On 12/11/17 22:21, Pádraig Brady wrote: > On 12/11/17 21:52, Paul Eggert wrote: >> Why doesn't lseek work for this? > > Good call, it probably would. > Something like the following is more acceptable > since it adds very little complexity:
Full patch attached with tests. cheers, Pádraig
>From 691b547c6ad68ee17cafe161b55138b962c3cd7a Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?P=C3=A1draig=20Brady?= <p...@draigbrady.com> Date: Sun, 12 Nov 2017 23:06:08 -0800 Subject: [PATCH] tail: seek to the end of block devices * src/tail.c (tail_bytes): Try lseek(..., SEEK_END) when we can't determine the file size. * tests/tail-2/end-of-device.sh: Add a new root only test. * tests/local.mk: Reference the new test. * NEWS: Mention the improvement. Paul Eggert suggested using lseek() (rather than ioctl(BLKGETSIZE64)). Fixes https://bugs.gnu.org/29259 --- NEWS | 5 +++++ src/tail.c | 27 +++++++++++++++--------- tests/local.mk | 1 + tests/tail-2/end-of-device.sh | 48 +++++++++++++++++++++++++++++++++++++++++++ 4 files changed, 71 insertions(+), 10 deletions(-) create mode 100755 tests/tail-2/end-of-device.sh diff --git a/NEWS b/NEWS index 3e6704d..bc5d5be 100644 --- a/NEWS +++ b/NEWS @@ -31,6 +31,11 @@ GNU coreutils NEWS -*- outline -*- timeout would have then waited for the time limit to expire. [bug introduced in coreutils-8.27] +** Improvements + + tail --bytes=NUM will efficiently seek to the end of block devices, + rather than reading from the start. + ** Build-related Default man pages are now distributed which are used if perl is diff --git a/src/tail.c b/src/tail.c index 1c7418d..c6de7f1 100644 --- a/src/tail.c +++ b/src/tail.c @@ -1830,12 +1830,11 @@ tail_bytes (const char *pretty_filename, int fd, uintmax_t n_bytes, if (from_start) { - if ( ! presume_input_pipe - && S_ISREG (stats.st_mode) && n_bytes <= OFF_T_MAX) - { - xlseek (fd, n_bytes, SEEK_CUR, pretty_filename); - *read_pos += n_bytes; - } + if (! presume_input_pipe && n_bytes <= OFF_T_MAX + && ((S_ISREG (stats.st_mode) + && xlseek (fd, n_bytes, SEEK_CUR, pretty_filename) >= 0) + || lseek (fd, n_bytes, SEEK_CUR) != -1)) + *read_pos += n_bytes; else { int t = start_bytes (pretty_filename, fd, n_bytes, read_pos); @@ -1846,12 +1845,20 @@ tail_bytes (const char *pretty_filename, int fd, uintmax_t n_bytes, } else { - off_t end_pos = ((! presume_input_pipe && usable_st_size (&stats) - && n_bytes <= OFF_T_MAX) - ? stats.st_size : -1); + off_t end_pos = -1; + off_t current_pos = -1; + + if (! presume_input_pipe && n_bytes <= OFF_T_MAX) + { + if (usable_st_size (&stats)) + end_pos = stats.st_size; + else if ((current_pos = lseek (fd, -n_bytes, SEEK_END)) != -1) + end_pos = current_pos + n_bytes; + } if (end_pos <= ST_BLKSIZE (stats)) return pipe_bytes (pretty_filename, fd, n_bytes, read_pos); - off_t current_pos = xlseek (fd, 0, SEEK_CUR, pretty_filename); + if (current_pos == -1) + current_pos = xlseek (fd, 0, SEEK_CUR, pretty_filename); if (current_pos < end_pos) { off_t bytes_remaining = end_pos - current_pos; diff --git a/tests/local.mk b/tests/local.mk index 74426f6..8ee7c50 100644 --- a/tests/local.mk +++ b/tests/local.mk @@ -135,6 +135,7 @@ all_root_tests = \ tests/rm/one-file-system.sh \ tests/rm/read-only.sh \ tests/tail-2/append-only.sh \ + tests/tail-2/end-of-device.sh \ tests/touch/now-owned-by-other.sh ALL_RECURSIVE_TARGETS += check-root diff --git a/tests/tail-2/end-of-device.sh b/tests/tail-2/end-of-device.sh new file mode 100755 index 0000000..9123c80 --- /dev/null +++ b/tests/tail-2/end-of-device.sh @@ -0,0 +1,48 @@ +#!/bin/sh +# Ensure that tail seeks to the end of a device + +# Copyright (C) 2017 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 <https://www.gnu.org/licenses/>. + +. "${srcdir=.}/tests/init.sh"; path_prepend_ ./src +print_ver_ tail + +# need write access to local device +# (even though we don't actually write anything) +require_root_ +require_local_dir_ + +get_device_size() { + BLOCKDEV=blockdev + $BLOCKDEV -V >/dev/null 2>&1 || BLOCKDEV=/sbin/blockdev + $BLOCKDEV --getsize64 "$1" +} + +# Get path to device the current dir is on. +# Note df can only get fs size, not device size. +device=$(df --output=source . | tail -n1) || framework_failure_ + +dev_size=$(get_device_size "$device") || + skip_ "failed to determine size of $device" + +tail_offset=$(expr $dev_size - 1023) || + skip_ "failed to determine tail offset" + +timeout 10 tail -c 1024 "$device" > end1 || fail=1 +timeout 10 tail -c +"$tail_offset" "$device" > end2 || fail=1 +test $(wc -c < end1) = 1024 || fail=1 +cmp end1 end2 || fail=1 + +Exit $fail -- 2.9.3
