register: frame out basic driver registration

Signed-off-by: Vincent Batts <vbatts@hashbangbash.com>

driver/driver.go

@@ -2,5 +2,6 @@ package driver
 
 // Driver is the vfs level driver to operate on filesystem trees (i.e.
 // copy-on-write filesystems, etc)
-type Driver struct {
+type Driver interface {
+	Init(rootPath string) error
 }

register.go

@@ -1,15 +1,42 @@
 package storage
 
-import "git.thisco.de/vbatts/image-storage-interfaces/driver"
+import (
+	"sort"
+	"sync"
+
+	"git.thisco.de/vbatts/image-storage-interfaces/driver"
+)
+
+var (
+	storageDriversMu sync.RWMutex
+	storageDrivers   = make(map[string]driver.Driver)
+)
 
 // Drivers returns a sorted list of the registered drivers
 func Drivers() []string {
-	return nil
+	storageDriversMu.Lock()
+	defer storageDriversMu.Unlock()
+	var list []string
+	for name, _ := range storageDrivers {
+		list = append(list, name)
+	}
+	sort.Strings(list)
+	return list
 }
 
 // Register makes a database driver available by the provided name. If Register
 // is called twice with the same name or if driver is nil, it panics.
 func Register(name string, driver driver.Driver) {
+	storageDriversMu.Lock()
+	defer storageDriversMu.Unlock()
+	if driver == nil {
+		panic("storage: Register driver is nil")
+	}
+	if _, dup := storageDrivers[name]; dup {
+		panic("storage: Register called twice for driver " + name)
+	}
+	storageDrivers[name] = driver
+
 }
 
 // OpenHandle a storage handle using the specified storage driver name and a

register_test.go

@@ -0,0 +1,21 @@
+package storage
+
+import "testing"
+
+type memDriver struct {
+}
+
+func (md memDriver) Init(rootPath string) error {
+	return nil
+}
+
+func TestRegister(t *testing.T) {
+	originalLen := len(Drivers())
+
+	// TODO assert this does not panic? (and later assert that one _does_ panic?)
+	Register("mem", memDriver{})
+
+	if nowLen := len(Drivers()); nowLen != originalLen+1 {
+		t.Errorf("expected registered driver to be reflected, but was not. original: %d; now: %d", originalLen, nowLen)
+	}
+}