If the directory name has the form 'x:' followed by one or more
slashes or backslashes, and if there's at least one backslash, assume
that the user is referring to 'x:\', the root directory of drive x,
and don't strip the backslash.

Previously all trailing slashes and backslashes were stripped, and the
name was treated as a relative file name containing a literal colon.

Addresses https://cygwin.com/ml/cygwin/2019-08/msg00334.html.
---
 winsup/cygwin/dir.cc | 33 ++++++++++++++++++++++++++++-----
 1 file changed, 28 insertions(+), 5 deletions(-)

diff --git a/winsup/cygwin/dir.cc b/winsup/cygwin/dir.cc
index 29a9dfa83..3429fe022 100644
--- a/winsup/cygwin/dir.cc
+++ b/winsup/cygwin/dir.cc
@@ -313,15 +313,27 @@ mkdir (const char *dir, mode_t mode)
       /* Following Linux, and intentionally ignoring POSIX, do not
         resolve the last component of DIR if it is a symlink, even if
         DIR has a trailing slash.  Achieve this by stripping trailing
-        slashes or backslashes.  */
+        slashes or backslashes.
+
+        Exception: If DIR == 'x:' followed by one or more slashes or
+        backslashes, and if there's at least one backslash, assume
+        that the user is referring to the root directory of drive x.
+        Retain one backslash in this case.  */
       if (isdirsep (dir[strlen (dir) - 1]))
        {
          /* This converts // to /, but since both give EEXIST, we're okay.  */
          char *buf;
          char *p = stpcpy (buf = tp.c_get (), dir) - 1;
+         bool msdos = false;
          dir = buf;
          while (p > dir && isdirsep (*p))
-           *p-- = '\0';
+           {
+             if (*p == '\\')
+               msdos = true;
+             *p-- = '\0';
+           }
+         if (msdos && p == dir + 1 && isdrive (dir))
+           p[1] = '\\';
        }
       if (!(fh = build_fh_name (dir, PC_SYM_NOFOLLOW)))
        __leave;   /* errno already set */;
@@ -360,20 +372,31 @@ rmdir (const char *dir)
          set_errno (ENOENT);
          __leave;
        }
-
       /* Following Linux, and intentionally ignoring POSIX, do not
         resolve the last component of DIR if it is a symlink, even if
         DIR has a trailing slash.  Achieve this by stripping trailing
-        slashes or backslashes.  */
+        slashes or backslashes.
+
+        Exception: If DIR == 'x:' followed by one or more slashes or
+        backslashes, and if there's at least one backslash, assume
+        that the user is referring to the root directory of drive x.
+        Retain one backslash in this case.  */
       if (isdirsep (dir[strlen (dir) - 1]))
        {
          /* This converts // to /, but since both give ENOTEMPTY,
             we're okay.  */
          char *buf;
          char *p = stpcpy (buf = tp.c_get (), dir) - 1;
+         bool msdos = false;
          dir = buf;
          while (p > dir && isdirsep (*p))
-           *p-- = '\0';
+           {
+             if (*p == '\\')
+               msdos = true;
+             *p-- = '\0';
+           }
+         if (msdos && p == dir + 1 && isdrive (dir))
+           p[1] = '\\';
        }
       if (!(fh = build_fh_name (dir, PC_SYM_NOFOLLOW)))
        __leave;   /* errno already set */;
-- 
2.21.0

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