Fix: Don't strip $libdir from nested module_pathnames

This patch fixes a bug in how 'load_external_function' handles
'$libdir/ prefixes in module paths.

Previously, 'load_external_function' would unconditionally strip
'$libdir/' from the beginning of the 'filename' string.  This caused
an issue when the path was nested, such as "$libdir/nested/my_lib".
Stripping the prefix resulted in a path of "nested/my_lib", which
would fail to be found by the expand_dynamic_library_name function
because the original '$libdir' macro was removed.

To fix this, the code now checks for the presence of an additional
directory separator ('/' or '\') after the '$libdir/' prefix.  The
prefix is only stripped if the remaining string does not contain a
separator.  This ensures that simple filenames like '"$libdir/my_lib"'
are correctly handled, while nested paths are left intact for
'expand_dynamic_library_name' to process correctly.

Reported-by: Dilip Kumar <dilipbalaut@gmail.com>
Co-authored-by: Matheus Alcantara <matheusssilv97@gmail.com>
Co-authored-by: Dilip Kumar <dilipbalaut@gmail.com>
Reviewed-by: Srinath Reddy Sadipiralla <srinath2133@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/CAFiTN-uKNzAro4tVwtJhF1UqcygfJ%2BR%2BRL%3Db-_ZMYE3LdHoGhA%40mail.gmail.com

src/backend/utils/fmgr/dfmgr.c

@@ -100,12 +100,19 @@ load_external_function(const char *filename, const char *funcname,
 	void	   *retval;
 
 	/*
-	 * If the value starts with "$libdir/", strip that.  This is because many
+	 * For extensions with hardcoded '$libdir/' library names, we strip the
-	 * extensions have hardcoded '$libdir/foo' as their library name, which
+	 * prefix to allow the library search path to be used. This is done only
-	 * prevents using the path.
+	 * for simple names (e.g., "$libdir/foo"), not for nested paths (e.g.,
+	 * "$libdir/foo/bar").
+	 *
+	 * For nested paths, 'expand_dynamic_library_name' directly expands the
+	 * '$libdir' macro, so we leave them untouched.
 	 */
 	if (strncmp(filename, "$libdir/", 8) == 0)
-		filename += 8;
+	{
+		if (first_dir_separator(filename + 8) == NULL)
+			filename += 8;
+	}
 
 	/* Expand the possibly-abbreviated filename to an exact path name */
 	fullname = expand_dynamic_library_name(filename);