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_test.go

@@ -3,6 +3,7 @@ package filesystem
 import (
 	"io/ioutil"
 	"os"
+	"reflect"
 	"testing"
 
 	storagedriver "github.com/docker/distribution/registry/storage/driver"
@@ -20,7 +21,93 @@ func init() {
 	}
 	defer os.Remove(root)
 
+	driver, err := FromParameters(map[string]interface{}{
+		"rootdirectory": root,
+	})
+	if err != nil {
+		panic(err)
+	}
+
 	testsuites.RegisterSuite(func() (storagedriver.StorageDriver, error) {
-		return New(root), nil
+		return driver, nil
 	}, testsuites.NeverSkip)
 }
+
+func TestFromParametersImpl(t *testing.T) {
+
+	tests := []struct {
+		params   map[string]interface{} // techincally the yaml can contain anything
+		expected DriverParameters
+		pass     bool
+	}{
+		// check we use default threads and root dirs
+		{
+			params: map[string]interface{}{},
+			expected: DriverParameters{
+				RootDirectory: defaultRootDirectory,
+				MaxThreads:    defaultMaxThreads,
+			},
+			pass: true,
+		},
+		// Testing initiation with a string maxThreads which can't be parsed
+		{
+			params: map[string]interface{}{
+				"maxthreads": "fail",
+			},
+			expected: DriverParameters{},
+			pass:     false,
+		},
+		{
+			params: map[string]interface{}{
+				"maxthreads": "100",
+			},
+			expected: DriverParameters{
+				RootDirectory: defaultRootDirectory,
+				MaxThreads:    uint64(100),
+			},
+			pass: true,
+		},
+		{
+			params: map[string]interface{}{
+				"maxthreads": 100,
+			},
+			expected: DriverParameters{
+				RootDirectory: defaultRootDirectory,
+				MaxThreads:    uint64(100),
+			},
+			pass: true,
+		},
+		// check that we use minimum thread counts
+		{
+			params: map[string]interface{}{
+				"maxthreads": 1,
+			},
+			expected: DriverParameters{
+				RootDirectory: defaultRootDirectory,
+				MaxThreads:    minThreads,
+			},
+			pass: true,
+		},
+	}
+
+	for _, item := range tests {
+		params, err := fromParametersImpl(item.params)
+
+		if !item.pass {
+			// We only need to assert that expected failures have an error
+			if err == nil {
+				t.Fatalf("expected error configuring filesystem driver with invalid param: %+v", item.params)
+			}
+			continue
+		}
+
+		if err != nil {
+			t.Fatalf("unexpected error creating filesystem driver: %s", err)
+		}
+		// Note that we get a pointer to params back
+		if !reflect.DeepEqual(*params, item.expected) {
+			t.Fatalf("unexpected params from filesystem driver. expected %+v, got %+v", item.expected, params)
+		}
+	}
+
+}