Use SEEK_HOLE for hole detection

Based on patch by Pavel Raiskup.

Use SEEK_HOLE/SEEK_DATA feature of lseek on systems that support
it.  This can make archiving of sparse files much faster.

Implement the --hole-detection option to allow users to select
hole-detection method.

* src/common.h (hole_detection_method): New enum.
(hole_detection): New global.
* src/sparse.c  (sparse_scan_file_wholesparse): New function as a
method for detecting sparse files without any data.
(sparse_scan_file_raw): Rename from sparse_scan_file; with edits.
(sparse_scan_file_seek): New function.
(sparse_scan_file): Reimplement function.
* src/tar.c: New option --hole-detection

* tests/checkseekhole.c: New file.
* tests/.gitignore: Mention two test binaries.
* tests/Makefile.am: Add new tests.
* tests/testsuite.at (AT_SEEKHOLE_PREREQ): New macro.
Include sparse06.at.
* tests/sparse06.at: New test case.
* tests/sparse02.at: Force raw hole-detection method.
* tests/sparsemv.at: Likewise.
* tests/sparsemvp.at: Likewise.

* doc/tar.1: Document --hole-detection option.
* doc/tar.texi: Document hole-detection algorithms and
command-line options.
* NEWS: Document hole-detection.

tests/sparse06.at

@@ -0,0 +1,56 @@
+# Process this file with autom4te to create testsuite. -*- Autotest -*-
+#
+# Test suite for GNU tar.
+# Copyright 2014 Free Software Foundation, Inc.
+
+# This file is part of GNU tar.
+
+# GNU tar 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.
+
+# GNU tar 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/>.
+
+AT_SETUP([storing sparse file using seek method])
+AT_KEYWORDS([sparse sparse06])
+
+m4_define([check_pattern],[
+rm -rf out archive.tar smallsparse && mkdir out
+genfile --sparse --file smallsparse $1
+tar -cSf archive.tar smallsparse
+tar -xf archive.tar -C out
+cmp smallsparse out/smallsparse
+])
+
+AT_TAR_CHECK([
+AT_SEEKHOLE_PREREQ
+AT_TIMEOUT_PREREQ
+
+TAR_OPTIONS="$TAR_OPTIONS --hole-detection=seek"
+genfile --sparse --file bigsparse 0 ABC 8G DEF
+timeout 2 tar -cSf a bigsparse
+test $? -eq 0 || exit 1
+
+check_pattern([0 ABC])
+check_pattern([0 ABC 10M])
+check_pattern([0 ABC 10M DEF])
+
+check_pattern([10M])
+check_pattern([10M ABC])
+check_pattern([10M ABC 20M])
+
+check_pattern([10M DEF 20M GHI 30M JKL 40M])
+
+],
+[0],,
+[genfile: created file is not sparse
+],,,[posix])
+
+AT_CLEANUP