seq: detect and report I/O errors immediately

Ensure I/O errors are detected (and terminate seq), preventing seq
from infloop (or running for long time with a large
range) upon write errors or ignored SIGPIPE. Examples:

     seq 1 inf > /dev/full             (seq_fast)
     seq 1.1 0.1 inf >/dev/full        (print_numbers)

* src/seq.c (io_error): A new function to diagnose appropriate
stdio errors and exit the program with failure status.
(seq_fast, print_numbers): Explicitly check for write errors
and terminate the program with diagnostic.
* tests/misc/seq-io-errors.sh: Test error detection with /dev/full.
* tests/misc/seq-epipe.sh: Test error detection with broken pipes.
* tests/local.mk: Add new tests.
* NEWS: Mention the fix.

tests/misc/seq-io-errors.sh

@@ -0,0 +1,46 @@
+#!/bin/sh
+# Test for proper detection of I/O errors in seq
+
+# Copyright (C) 2016 Free Software Foundation, Inc.
+
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+
+# This program 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 General Public License for more details.
+
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+. "${srcdir=.}/tests/init.sh"; path_prepend_ ./src
+print_ver_ seq
+
+if ! test -w /dev/full || ! test -c /dev/full; then
+  skip_ '/dev/full is required'
+fi
+
+# Run 'seq' with a timeout, preventing infinite-loop run.
+# expected returned codes:
+#  1     - seq detected an I/O error and exited with an error.
+#  124   - timed-out (seq likely infloop)
+#  other - unexpected error
+timed_seq_fail() { timeout 10 seq "$@" >/dev/full 2>/dev/null; }
+
+
+# Test infinite sequence, using fast-path method (seq_fast).
+returns_ 1 timed_seq_fail 1 inf || fail=1
+
+# Test infinite sequence, using slow-path method (print_numbers).
+returns_ 1 timed_seq_fail 1.1 .1 inf || fail=1
+
+# Test non-infinite sequence, using slow-path method (print_numbers).
+# (despite being non-infinite, the entire sequence should take long time to
+#  print. Thus, either an I/O error is detected immediately, or seq will
+#  timeout).
+returns_ 1 timed_seq_fail 1 0.0001 99999999 || fail=1
+
+Exit $fail