Implement regulator in filesystem driver

This commit refactors base.regulator into the 2.4 interfaces and adds a
filesystem configuration option `maxthreads` to configure the regulator.

By default `maxthreads` is set to 100. This means the FS driver is
limited to 100 concurrent blocking file operations. Any subsequent
operations will block in Go until previous filesystem operations
complete.

This ensures that the registry can never open thousands of simultaneous
threads from os filesystem operations.

Note that `maxthreads` can never be less than 25.

Add test case covering parsable string maxthreads

Signed-off-by: Tony Holdstock-Brown <tony@docker.com>

docs/storage/driver/filesystem/driver.go

@@ -8,6 +8,8 @@ import (
 	"io/ioutil"
 	"os"
 	"path"
+	"reflect"
+	"strconv"
 	"time"
 
 	"github.com/docker/distribution/context"
@@ -16,8 +18,23 @@ import (
 	"github.com/docker/distribution/registry/storage/driver/factory"
 )
 
-const driverName = "filesystem"
-const defaultRootDirectory = "/var/lib/registry"
+const (
+	driverName           = "filesystem"
+	defaultRootDirectory = "/var/lib/registry"
+	defaultMaxThreads    = uint64(100)
+
+	// minThreads is the minimum value for the maxthreads configuration
+	// parameter. If the driver's parameters are less than this we set
+	// the parameters to minThreads
+	minThreads = uint64(25)
+)
+
+// DriverParameters represents all configuration options available for the
+// filesystem driver
+type DriverParameters struct {
+	RootDirectory string
+	MaxThreads    uint64
+}
 
 func init() {
 	factory.Register(driverName, &filesystemDriverFactory{})
@@ -27,7 +44,7 @@ func init() {
 type filesystemDriverFactory struct{}
 
 func (factory *filesystemDriverFactory) Create(parameters map[string]interface{}) (storagedriver.StorageDriver, error) {
-	return FromParameters(parameters), nil
+	return FromParameters(parameters)
 }
 
 type driver struct {
@@ -47,25 +64,67 @@ type Driver struct {
 // FromParameters constructs a new Driver with a given parameters map
 // Optional Parameters:
 // - rootdirectory
-func FromParameters(parameters map[string]interface{}) *Driver {
-	var rootDirectory = defaultRootDirectory
+// - maxthreads
+func FromParameters(parameters map[string]interface{}) (*Driver, error) {
+	params, err := fromParametersImpl(parameters)
+	if err != nil || params == nil {
+		return nil, err
+	}
+	return New(*params), nil
+}
+
+func fromParametersImpl(parameters map[string]interface{}) (*DriverParameters, error) {
+	var (
+		err           error
+		maxThreads    = defaultMaxThreads
+		rootDirectory = defaultRootDirectory
+	)
+
 	if parameters != nil {
-		rootDir, ok := parameters["rootdirectory"]
-		if ok {
+		if rootDir, ok := parameters["rootdirectory"]; ok {
 			rootDirectory = fmt.Sprint(rootDir)
 		}
+
+		// Get maximum number of threads for blocking filesystem operations,
+		// if specified
+		threads := parameters["maxthreads"]
+		switch v := threads.(type) {
+		case string:
+			if maxThreads, err = strconv.ParseUint(v, 0, 64); err != nil {
+				return nil, fmt.Errorf("maxthreads parameter must be an integer, %v invalid", threads)
+			}
+		case uint64:
+			maxThreads = v
+		case int, int32, int64:
+			maxThreads = uint64(reflect.ValueOf(v).Convert(reflect.TypeOf(threads)).Int())
+		case uint, uint32:
+			maxThreads = reflect.ValueOf(v).Convert(reflect.TypeOf(threads)).Uint()
+		case nil:
+			// do nothing
+		default:
+			return nil, fmt.Errorf("invalid value for maxthreads: %#v", threads)
+		}
+
+		if maxThreads < minThreads {
+			maxThreads = minThreads
+		}
 	}
-	return New(rootDirectory)
+
+	params := &DriverParameters{
+		RootDirectory: rootDirectory,
+		MaxThreads:    maxThreads,
+	}
+	return params, nil
 }
 
 // New constructs a new Driver with a given rootDirectory
-func New(rootDirectory string) *Driver {
-	fsDriver := &driver{rootDirectory: rootDirectory}
+func New(params DriverParameters) *Driver {
+	fsDriver := &driver{rootDirectory: params.RootDirectory}
 
 	return &Driver{
 		baseEmbed: baseEmbed{
 			Base: base.Base{
-				StorageDriver: base.NewRegulator(fsDriver, 100),
+				StorageDriver: base.NewRegulator(fsDriver, params.MaxThreads),
 			},
 		},
 	}