conmon: implement logging to logPath

This adds a very simple implementation of logging within conmon, where
every buffer read from the masterfd of the container is also written to
the log file (with errors during writing to the log file ignored).

Signed-off-by: Aleksa Sarai <asarai@suse.de>

server/container_create.go

@@ -332,6 +332,22 @@ func (s *Server) createSandboxContainer(ctx context.Context, containerID string,
 	}
 
 	logPath := containerConfig.LogPath
+	if logPath == "" {
+		// TODO: Should we use sandboxConfig.GetLogDirectory() here?
+		logPath = filepath.Join(sb.logDir, containerID+".log")
+	}
+	if !filepath.IsAbs(logPath) {
+		// XXX: It's not really clear what this should be versus the sbox logDirectory.
+		logrus.Warnf("requested logPath for ctr id %s is a relative path: %s", containerID, logPath)
+		logPath = filepath.Join(sb.logDir, logPath)
+	}
+
+	logrus.WithFields(logrus.Fields{
+		"sbox.logdir": sb.logDir,
+		"ctr.logfile": containerConfig.LogPath,
+		"log_path":    logPath,
+	}).Debugf("setting container's log_path")
+
 	specgen.SetProcessTerminal(containerConfig.Tty)
 
 	linux := containerConfig.GetLinux()