dd: use more robust SIGUSR1 handling

* src/dd.c (ifd_reopen): A new wrapper to ensure we
don't exit upon receiving a SIGUSR1 in a blocking open()
on a fifo for example.
(iftruncate): Likewise for ftruncate().
(iread): Process signals also after a short read.
(install_signal_handlers): Install SIGINFO/SIGUSR1 handler
even if set to SIG_IGN, as this is what the parent can easily
set from a shell script that can send SIGUSR1 without the
possiblity of inadvertently killing the dd process.
* doc/coreutils.texi (dd invocation): Improve the example to
show robust usage wrt signal races and short reads.
* tests/dd/stats.sh: A new test for various signal races.
* tests/local.mk: Reference the new test.
* NEWS: Mention the fix.

1 files changed, 57 insertions, 0 deletions

diff --git a/tests/dd/stats.sh b/tests/dd/stats.sh
new file mode 100755
index 000000000..386752e77
--- /dev/null
+++ b/tests/dd/stats.sh
@@ -0,0 +1,57 @@
+#!/bin/sh
+# Check robust handling of SIG{INFO,USR1}
+
+# Copyright (C) 2014 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/>.
+
+. "${srcdir=.}/tests/init.sh"; path_prepend_ ./src
+print_ver_ dd
+
+env kill -l | grep '^INFO$' && SIGINFO='INFO' || SIGINFO='USR1'
+
+# This to avoid races in the USR1 case
+# as the dd process will terminate by default until
+# it has its handler enabled.
+trap '' $SIGINFO
+
+mkfifo_or_skip_ fifo
+
+for open in '' '1'; do
+  # Run dd with the fullblock iflag to avoid short reads
+  # which can be triggered by reception of signals
+  dd iflag=fullblock if=/dev/zero of=fifo count=100 bs=5000000 2>err & pid=$!
+
+  # Note if we sleep here we give dd a chance to exec and block on open.
+  # Otherwise we're probably testing SIG_IGN in the forked shell or early dd.
+  test "$open" && sleep .1
+
+  # dd will block on open until fifo is opened for reading.
+  # Timeout in case dd goes away erroneously which we check for below.
+  timeout 10 sh -c 'wc -c < fifo > nwritten' &
+
+  # Send lots of signals immediately to ensure dd not killed due
+  # to race setting handler, or blocking on open of fifo.
+  # Many signals also check that short reads are handled.
+  until ! kill -s $SIGINFO $pid 2>/dev/null; do
+    sleep .01
+  done
+
+  wait
+
+  # Ensure all data processed and at least last status written
+  grep '500000000 bytes .* copied' err || { cat err; fail=1; }
+done
+
+Exit $fail