access, euidaccess tests: Avoid test failure in Cygwin 3.5.5

[Bruno Haible]

The GitHub CI reports these test failures:

FAIL: test-access
=================

../../gltests/test-access.h:101: assertion 'func (BASE "f2", X_OK) == -1' failed
../../gltests/test-access.h:102: assertion 'errno == EACCES' failed
FAIL test-access.exe (exit status: 1)

FAIL: test-euidaccess
=====================

../../gltests/test-access.h:101: assertion 'func (BASE "f2", X_OK) == -1' failed
../../gltests/test-access.h:102: assertion 'errno == EACCES' failed
FAIL test-euidaccess.exe (exit status: 1)

The cause is that
  - GitHub apparently recently upgraded to use Cygwin 3.5.5.
  - The GitHub CI runs with Administrator permissions.
  - The behaviour of the access() function in these circumstances has changed
    in Cygwin 3.5.5, see
    <https://sourceware.org/pipermail/cygwin/2024-December/256972.html>

This patch adds a workaround.


2024-12-24  Bruno Haible  <bruno@clisp.org>

        access, euidaccess tests: Avoid test failure in Cygwin 3.5.5.
        * tests/test-access.h [__CYGWIN__]: Include <grp.h>, <string.h>,
        <unistd.h>.
        (is_administrator): New function.
        (is_root): New macro.
        (test_access): Reenable F_OK test for non-administrators on Cygwin.
        Disable X_OK test for administrators on Cygwin.

diff --git a/tests/test-access.h b/tests/test-access.h
index ece89beb6b..a5223356a5 100644
--- a/tests/test-access.h
+++ b/tests/test-access.h
@@ -19,6 +19,38 @@
 # define geteuid() ROOT_UID
 #endif
 
+#if defined __CYGWIN__
+/* Cygwin uses the security model from Windows.  */
+# include <grp.h>
+# include <string.h>
+# include <unistd.h>
+/* Test whether the current user is in the 'Administrators' group.
+   We cannot use the group name "Administrators", due to internationalization.
+   Therefore test for the SID "S-1-5-32-544" (cf.
+   <https://learn.microsoft.com/en-us/windows/win32/secauthz/well-known-sids>).
+ */
+static int
+is_administrator (void)
+{
+  gid_t list[100];
+  int i;
+  int count = getgroups (sizeof (list) / sizeof (list[0]), list);
+  if (count < 0)
+    return 0;
+  for (i = 0; i < count; i++)
+    {
+      gid_t id = list[i];
+      struct group *details = getgrgid (id);
+      if (details != NULL && strcmp (details->gr_passwd, "S-1-5-32-544") == 0)
+        return 1;
+    }
+  return 0;
+}
+# define is_root() is_administrator ()
+#else
+# define is_root() (geteuid () == ROOT_UID)
+#endif
+
 static void
 test_access (int (*func) (const char * /*file*/, int /*mode*/))
 {
@@ -79,22 +111,22 @@ test_access (int (*func) (const char * /*file*/, int 
/*mode*/))
 
     /* On Cygwin, for users that are in the 'Administrators' group,
        W_OK is allowed.  */
-#if !defined __CYGWIN__
-    if (geteuid () != ROOT_UID)
+    if (! is_root ())
       {
         errno = 0;
         ASSERT (func (BASE "f2", W_OK) == -1);
         ASSERT (errno == EACCES);
       }
-#endif
 
 #if defined _WIN32 && !defined __CYGWIN__
     /* X_OK works like R_OK.  */
     ASSERT (func (BASE "f2", X_OK) == 0);
 #else
-    /* On Solaris, for the root user, X_OK is allowed.  */
-# if defined __sun
-    if (geteuid () != ROOT_UID)
+    /* On Solaris, for the root user, X_OK is allowed.
+       On Cygwin 3.5.5, for users that are in the 'Administrators' group,
+       X_OK is allowed.  */
+# if defined __sun || defined __CYGWIN__
+    if (! is_root ())
 # endif
       {
         errno = 0;