io: Add FUSE-based test for fchmod

Test all mode arguments, and that extra bits are ignored
as required by POSIX.

Reviewed-by: DJ Delorie <dj@redhat.com>

io/Makefile

@@ -189,6 +189,7 @@ tests := \
   tst-copy_file_range \
   tst-faccessat \
   tst-fchmod-errors \
+  tst-fchmod-fuse \
   tst-fchmodat \
   tst-fchownat \
   tst-fcntl \

io/tst-fchmod-fuse.c

@@ -0,0 +1,114 @@
+/* FUSE-based test for fchmod.
+   Copyright (C) 2024 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <support/fuse.h>
+
+#include <errno.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <support/check.h>
+#include <support/support.h>
+#include <support/xunistd.h>
+
+/* Set from do_test to indicate the expected incoming mode change request.  */
+static _Atomic int expected_mode;
+
+static void
+fuse_thread (struct support_fuse *f, void *closure)
+{
+  struct fuse_in_header *inh;
+  while ((inh = support_fuse_next (f)) != NULL)
+    {
+      if (support_fuse_handle_mountpoint (f)
+          || (inh->nodeid == 1 && support_fuse_handle_directory (f)))
+        continue;
+      switch (inh->opcode)
+        {
+        case FUSE_LOOKUP:
+          {
+            char *name = support_fuse_cast (LOOKUP, inh);
+            TEST_COMPARE_STRING (name, "file");
+            struct fuse_entry_out *out
+              = support_fuse_prepare_entry (f, 2);
+            out->attr.mode = S_IFREG | 0600;
+            support_fuse_reply_prepared (f);
+          }
+          break;
+        case FUSE_OPEN:
+          {
+            TEST_COMPARE (inh->nodeid, 2);
+            struct fuse_open_in *p = support_fuse_cast (OPEN, inh);
+            TEST_COMPARE (p->flags & O_ACCMODE, O_RDWR);
+            struct fuse_open_out out = { 0, };
+            support_fuse_reply (f, &out, sizeof (out));
+          }
+          break;
+        case FUSE_SETATTR:
+          {
+            TEST_COMPARE (inh->nodeid, 2);
+            struct fuse_setattr_in *p = support_fuse_cast (SETATTR, inh);
+            TEST_COMPARE (p->valid , FATTR_MODE);
+            TEST_COMPARE (p->mode, S_IFREG | expected_mode);
+            struct fuse_attr_out *out = support_fuse_prepare_attr (f);
+            out->attr.mode = S_IFREG | p->mode;
+            support_fuse_reply_prepared (f);
+          }
+          break;
+        case FUSE_FLUSH:
+          support_fuse_reply_empty (f);
+          break;
+        default:
+          support_fuse_reply_error (f, EIO);
+        }
+    }
+}
+
+/* Test all mode values with the specified extra bits.  */
+static void
+test_with_bits (int fd, unsigned int extra_bits)
+{
+  for (int do_mode = 0; do_mode <= 07777; ++do_mode)
+    {
+      expected_mode = do_mode;
+      TEST_COMPARE (fchmod (fd, extra_bits | do_mode), 0);
+    }
+}
+
+static int
+do_test (void)
+{
+  support_fuse_init ();
+
+  struct support_fuse *f = support_fuse_mount (fuse_thread, NULL);
+  char *path = xasprintf ("%s/file", support_fuse_mountpoint (f));
+  int fd = xopen (path, O_RDWR, 0600);
+  free (path);
+
+  test_with_bits (fd, 0);
+  /* POSIX requires that the extra bits are ignored.  */
+  test_with_bits (fd, S_IFREG);
+  test_with_bits (fd, S_IFDIR);
+  test_with_bits (fd, ~07777);
+
+  xclose (fd);
+  support_fuse_unmount (f);
+
+  return 0;
+}
+
+#include <support/test-driver.c>