Add a sysfs container

However, do not try to change memory cgroups from it; this needs to be in `init`.

Signed-off-by: Justin Cormack <justin.cormack@docker.com>

pkg/sysfs/main.go

@@ -0,0 +1,72 @@
+package main
+
+import (
+	"bytes"
+	"flag"
+	"fmt"
+	"io/ioutil"
+	"log"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+var (
+	configDir string
+	sysfsDir  string
+)
+
+func init() {
+	flag.StringVar(&configDir, "configDir", "/etc/sysfs.d", "directory with config files")
+	flag.StringVar(&sysfsDir, "sysfsDir", "/sys", "mount point for sysfs")
+}
+
+func sysfs(line []byte) error {
+	// parse line into a string of expected form X/Y/Z=VALUE
+	sysfsLineKV := strings.Split(string(line[:]), "=")
+	if len(sysfsLineKV) != 2 {
+		if len(sysfsLineKV) >= 1 && len(sysfsLineKV[0]) >= 1 && strings.Trim(sysfsLineKV[0], " ")[:1] == "#" {
+			return nil
+		}
+		return fmt.Errorf("Cannot parse %s", string(line))
+	}
+	// trim any extra whitespace
+	sysfsSetting, sysfsValue := strings.Trim(sysfsLineKV[0], " "), strings.Trim(sysfsLineKV[1], " ")
+	sysfsFile := filepath.Join(sysfsDir, sysfsSetting)
+	file, err := os.OpenFile(sysfsFile, os.O_WRONLY, 0)
+	if err != nil {
+		return fmt.Errorf("Cannot open %s: %s", sysfsFile, err)
+	}
+	defer file.Close()
+	_, err = file.Write([]byte(sysfsValue))
+	if err != nil {
+		return fmt.Errorf("Cannot write to %s: %s", sysfsFile, err)
+	}
+	return nil
+}
+
+func main() {
+	flag.Parse()
+
+	files, err := ioutil.ReadDir(configDir)
+	if err != nil {
+		log.Fatalf("Cannot read directory %s: %s", configDir, err)
+	}
+
+	for _, file := range files {
+		contents, err := ioutil.ReadFile(filepath.Join(configDir, file.Name()))
+		if err != nil {
+			log.Fatalf("Cannot read file %s: %s", file.Name(), err)
+		}
+		lines := bytes.Split(contents, []byte("\n"))
+		for _, line := range lines {
+			if len(line) == 0 {
+				continue
+			}
+			err = sysfs(line)
+			if err != nil {
+				log.Fatal(err)
+			}
+		}
+	}
+}