HANDLE h;
+  h = CreateFileW (installation_root_buf, GENERIC_READ, FILE_SHARE_VALID_FLAGS,
+		   &sec_none, OPEN_EXISTING, 0, 0);
+  if (h != INVALID_HANDLE_VALUE)
+    {
+      GetFinalPathNameByHandleW (h, installation_root_buf, PATH_MAX,
+				 FILE_NAME_NORMALIZED);
+      CloseHandle (h);
+    }
+
   PWCHAR p = installation_root_buf;
   if (wcsncasecmp (p, L"\\\\", 2))	/* Normal drive letter path */
     {