[BZ 1190] Make EOF sticky in stdio.

C99 specifies that the EOF condition on a file is "sticky": once EOF
has been encountered, all subsequent reads should continue to return
EOF until the file is closed or something clears the "end-of-file
indicator" (e.g. fseek, clearerr).  This is arguably a change from
C89, where the wording was ambiguous; the BSDs always had sticky EOF,
but the System V lineage would attempt to read from the underlying fd
again.  GNU libc has followed System V for as long as we've been
using libio, but nowadays C99 conformance and BSD compatibility are
more important than System V compatibility.

You might wonder if changing the _underflow impls is sufficient to
apply the C99 semantics to all of the many stdio functions that
perform input.  It should be enough to cover all paths to _IO_SYSREAD,
and the only other functions that call _IO_SYSREAD are the _seekoff
impls, which is OK because seeking clears EOF, and the _xsgetn impls,
which, as far as I can tell, are unused within glibc.

The test programs in this patch use a pseudoterminal to set up the
necessary conditions.  To facilitate this I added a new test-support
function that sets up a pair of pty file descriptors for you; it's
almost the same as BSD openpty, the only differences are that it
allocates the optionally-returned tty pathname with malloc, and that
it crashes if anything goes wrong.

	[BZ #1190]
        [BZ #19476]
	* libio/fileops.c (_IO_new_file_underflow): Return EOF immediately
	if the _IO_EOF_SEEN bit is already set; update commentary.
	* libio/oldfileops.c (_IO_old_file_underflow): Likewise.
	* libio/wfileops.c (_IO_wfile_underflow): Likewise.

	* support/support_openpty.c, support/tty.h: New files.
	* support/Makefile (libsupport-routines): Add support_openpty.

	* libio/tst-fgetc-after-eof.c, wcsmbs/test-fgetwc-after-eof.c:
	New test cases.
	* libio/Makefile (tests): Add tst-fgetc-after-eof.
	* wcsmbs/Makefile (tests): Add tst-fgetwc-after-eof.

1 files changed, 45 insertions, 0 deletions

diff --git a/support/tty.h b/support/tty.h
new file mode 100644
index 0000000000..1d37c42279
--- /dev/null
+++ b/support/tty.h
@@ -0,0 +1,45 @@
+/* Support functions related to (pseudo)terminals.
+   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/>.  */
+
+#ifndef _SUPPORT_TTY_H
+#define _SUPPORT_TTY_H 1
+
+struct termios;
+struct winsize;
+
+/** Open a pseudoterminal pair.  The outer fd is written to the address
+    A_OUTER and the inner fd to A_INNER.
+
+    If A_NAME is not NULL, it will be set to point to a string naming
+    the /dev/pts/NNN device corresponding to the inner fd; space for
+    this string is allocated with malloc and should be freed by the
+    caller when no longer needed.  (This is different from the libutil
+    function 'openpty'.)
+
+    If TERMP is not NULL, the terminal parameters will be initialized
+    according to the termios structure it points to.
+
+    If WINP is not NULL, the terminal window size will be set
+    accordingly.
+
+    Terminates the process on failure (like xmalloc).  */
+extern void support_openpty (int *a_outer, int *a_inner, char **a_name,
+                             const struct termios *termp,
+                             const struct winsize *winp);
+
+#endif