libio: Flush stream at freopen (BZ#21037)

As POSIX states [1] a freopen call should first flush the stream as if by a
call fflush.  C99 (n1256) and C11 (n1570) only states the function should
first close any file associated with the specific stream.  Although current
implementation only follow C specification, current BSD and other libc
implementation (musl) are in sync with POSIX and fflush the stream.

This patch change freopen{64} to fflush the stream before actually reopening
it (or returning if the stream does not support reopen).  It also changes the
Linux implementation to avoid a dynamic allocation on 'fd_to_filename'.

Checked on x86_64-linux-gnu.

	[BZ #21037]
	* libio/Makefile (tests): Add tst-memstream4 and tst-wmemstream4.
	* libio/freopen.c (freopen): Sync stream before reopen and adjust to
	new fd_to_filename interface.
	* libio/freopen64.c (freopen64): Likewise.
	* libio/tst-memstream.h: New file.
	* libio/tst-memstream4.c: Likewise.
	* libio/tst-wmemstream4.c: Likewise.
	* sysdeps/generic/fd_to_filename.h (fd_to_filename): Change signature.
	* sysdeps/unix/sysv/linux/fd_to_filename.h (fd_to_filename): Likewise
	and remove internal dynamic allocation.

[1] http://pubs.opengroup.org/onlinepubs/9699919799/

1 files changed, 64 insertions, 0 deletions

diff --git a/libio/tst-memstream4.c b/libio/tst-memstream4.c
new file mode 100644
index 0000000000..a83a8bfed4
--- /dev/null
+++ b/libio/tst-memstream4.c
@@ -0,0 +1,64 @@
+/* Test for open_memstream BZ #21037.
+   Copyright (C) 2018 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/>.  */
+
+#include "tst-memstream.h"
+
+static void
+mcheck_abort (enum mcheck_status ev)
+{
+  printf ("mecheck failed with status %d\n", (int) ev);
+  exit (1);
+}
+
+static int
+do_test (void)
+{
+  mcheck_pedantic (mcheck_abort);
+
+  /* Check if freopen proper fflush the stream.  */
+  {
+    CHAR_T old[] = W("old");
+    CHAR_T *buf = old;
+    size_t size;
+
+    FILE *fp = OPEN_MEMSTREAM (&buf, &size);
+    TEST_VERIFY_EXIT (fp != NULL);
+
+    FPUTS (W("new"), fp);
+    /* The stream buffer pointer should be updated with only a fflush or
+       fclose.  */
+    TEST_COMPARE (STRCMP (buf, old), 0);
+
+    /* The old stream should be fflush the stream, even for an invalid
+       streams.  */
+    FILE *nfp = freopen ("invalid-file", "r", fp);
+    TEST_VERIFY_EXIT (nfp == NULL);
+
+    TEST_VERIFY (STRCMP (buf, W("new")) == 0);
+    TEST_COMPARE_BLOB (buf, STRLEN (buf) * sizeof (CHAR_T),
+		       W("new"), STRLEN (W("new")) * sizeof (CHAR_T));
+
+    TEST_COMPARE (fclose (fp), 0);
+
+    free (buf);
+  }
+
+  return 0;
+}
+
+#include <support/test-driver.c>