projects/logging: start memlogd from init.d

Previously memlogd would always run in the foreground. This patch
adds a `-daemonize` option which binds the /var/run sockets, forks
and execs itself and immediately returns. Therefore the program won't
block (important for an init.d script) but guarantees the sockets will
be available for any program started afterwards.

This also removes the alpine base from the memlogd image as `init`
"containers" are treated as simple file overlays.

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

projects/logging/examples/logging.yml

@@ -6,7 +6,7 @@ init:
   - linuxkit/runc:v0.4
   - linuxkit/containerd:27a4c84cc8fab2d5ea04342546ecd20453ec99b3
   - linuxkit/ca-certificates:v0.4
-  - linuxkit/memlogd:9b5834189f598f43c507f6938077113906f51012
+  - linuxkit/memlogd:4c16d6720075bcb2aeb298a8ee2d08d74aa20512
 onboot:
   - name: sysctl
     image: linuxkit/sysctl:v0.4

projects/logging/pkg/memlogd/Dockerfile

@@ -1,20 +1,3 @@
-FROM linuxkit/alpine:1b05307ae8152e3d38f79e297b0632697a30c65c AS mirror
-
-RUN mkdir -p /out/etc/apk && cp -r /etc/apk/* /out/etc/apk/
-RUN apk add --no-cache --initdb -p /out \
-    alpine-baselayout \
-    busybox \
-    e2fsprogs \
-    e2fsprogs-extra \
-    btrfs-progs \
-    xfsprogs \
-    xfsprogs-extra \
-    musl \
-    sfdisk \
-    util-linux \
-    && true
-RUN rm -rf /out/etc/apk /out/lib/apk /out/var/cache
-
 FROM linuxkit/alpine:1b05307ae8152e3d38f79e297b0632697a30c65c AS build
 
 RUN apk add --no-cache go musl-dev
@@ -29,8 +12,8 @@ FROM scratch
 ENTRYPOINT []
 CMD []
 WORKDIR /
-COPY --from=mirror /out/ /
 COPY --from=build /go/bin/memlogd usr/bin/memlogd
 COPY --from=build /go/bin/logread usr/bin/logread
 COPY --from=build /go/bin/logwrite usr/bin/logwrite
-CMD ["/usr/bin/memlogd"]
+# We'll start from init.d
+COPY etc/ /etc/

projects/logging/pkg/memlogd/cmd/memlogd/main.go

@@ -11,6 +11,7 @@ import (
 	"log"
 	"net"
 	"os"
+	"os/exec"
 	"sync"
 	"syscall"
 	"time"
@@ -254,6 +255,7 @@ func main() {
 	var passedLogFD int
 	var linesInBuffer int
 	var lineMaxLength int
+	var daemonize bool
 
 	flag.StringVar(&socketQueryPath, "socket-query", "/var/run/memlogdq.sock", "unix domain socket for responding to log queries. Overridden by -fd-query")
 	flag.StringVar(&socketLogPath, "socket-log", "/var/run/linuxkit-external-logging.sock", "unix domain socket to listen for new fds to add to log. Overridden by -fd-log")
@@ -261,7 +263,7 @@ func main() {
 	flag.IntVar(&passedQueryFD, "fd-query", -1, "an existing SOCK_STREAM for receiving log read requets. Overrides -socket-query.")
 	flag.IntVar(&linesInBuffer, "max-lines", 5000, "Number of log lines to keep in memory")
 	flag.IntVar(&lineMaxLength, "max-line-len", 1024, "Maximum line length recorded. Additional bytes are dropped.")
-
+	flag.BoolVar(&daemonize, "daemonize", false, "Bind sockets and then daemonize.")
 	flag.Parse()
 
 	var connLogFd *net.UnixConn
@@ -302,6 +304,28 @@ func main() {
 	}
 	defer connQuery.Close()
 
+	if daemonize {
+		child := exec.Command(os.Args[0],
+			"-fd-log", "3", // connLogFd in ExtraFiles below
+			"-fd-query", "4", // connQuery in ExtraFiles below
+			"-max-lines", fmt.Sprintf("%d", linesInBuffer),
+			"-max-line-len", fmt.Sprintf("%d", lineMaxLength),
+		)
+		connLogFile, err := connLogFd.File()
+		if err != nil {
+			log.Fatalf("The -fd-log cannot be represented as a *File: %s", err)
+		}
+		connQueryFile, err := connQuery.File()
+		if err != nil {
+			log.Fatalf("The -fd-query cannot be represented as a *File: %s", err)
+		}
+		child.ExtraFiles = append(child.ExtraFiles, connLogFile, connQueryFile)
+		if err := child.Start(); err != nil {
+			log.Fatalf("Failed to re-exec: %s", err)
+		}
+		os.Exit(0)
+	}
+
 	logCh := make(chan logEntry)
 	fdMsgChan := make(chan fdMessage)
 	queryMsgChan := make(chan queryMessage)

projects/logging/pkg/memlogd/etc/init.d/002-logging

@@ -0,0 +1,3 @@
+#!/bin/sh
+
+/usr/bin/memlogd -daemonize