stdio-common: Add tst-memstream-string for open_memstream overflow

This code path is exercised indirectly by some of the DNS stub
resolver tests, via their own use of xopen_memstream for constructing
strings describing result data.  The relative lack of test suite
coverage became apparent when these tests starting failing after a
printf changes uncovered bug 28949.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

stdio-common/tst-memstream-string.c

@@ -0,0 +1,77 @@
+/* Test writing differently sized strings to a memstream.
+   Copyright (C) 2022 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 <stdlib.h>
+#include <string.h>
+#include <support/check.h>
+#include <support/support.h>
+#include <support/xmemstream.h>
+
+/* Returns a printable ASCII character based on INDEX.   */
+static inline char
+char_from_index (unsigned int index)
+{
+  return ' ' + (index % 95);
+}
+
+enum { result_size = 25000 };
+
+static void
+run_one_size (unsigned int chunk_size)
+{
+  char *chunk = xmalloc (chunk_size + 1);
+
+  struct xmemstream mem;
+  xopen_memstream (&mem);
+  unsigned int written = 0;
+  for (unsigned int i = 0; i < result_size; )
+    {
+      unsigned int to_print = result_size - i;
+      if (to_print > chunk_size)
+        to_print = chunk_size;
+      for (unsigned int j = 0; j < to_print; ++j)
+        chunk[j] = char_from_index(i + j);
+      chunk[to_print] = '\0';
+      fprintf (mem.out, "%s", chunk); /* Needs -fno-builtin-fprintf.  */
+      i += to_print;
+      written += strlen(chunk);
+    }
+  xfclose_memstream (&mem);
+
+  TEST_COMPARE (written, result_size);
+  TEST_COMPARE (mem.length, result_size);
+  TEST_COMPARE (strlen (mem.buffer), result_size);
+
+  for (unsigned int i = 0; i < result_size; ++i)
+    TEST_COMPARE (mem.buffer[i], char_from_index (i));
+
+  free (mem.buffer);
+  free (chunk);
+}
+
+static int
+do_test (void)
+{
+  for (unsigned int chunk_size = 1; chunk_size <= 30; ++ chunk_size)
+    run_one_size (chunk_size);
+
+  return 0;
+}
+
+#include <support/test-driver.c>