Allow for larger preset property values, do not override

Signed-off-by: Jim Carroll <jim.carroll@docker.com>

drivers/overlay/ostweaks_linux.go

@@ -1,17 +1,41 @@
 package overlay
 
 import (
+	"fmt"
 	"io/ioutil"
 	"path"
+	"strconv"
 	"strings"
 
 	"github.com/sirupsen/logrus"
 )
 
-var sysctlConf = map[string]string{
-	"net.ipv4.neigh.default.gc_thresh1": "8192",
-	"net.ipv4.neigh.default.gc_thresh2": "49152",
-	"net.ipv4.neigh.default.gc_thresh3": "65536",
+type conditionalCheck func(val1, val2 string) bool
+
+type osValue struct {
+	value   string
+	checkFn conditionalCheck
+}
+
+var osConfig = map[string]osValue{
+	"net.ipv4.neigh.default.gc_thresh1": {"8192", checkHigher},
+	"net.ipv4.neigh.default.gc_thresh2": {"49152", checkHigher},
+	"net.ipv4.neigh.default.gc_thresh3": {"65536", checkHigher},
+}
+
+func newValIsHigher(val1, val2 string, check conditionalCheck) bool {
+	if check == nil || check(val1, val2) {
+		fmt.Println("Yep doing something")
+		return true
+	}
+	fmt.Println("Nope")
+	return false
+}
+
+func checkHigher(val1, val2 string) bool {
+	val1Int, _ := strconv.ParseInt(val1, 10, 32)
+	val2Int, _ := strconv.ParseInt(val2, 10, 32)
+	return val1Int < val2Int
 }
 
 // writeSystemProperty writes the value to a path under /proc/sys as determined from the key.
@@ -21,10 +45,28 @@ func writeSystemProperty(key, value string) error {
 	return ioutil.WriteFile(path.Join("/proc/sys", keyPath), []byte(value), 0644)
 }
 
+func readSystemProperty(key string) ([]byte, error) {
+	keyPath := strings.Replace(key, ".", "/", -1)
+	return ioutil.ReadFile(path.Join("/proc/sys", keyPath))
+}
+
 func applyOStweaks() {
-	for k, v := range sysctlConf {
-		if err := writeSystemProperty(k, v); err != nil {
-			logrus.Errorf("error setting the kernel parameter %s = %s, err: %s", k, v, err)
+	for k, v := range osConfig {
+		// read the existing property from disk
+		oldv, err := readSystemProperty(k)
+
+		if err != nil {
+			logrus.Errorf("error reading the kernel parameter %s = %s, err: %s", k, v, err)
+			continue
+		}
+
+		if newValIsHigher(string(oldv), v.value, v.checkFn) {
+			// write new prop value to disk
+			if err := writeSystemProperty(k, v.value); err != nil {
+				logrus.Errorf("error setting the kernel parameter %s = %s, err: %s", k, v.value, err)
+				continue
+			}
+			logrus.Debugf("updating kernel parameter %s = %s (was %s)", k, v.value, oldv)
 		}
 	}
 }