Add regulator to GCS

Signed-off-by: Huu Nguyen <whoshuu@gmail.com>

registry/storage/driver/base/regulator.go

@@ -2,7 +2,10 @@ package base
 
 import (
 	"context"
+	"fmt"
 	"io"
+	"reflect"
+	"strconv"
 	"sync"
 
 	storagedriver "github.com/docker/distribution/registry/storage/driver"
@@ -15,6 +18,46 @@ type regulator struct {
 	available uint64
 }
 
+// GetLimitFromParameter takes an interface type as decoded from the YAML
+// configuration and returns a uint64 representing the maximum number of
+// concurrent calls given a minimum limit and default.
+//
+// If the parameter supplied is of an invalid type this returns an error.
+func GetLimitFromParameter(param interface{}, min, def uint64) (uint64, error) {
+	limit := def
+
+	switch v := param.(type) {
+	case string:
+		var err error
+		if limit, err = strconv.ParseUint(v, 0, 64); err != nil {
+			return limit, fmt.Errorf("parameter must be an integer, '%v' invalid", param)
+		}
+	case uint64:
+		limit = v
+	case int, int32, int64:
+		val := reflect.ValueOf(v).Convert(reflect.TypeOf(param)).Int()
+		// if param is negative casting to uint64 will wrap around and
+		// give you the hugest thread limit ever. Let's be sensible, here
+		if val > 0 {
+			limit = uint64(val)
+		} else {
+			limit = min
+		}
+	case uint, uint32:
+		limit = reflect.ValueOf(v).Convert(reflect.TypeOf(param)).Uint()
+	case nil:
+		// use the default
+	default:
+		return 0, fmt.Errorf("invalid value '%#v'", param)
+	}
+
+	if limit < min {
+		return min, nil
+	}
+
+	return limit, nil
+}
+
 // NewRegulator wraps the given driver and is used to regulate concurrent calls
 // to the given storage driver to a maximum of the given limit. This is useful
 // for storage drivers that would otherwise create an unbounded number of OS