Add pkg/logwrite which writes and rotates log files

This process connects to memlogd and streams logs to individual files,
one per log. It keeps track of how many bytes have been written to each
file and rotates when the file size exceeds a defined threshold.

By default the maximum size of each file before rotation is 1MiB and
we keep up to 10 files per log.

Signed-off-by: David Scott <dave.scott@docker.com>

test/cases/040_packages/030_logwrite/check.sh

@@ -0,0 +1,14 @@
+#!/bin/sh
+
+for i in $(seq 1 20); do
+	if [ -e /var/log/fill-the-logs.out.0 ]; then
+		printf "logwrite test suite PASSED\n" > /dev/console
+		/sbin/poweroff -f
+	fi
+	sleep 1
+done
+
+printf "logwrite test suite FAILED\n" > /dev/console
+echo "contents of /var/log:" > /dev/console
+ls -l /var/log > /dev/console
+/sbin/poweroff -f

test/cases/040_packages/030_logwrite/test.sh

@@ -0,0 +1,24 @@
+#!/bin/sh
+# SUMMARY: Check that the logwrite package works
+# LABELS:
+# REPEAT:
+
+set -e
+
+# Source libraries. Uncomment if needed/defined
+#. "${RT_LIB}"
+. "${RT_PROJECT_ROOT}/_lib/lib.sh"
+NAME=logwrite
+
+clean_up() {
+	rm -rf ${NAME}-*
+}
+trap clean_up EXIT
+
+# Test code goes here
+linuxkit build -disable-content-trust -format kernel+initrd -name "${NAME}" test.yml
+RESULT="$(linuxkit run ${NAME})"
+echo "${RESULT}"
+echo "${RESULT}" | grep -q "suite PASSED"
+
+exit 0

test/cases/040_packages/030_logwrite/test.yml

@@ -0,0 +1,35 @@
+kernel:
+  image: linuxkit/kernel:4.14.53
+  cmdline: "console=ttyS0 console=ttyAMA0"
+init:
+  - linuxkit/init:6cc1442112980c889230b6449df09d5b48de6854
+  - linuxkit/runc:v0.4
+  - linuxkit/containerd:f2bc1bda1ab18146967fa1a149800aaf14bee81b
+  - linuxkit/ca-certificates:v0.4
+  - linuxkit/memlogd:883f0d46e7d3ae2d787e8acb496da115a4707cbc
+services:
+# A service which generates logs of log messages
+  - name: fill-the-logs
+    image: alpine
+    command: ["/bin/sh", "-c", "while /bin/true; do echo hello $(date); done" ]
+  - name: write-and-rotate-logs
+    image: linuxkit/logwrite:7859c102a963828fd9c5aa3837db9600483220c7
+    command: ["/usr/bin/logwrite", "-max-log-size", "1024"]
+  - name: check-the-logs
+    image: alpine:3.8
+    binds:
+      - /check.sh:/check.sh
+      - /dev/console:/dev/console
+      - /var/log:/var/log
+    command: ["sh", "./check.sh"]
+    pid: host
+    capabilities:
+      - CAP_SYS_BOOT
+files:
+  - path: check.sh
+    source: ./check.sh
+trust:
+  org:
+    - linuxkit
+  image:
+    - alpine:3.8