dd: add a flag to discard cached data

* src/dd.c (FFS_MASK): A new macro (Find First Set) refactored
from the following enum as it's now used twice.
(usage): Mention the new 'nocache' flag.
(cache_round): A new function to help ignore requests
to drop cache, that are less than page_size.
(invalidate_cache): A new function to call posix_fadvise()
with the appropriate offset and length.  Note we don't
use fdadvise() so we can detect errors when count=0.
(dd_copy): Call invalidate_cache() for the portions read.
(iwrite): Likewise for the portions written.
(main): Call invalidate_cache for page_size slop or
for full file when count=0.
* cfg.mk (sc_dd_O_FLAGS): Adjust to pass.
* doc/coreutils.texi (dd invocation): Describe the 'nocache' flag,
and give some examples of how it can be used.
* tests/dd/nocache: A new test.
* tests/Makefile.am: Reference the new test.
* NEWS: Mention the new feature.

1 files changed, 58 insertions, 0 deletions

diff --git a/tests/dd/nocache b/tests/dd/nocache
new file mode 100755
index 000000000..b0d4cc107
--- /dev/null
+++ b/tests/dd/nocache
@@ -0,0 +1,58 @@
+#!/bin/sh
+# Ensure dd handles the 'nocache' flag
+
+# Copyright (C) 2011 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/>.
+
+. "${srcdir=.}/init.sh"; path_prepend_ ../src
+print_ver_ dd
+
+# This should not call posix_fadvise
+dd iflag=nocache oflag=nocache if=/dev/null of=/dev/null || fail=1
+
+# We should get an error for trying to process a pipe
+dd iflag=nocache count=0 && fail=1
+
+# O_DIRECT is orthogonal to drop cache so mutually exclusive
+dd iflag=nocache,direct if=/dev/null && fail=1
+
+# The rest ensure that the documented uses cases
+# proceed without error
+for f in ifile ofile; do
+  dd if=/dev/zero of=$f conv=fdatasync count=100 || framework_failure
+done
+
+# Advise to drop cache for whole file
+if ! dd if=ifile iflag=nocache count=0 2>err; then
+  if grep -F 'Operation not supported' err >/dev/null; then
+     warn_ 'skipping part; this file system lacks support for posix_fadvise()'
+     skip=1
+  else
+    fail=1
+  fi
+fi
+
+if test "$skip" != 1; then
+  # Ensure drop cache for whole file
+  dd of=ofile oflag=nocache conv=notrunc,fdatasync count=0 || fail=1
+
+  # Drop cache for part of file
+  dd if=ifile iflag=nocache skip=10 count=10 of=/dev/null || fail=1
+
+  # Stream data just using readahead cache
+  dd if=ifile of=ofile iflag=nocache oflag=nocache || fail=1
+fi
+
+Exit $fail