[newlib-cygwin/cygwin-3_3-branch] Cygwin: path: Convert UNC path prefix back to drive letter.

[Takashi Yano <tyan0@sourceware.org>]

https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=1b40f151cd5ad86241e98fe3b60d4700f04bfb99

commit 1b40f151cd5ad86241e98fe3b60d4700f04bfb99
Author: Takashi Yano <takashi.yano@nifty.ne.jp>
Date:   Wed Dec 8 20:15:45 2021 +0900

    Cygwin: path: Convert UNC path prefix back to drive letter.
    
    - When symlink_info::check() is called with the path having drive
      letter and UNC path is mounted to the drive, the path is replaced
      with UNC path. With this patch, UNC path prefix is converted back
      to drive letter.  This fixes the issue:
      https://cygwin.com/pipermail/cygwin/2021-November/250087.html
      https://cygwin.com/pipermail/cygwin/2021-December/250103.html

Diff:
---
 winsup/cygwin/path.cc       | 37 ++++++++++++++++++++++++++++++++++++-
 winsup/cygwin/release/3.3.4 |  5 +++++
 2 files changed, 41 insertions(+), 1 deletion(-)

diff --git a/winsup/cygwin/path.cc b/winsup/cygwin/path.cc
index baf04ce89..eb1255849 100644
--- a/winsup/cygwin/path.cc
+++ b/winsup/cygwin/path.cc
@@ -3492,10 +3492,45 @@ restart:
 	    {
 	      UNICODE_STRING fpath;
 
-	      RtlInitCountedUnicodeString (&fpath, fpbuf, ret * sizeof (WCHAR));
+	      /* If incoming path has no trailing backslash, but final path
+		 has one, drop trailing backslash from final path so the
+		 below string comparison has a chance to succeed. */
+	      if (upath.Buffer[(upath.Length - 1) / sizeof (WCHAR)] != L'\\'
+                  && fpbuf[ret - 1] == L'\\')
+                fpbuf[--ret] = L'\0';
 	      fpbuf[1] = L'?';	/* \\?\ --> \??\ */
+	      RtlInitCountedUnicodeString (&fpath, fpbuf, ret * sizeof (WCHAR));
 	      if (!RtlEqualUnicodeString (&upath, &fpath, !!ci_flag))
 	        {
+		  /* Check if the final path is an UNC path and the incoming
+		     path isn't.  If so... */
+		  if (RtlEqualUnicodePathPrefix (&fpath, &ro_u_uncp, TRUE)
+		      && !RtlEqualUnicodePathPrefix (&upath, &ro_u_uncp, TRUE))
+		    {
+		      /* ...get the remote path from the volume path name,
+			 replace remote path with drive letter, check again. */
+		      WCHAR remote[MAX_PATH];
+
+		      fpbuf[1] = L'\\';
+		      BOOL r = GetVolumePathNameW (fpbuf, remote, MAX_PATH);
+		      fpbuf[1] = L'?';
+		      if (r)
+			{
+			  int remlen = wcslen (remote);
+			  if (remote[remlen - 1] == L'\\')
+			    remlen--;
+			  /* Hackfest */
+			  fpath.Buffer[4] = upath.Buffer[4]; /* Drive letter */
+			  fpath.Buffer[5] = L':';
+			  WCHAR *to = fpath.Buffer + 6;
+			  WCHAR *from = to + remlen - 6;
+			  memmove (to, from,
+				   (wcslen (from) + 1) * sizeof (WCHAR));
+			  fpath.Length -= (from - to) * sizeof (WCHAR);
+			  if (RtlEqualUnicodeString (&upath, &fpath, !!ci_flag))
+			    goto file_not_symlink;
+			}
+		    }
 		  issymlink = true;
 		  /* upath.Buffer is big enough and unused from this point on.
 		     Reuse it here, avoiding yet another buffer allocation. */
diff --git a/winsup/cygwin/release/3.3.4 b/winsup/cygwin/release/3.3.4
index f1c32a1a5..4cbfba8eb 100644
--- a/winsup/cygwin/release/3.3.4
+++ b/winsup/cygwin/release/3.3.4
@@ -4,3 +4,8 @@ Bug Fixes
 - Fix a bug in fhandler_dev_clipboard::read() that the second read
   fails with 'Bad address'.
   Addresses: https://cygwin.com/pipermail/cygwin/2021-December/250141.html
+
+- Convert UNC path prefix back to drive letter in symlink_info::check().
+  This solves the following issues:
+  Addresses: https://cygwin.com/pipermail/cygwin/2021-November/250087.html
+             https://cygwin.com/pipermail/cygwin/2021-December/250103.html