Add a test for virt-tail.

cat/Makefile.am

@@ -27,6 +27,7 @@ EXTRA_DIST = \
 	virt-log.pod \
 	test-virt-ls.sh \
 	virt-ls.pod \
+	test-virt-tail.sh \
 	virt-tail.pod
 
 bin_PROGRAMS = virt-cat virt-filesystems virt-log virt-ls virt-tail
@@ -234,7 +235,8 @@ TESTS += \
 	test-virt-cat.sh \
 	test-virt-filesystems.sh \
 	test-virt-log.sh \
-	test-virt-ls.sh
+	test-virt-ls.sh \
+	test-virt-tail.sh
 endif ENABLE_APPLIANCE
 
 check-valgrind:

cat/test-virt-tail.sh

@@ -0,0 +1,116 @@
+#!/bin/bash -
+# libguestfs
+# Copyright (C) 2016 Red Hat 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 2 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, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+
+# To test virt-tail, we run a guestfish instance which creates a disk
+# and a file in that disk.  We then run virt-tail in parallel.  Back
+# in the guestfish instance we append to the file, and we check that
+# the addenda are displayed by virt-tail.
+
+export LANG=C
+set -e
+set -x
+
+# Libvirt screws with the SELinux labels, preventing guestfish from
+# continuing to write to the original disk.  Therefore only run this
+# test when using direct access.
+if [ "$(guestfish get-backend)" != "direct" ]; then
+    echo "$0: test skipped because default backend is not 'direct'"
+    exit 77
+fi
+
+out=test-virt-tail.out
+disk=test-virt-tail.disk
+
+rm -f $out $disk
+
+tailpid=0
+
+eval `guestfish --listen`
+
+# Clean up if the script is killed or exits early.
+cleanup ()
+{
+    status=$?
+    set +e
+    guestfish --remote exit
+    if [ "$tailpid" -gt 0 ]; then kill "$tailpid"; fi
+
+    # Don't delete the output files if non-zero exit.
+    if [ "$status" -eq 0 ]; then rm -f $disk $out; fi
+
+    exit $status
+}
+trap cleanup INT QUIT TERM EXIT ERR
+
+# Create the output disk.
+guestfish --remote sparse $disk 10M
+guestfish --remote run
+guestfish --remote part-disk /dev/sda mbr
+guestfish --remote mkfs ext2 /dev/sda1
+guestfish --remote mount /dev/sda1 /
+
+# Create the file to be tailed with a single full line of content.
+guestfish --remote write /tail 'line 1
+'
+guestfish --remote sync
+
+# Run virt-tail in the background
+$VG virt-tail -a $disk -m /dev/sda1 /tail > $out &
+tailpid=$!
+
+# Wait for the first line of the tailed file to appear.
+# Note we can wait up to 10 minutes here to deal with slow machines.
+for retry in `seq 0 60`; do
+    if grep -sq "line 1" $out; then break; fi
+    sleep 10;
+done
+if [ "$retry" -ge 60 ]; then
+    echo "$0: error: initial line of output did not appear"
+    exit 1
+fi
+
+# Write some more lines to the file.
+guestfish --remote write-append /tail 'line 2
+line 3
+'
+guestfish --remote sync
+
+# Wait for new content to appear.
+for retry in `seq 0 60`; do
+    if grep -sq "line 3" $out; then break; fi
+    sleep 10;
+done
+if [ "$retry" -ge 60 ]; then
+    echo "$0: error: continued output did not appear"
+    exit 1
+fi
+
+# Delete the file.  This should cause virt-tail to exit gracefully.
+guestfish --remote rm /tail
+
+# Wait for virt-tail to finish and check the status.
+wait "$tailpid"
+tailstatus=$?
+tailpid=0
+if [ "$tailstatus" -ne 0 ]; then
+    echo "$0: error: non-zero exit status from virt-tail: $tailstatus"
+    exit 1
+fi
+
+# cleanup() is called implicitly which cleans up everything.
+exit 0