[newlib-cygwin/cygwin-3_3-branch] Cygwin: path: Fix UNC path handling for SMB3 mounted to a drive.

[Takashi Yano <tyan0@sourceware.org>]

https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=71d5490a3a2d769e1893f11c98edb06468ad6af4

commit 71d5490a3a2d769e1893f11c98edb06468ad6af4
Author: Takashi Yano <takashi.yano@nifty.ne.jp>
Date:   Thu Feb 3 12:00:14 2022 +0900

    Cygwin: path: Fix UNC path handling for SMB3 mounted to a drive.
    
    - If an UNC path is mounted to a drive using SMB3.11, accessing to
      the drive fails with error "Too many levels of symbolic links."
      This patch fixes the issue.

Diff:
---
 winsup/cygwin/path.cc | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/winsup/cygwin/path.cc b/winsup/cygwin/path.cc
index 87ac2404a..4ad4e0821 100644
--- a/winsup/cygwin/path.cc
+++ b/winsup/cygwin/path.cc
@@ -3495,10 +3495,19 @@ restart:
 
 	      /* 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. */
+		 below string comparison has a chance to succeed.
+		 On the contrary, if incoming path has trailing backslash,
+		 but final path does not have one, add trailing backslash
+		 to the final path. */
 	      if (upath.Buffer[(upath.Length - 1) / sizeof (WCHAR)] != L'\\'
-                  && fpbuf[ret - 1] == L'\\')
+		  && fpbuf[ret - 1] == L'\\')
                 fpbuf[--ret] = L'\0';
+	      if (upath.Buffer[(upath.Length - 1) / sizeof (WCHAR)] == L'\\'
+		  && fpbuf[ret - 1] != L'\\' && ret < NT_MAX_PATH - 1)
+		{
+		  fpbuf[ret++] = L'\\';
+		  fpbuf[ret] = L'\0';
+		}
 	      fpbuf[1] = L'?';	/* \\?\ --> \??\ */
 	      RtlInitCountedUnicodeString (&fpath, fpbuf, ret * sizeof (WCHAR));
 	      if (!RtlEqualUnicodeString (&upath, &fpath, !!ci_flag))