service: log stdout and stderr to separate files

Previously we would pass the path `/var/log/service.log` for both
stdout and stderr to containerd. containerd would construct a dict
with the paths as keys[1] and, due to the duplicate key, would only
open one of the files and start one `io.Copy` instance. Writes to
the other stream would be buffered by the pipe connected to
containerd-shim and would eventually block.

If we modified containerd to open the file twice and start 2
`io.Copy` instances, we would end up with the two streams interleaved
together. It seems cleaner to keep the streams separate; therefore
this patch logs stdout to `/var/log/service.out.log` and stderr to
`/var/log/service.err.log`.

[1]
49437711c3/linux/shim/io.go (L51)

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

pkg/init/cmd/service/start.go

@@ -121,17 +121,21 @@ func start(service, sock, basePath, dumpSpec string) (string, uint32, string, er
 	}
 
 	io := func(id string) (containerd.IO, error) {
-		logfile := filepath.Join("/var/log", service+".log")
+		stdoutFile := filepath.Join("/var/log", service+".out.log")
-		// We just need this to exist.
+		stderrFile := filepath.Join("/var/log", service+".err.log")
-		if err := ioutil.WriteFile(logfile, []byte{}, 0600); err != nil {
+		// We just need this to exist. If we cannot write to the directory,
-			// if we cannot write to log, discard output
+		// we'll discard output instead.
-			logfile = "/dev/null"
+		if err := ioutil.WriteFile(stdoutFile, []byte{}, 0600); err != nil {
+			stdoutFile = "/dev/null"
+		}
+		if err := ioutil.WriteFile(stderrFile, []byte{}, 0600); err != nil {
+			stderrFile = "/dev/null"
 		}
 		return &cio{
 			containerd.IOConfig{
 				Stdin:    "/dev/null",
-				Stdout:   logfile,
+				Stdout:   stdoutFile,
-				Stderr:   logfile,
+				Stderr:   stderrFile,
 				Terminal: false,
 			},
 		}, nil