Fix data races in MemMapFs

Also simplifiy the lock constructs.

Fixes #44
Fixes #45

memmap.go

diff --git a/memmap.go b/memmap.go
index 3a5a918..0b4edf4 100644
--- a/memmap.go
+++ b/memmap.go
@@ -29,47 +29,32 @@
 var mux = &sync.Mutex{}
 
 type MemMapFs struct {
-	data  map[string]File
-	mutex *sync.RWMutex
+	sync.RWMutex
+	data map[string]File
+	init sync.Once
 }
 
-func (m *MemMapFs) lock() {
-	mx := m.getMutex()
-	mx.Lock()
-}
-
-func (m *MemMapFs) unlock()  { m.getMutex().Unlock() }
-func (m *MemMapFs) rlock()   { m.getMutex().RLock() }
-func (m *MemMapFs) runlock() { m.getMutex().RUnlock() }
+var memfsInit sync.Once
 
 func (m *MemMapFs) getData() map[string]File {
-	if m.data == nil {
+	m.init.Do(func() {
 		m.data = make(map[string]File)
-
 		// Root should always exist, right?
 		// TODO: what about windows?
 		m.data[FilePathSeparator] = mem.CreateDir(FilePathSeparator)
-	}
+	})
 	return m.data
 }
 
-func (m *MemMapFs) getMutex() *sync.RWMutex {
-	mux.Lock()
-	if m.mutex == nil {
-		m.mutex = &sync.RWMutex{}
-	}
-	mux.Unlock()
-	return m.mutex
-}
 func (MemMapFs) Name() string { return "MemMapFS" }
 
 func (m *MemMapFs) Create(name string) (File, error) {
 	name = normalizePath(name)
-	m.lock()
+	m.Lock()
 	file := mem.CreateFile(name)
 	m.getData()[name] = file
 	m.registerWithParent(file)
-	m.unlock()
+	m.Unlock()
 	return file, nil
 }
 
@@ -152,9 +137,9 @@
 func (m *MemMapFs) Mkdir(name string, perm os.FileMode) error {
 	name = normalizePath(name)
 
-	m.rlock()
+	m.RLock()
 	x, ok := m.getData()[name]
-	m.runlock()
+	m.RUnlock()
 	if ok {
 		// Only return ErrFileExists if it's a file, not a directory.
 		i, err := x.Stat()
@@ -165,11 +150,11 @@
 			return err
 		}
 	} else {
-		m.lock()
+		m.Lock()
 		item := mem.CreateDir(name)
 		m.getData()[name] = item
 		m.registerWithParent(item)
-		m.unlock()
+		m.Unlock()
 	}
 	return nil
 }
@@ -195,14 +180,14 @@
 func (m *MemMapFs) Open(name string) (File, error) {
 	name = normalizePath(name)
 
-	m.rlock()
+	m.RLock()
 	f, ok := m.getData()[name]
 	ff, ok := f.(*mem.File)
 
 	if ok {
 		ff.Open()
 	}
-	m.runlock()
+	m.RUnlock()
 
 	if ok {
 		return f, nil
@@ -253,8 +238,8 @@
 func (m *MemMapFs) Remove(name string) error {
 	name = normalizePath(name)
 
-	m.lock()
-	defer m.unlock()
+	m.Lock()
+	defer m.Unlock()
 
 	if _, ok := m.getData()[name]; ok {
 		m.unRegisterWithParent(name)
@@ -267,20 +252,20 @@
 
 func (m *MemMapFs) RemoveAll(path string) error {
 	path = normalizePath(path)
-	m.lock()
+	m.Lock()
 	m.unRegisterWithParent(path)
-	m.unlock()
+	m.Unlock()
 
-	m.rlock()
-	defer m.runlock()
+	m.RLock()
+	defer m.RUnlock()
 
 	for p, _ := range m.getData() {
 		if strings.HasPrefix(p, path) {
-			m.runlock()
-			m.lock()
+			m.RUnlock()
+			m.Lock()
 			delete(m.getData(), p)
-			m.unlock()
-			m.rlock()
+			m.Unlock()
+			m.RLock()
 		}
 	}
 	return nil
@@ -294,20 +279,20 @@
 		return nil
 	}
 
-	m.rlock()
-	defer m.runlock()
+	m.RLock()
+	defer m.RUnlock()
 	if _, ok := m.getData()[oldname]; ok {
 		if _, ok := m.getData()[newname]; !ok {
-			m.runlock()
-			m.lock()
+			m.RUnlock()
+			m.Lock()
 			m.unRegisterWithParent(oldname)
 			file := m.getData()[oldname].(*mem.File)
 			delete(m.getData(), oldname)
 			mem.ChangeFileName(file, newname)
 			m.getData()[newname] = file
 			m.registerWithParent(file)
-			m.unlock()
-			m.rlock()
+			m.Unlock()
+			m.RLock()
 		} else {
 			return ErrDestinationExists
 		}
@@ -335,9 +320,9 @@
 
 	ff, ok := f.(*mem.File)
 	if ok {
-		m.lock()
+		m.Lock()
 		mem.SetMode(ff, mode)
-		m.unlock()
+		m.Unlock()
 	} else {
 		return errors.New("Unable to Chmod Memory File")
 	}
@@ -353,9 +338,9 @@
 
 	ff, ok := f.(*mem.File)
 	if ok {
-		m.lock()
+		m.Lock()
 		mem.SetModTime(ff, mtime)
-		m.unlock()
+		m.Unlock()
 	} else {
 		return errors.New("Unable to Chtime Memory File")
 	}