Disable single thread optimization for open_memstream

Single thread optimization is valid if at thread creation time the
optimization can be disabled.  This is in principle true for all
stream objects that user code can access (and thus needs locking),
using the same internal list as fflush(0) uses.  However in glibc
open_memstream is not on that list (BZ 21735) so the optimization
has to be disabled.

	* libio/memstream.c (__open_memstream): Set _IO_FLAGS2_NEED_LOCK.
	* libio/wmemstream.c (open_wmemstream): Likewise.
	* nptl/tst-memstream.c: New.

1 files changed, 101 insertions, 0 deletions

diff --git a/nptl/tst-memstream.c b/nptl/tst-memstream.c
new file mode 100644
index 0000000000..c88f708a50
--- /dev/null
+++ b/nptl/tst-memstream.c
@@ -0,0 +1,101 @@
+/* Test for open_memstream locking.
+   Copyright (C) 2017 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
+   <http://www.gnu.org/licenses/>.  */
+
+/* This test checks if concurrent writes to a FILE created with
+   open_memstream are correctly interleaved without loss or corruption
+   of data.  Large number of concurrent fputc operations are used
+   and in the end the bytes written to the memstream buffer are
+   counted to see if they all got recorded.
+
+   This is a regression test to see if the single threaded stdio
+   optimization broke multi-threaded open_memstream usage.  */
+
+#include <pthread.h>
+#include <stdio.h>
+#include <support/check.h>
+#include <support/xthread.h>
+
+enum
+{
+  thread_count = 2,
+  byte_count = 1000000,
+};
+
+struct closure
+{
+  FILE *fp;
+  char b;
+};
+
+static void *
+thread_func (void *closure)
+{
+  struct closure *args = closure;
+
+  for (int i = 0; i < byte_count; ++i)
+    fputc (args->b, args->fp);
+
+  return NULL;
+}
+
+int
+do_test (void)
+{
+  char *buffer = NULL;
+  size_t buffer_length = 0;
+  FILE *fp = open_memstream (&buffer, &buffer_length);
+  if (fp == NULL)
+    FAIL_RET ("open_memstream: %m");
+
+  pthread_t threads[thread_count];
+  struct closure args[thread_count];
+
+  for (int i = 0; i < thread_count; ++i)
+    {
+      args[i].fp = fp;
+      args[i].b = 'A' + i;
+      threads[i] = xpthread_create (NULL, thread_func, args + i);
+    }
+
+  for (int i = 0; i < thread_count; ++i)
+    xpthread_join (threads[i]);
+
+  fclose (fp);
+
+  if (buffer_length != thread_count * byte_count)
+    FAIL_RET ("unexpected number of written bytes: %zu (should be %d)",
+	      buffer_length, thread_count * byte_count);
+
+  /* Verify that each thread written its unique character byte_count times.  */
+  size_t counts[thread_count] = { 0, };
+  for (size_t i = 0; i < buffer_length; ++i)
+    {
+      if (buffer[i] < 'A' || buffer[i] >= 'A' + thread_count)
+	FAIL_RET ("written byte at %zu out of range: %d", i, buffer[i] & 0xFF);
+      ++counts[buffer[i] - 'A'];
+    }
+  for (int i = 0; i < thread_count; ++i)
+    if (counts[i] != byte_count)
+      FAIL_RET ("incorrect write count for thread %d: %zu (should be %d)", i,
+		counts[i], byte_count);
+
+  return 0;
+}
+
+#define TIMEOUT 100
+#include <support/test-driver.c>