Add a new fwrite test for read-only streams

Ensure that fwrite() behaves correctly even when the stream is
read-only.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>

stdio-common/Makefile

@@ -223,6 +223,7 @@ tests := \
   tst-freopen64-3 \
   tst-fseek \
   tst-fwrite \
+  tst-fwrite-ro \
   tst-getline \
   tst-getline-enomem \
   tst-gets \

stdio-common/tst-fwrite-ro.c

@@ -0,0 +1,65 @@
+/* Test fwrite on a read-only stream.
+   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 <stdio.h>
+#include <support/check.h>
+#include <support/temp_file.h>
+#include <support/xstdio.h>
+#include <support/xunistd.h>
+
+/* A small buffer size is enough to run this test.  */
+#define BUFSIZE 4
+
+static int
+do_test (void)
+{
+  int fd;
+  FILE *f;
+  struct stat64 st;
+
+  /* Create a temporary file and open it in read-only mode.  */
+  fd = create_temp_file ("tst-fwrite-ro", NULL);
+  TEST_VERIFY_EXIT (fd != -1);
+  f = fdopen (fd, "r");
+  TEST_VERIFY_EXIT (f != NULL);
+
+  /* Try to write to the temporary file with nmemb = 0, then check that
+     fwrite returns 0.  No errors are expected from this.  */
+  TEST_COMPARE (fwrite ("a", 1, 0, f), 0);
+  TEST_COMPARE (ferror (f), 0);
+
+  /* Try to write to the temporary file with size = 0, then check that
+     fwrite returns 0.  No errors are expected from this.  */
+  TEST_COMPARE (fwrite ("a", 0, 1, f), 0);
+  TEST_COMPARE (ferror (f), 0);
+
+  /* Try to write a single byte to the temporary file, then check that
+     fwrite returns 0.  Check if an error was reported.  */
+  TEST_COMPARE (fwrite ("a", 1, 1, f), 0);
+  TEST_COMPARE (ferror (f), 1);
+  clearerr (f);
+
+  xfstat64 (fd, &st);
+  TEST_COMPARE (st.st_size, 0);
+
+  xfclose (f);
+
+  return 0;
+}
+
+#include <support/test-driver.c>