[newlib-cygwin/main] Cygwin: dirname: fix handling of leading slashes

[Corinna Vinschen <corinna@sourceware.org>]

https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=2898a359b80c18fb0c3d4122b8f40daed49ba11e

commit 2898a359b80c18fb0c3d4122b8f40daed49ba11e
Author:     Corinna Vinschen <corinna@vinschen.de>
AuthorDate: Wed Mar 29 10:18:23 2023 +0200
Commit:     Corinna Vinschen <corinna@vinschen.de>
CommitDate: Wed Mar 29 10:23:05 2023 +0200

    Cygwin: dirname: fix handling of leading slashes
    
    Per https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/xbd_chap04.html:
    
      "A pathname that begins with two successive slashes may be interpreted
       in an implementation-defined manner, although more than two leading
       slashes shall be treated as a single slash."
    
    So more than 2 leading slashes are supposed to be folded into one,
    which our dirname neglected.  Fix that.
    
    Fixes: 24e8fc6872a3b ("* cygwin.din (basename): Export.")
    Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

Diff:
---
 winsup/cygwin/path.cc       | 2 +-
 winsup/cygwin/release/3.4.7 | 3 +++
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/winsup/cygwin/path.cc b/winsup/cygwin/path.cc
index eaf695b8182a..582238d1503f 100644
--- a/winsup/cygwin/path.cc
+++ b/winsup/cygwin/path.cc
@@ -5120,7 +5120,7 @@ dirname (char *path)
     return strcpy (buf, ".");
   if (isalpha (path[0]) && path[1] == ':')
     bs += 2;
-  else if (strspn (path, "/\\") > 1)
+  else if (strspn (path, "/\\") == 2)
     ++bs;
   c = strrchr (bs, '/');
   if ((d = strrchr (c ?: bs, '\\')) > c)
diff --git a/winsup/cygwin/release/3.4.7 b/winsup/cygwin/release/3.4.7
index a121d81b62fa..2c305ec5f320 100644
--- a/winsup/cygwin/release/3.4.7
+++ b/winsup/cygwin/release/3.4.7
@@ -6,3 +6,6 @@ Bug Fixes
 
 - kill(1): don't print spurious error message.
   Addresses: https://cygwin.com/pipermail/cygwin/2023-March/253291.html
+
+- Align behaviour of dirname in terms of leading slashes to POSIX:
+  https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html