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.

NEWS

@@ -15,6 +15,9 @@ GNU coreutils NEWS                                    -*- outline -*-
    stty --help no longer outputs extraneous gettext header lines
    for translated languages. [bug introduced in coreutils-8.24]
 
+   seq now immediately exits upon write errors.
+   [This bug was present in "the beginning".]
+
 ** Changes in behavior
 
    seq no longer accepts 0 value as increment, and now also rejects NaN

src/seq.c

@@ -278,6 +278,15 @@ long_double_format (char const *fmt, struct layout *layout)
       }
 }
 
+static void ATTRIBUTE_NORETURN
+io_error (void)
+{
+  /* FIXME: consider option to silently ignore errno=EPIPE */
+  error (0, errno, _("standard output"));
+  clearerr (stdout);
+  exit (EXIT_FAILURE);
+}
+
 /* Actually print the sequence of numbers in the specified range, with the
    given or default stepping and format.  */
 
@@ -295,7 +304,8 @@ print_numbers (char const *fmt, struct layout layout,
       for (i = 1; ; i++)
         {
           long double x0 = x;
-          printf (fmt, x);
+          if (printf (fmt, x) < 0)
+            io_error ();
           if (out_of_range)
             break;
           x = first + i * step;
@@ -336,10 +346,12 @@ print_numbers (char const *fmt, struct layout layout,
                 break;
             }
 
-          fputs (separator, stdout);
+          if (fputs (separator, stdout) == EOF)
+            io_error ();
         }
 
-      fputs (terminator, stdout);
+      if (fputs (terminator, stdout) == EOF)
+        io_error ();
     }
 }
 
@@ -506,14 +518,16 @@ seq_fast (char const *a, char const *b)
              output buffer so far, and reset to start of buffer.  */
           if (buf_end - (p_len + 1) < bufp)
             {
-              fwrite (buf, bufp - buf, 1, stdout);
+              if (fwrite (buf, bufp - buf, 1, stdout) != 1)
+                io_error ();
               bufp = buf;
             }
         }
 
       /* Write any remaining buffered output, and the terminator.  */
       *bufp++ = *terminator;
-      fwrite (buf, bufp - buf, 1, stdout);
+      if (fwrite (buf, bufp - buf, 1, stdout) != 1)
+        io_error ();
 
       IF_LINT (free (buf));
     }

tests/local.mk

@@ -235,6 +235,8 @@ all_tests =					\
   tests/misc/ptx.pl				\
   tests/misc/test.pl				\
   tests/misc/seq.pl				\
+  tests/misc/seq-epipe.sh			\
+  tests/misc/seq-io-errors.sh			\
   tests/misc/seq-long-double.sh			\
   tests/misc/seq-precision.sh			\
   tests/misc/head.pl				\

tests/misc/seq-epipe.sh

@@ -0,0 +1,46 @@
+#!/bin/sh
+# Test for proper detection of EPIPE with ignored SIGPIPE
+
+# 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
+
+# upon EPIPE with signals ignored, 'seq' should exit with an error.
+(trap '' PIPE;
+ timeout 10 sh -c '( (seq inf 2>err ; echo $?>code) | head -n1)'>/dev/null)
+
+# Exit-code must be 1, indicating 'write error'
+cat << \EOF > exp || framework_failure_
+1
+EOF
+if test -e code ; then
+  compare exp code || fail=1
+else
+  # 'exitcode' file was not created
+  warn_ "missing exit code file (seq failed to detect EPIPE?)"
+  fail=1
+fi
+
+# The error message must begin with "standard output:"
+# (but don't hard-code the strerror text)
+compare_dev_null_ /dev/null err
+if ! grep -qE '^seq: standard output: .+$' err ; then
+  warn_ "seq emitted incorrect error on EPIPE"
+  fail=1
+fi
+
+Exit $fail

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