engine: local: Add a vardir API to our local API collection

engine/local/local.go

@@ -54,7 +54,11 @@ type API struct {
 	Logf   func(format string, v ...interface{})
 
 	// Each piece of the API can take a handle here.
-	*Value
+	*Value // TODO: Rename to ValueImpl?
+
+	// VarDirImpl is the implementation for the VarDir API's. The API's are
+	// the collection of public methods that exist on this struct.
+	*VarDirImpl
 }
 
 // Init initializes the API before first use. It returns itself so it can be
@@ -67,6 +71,13 @@ func (obj *API) Init() *API {
 		Logf:   obj.Logf,
 	})
 
+	obj.VarDirImpl = &VarDirImpl{}
+	obj.VarDirImpl.Init(&VarDirInit{
+		Prefix: obj.Prefix,
+		Debug:  obj.Debug,
+		Logf:   obj.Logf,
+	})
+
 	return obj
 }
 
@@ -332,3 +343,81 @@ func valueRemove(ctx context.Context, prefix, key string) error {
 	}
 	return nil // ignore not found errors
 }
+
+// VarDirInit are the init values that the VarDir API needs to work correctly.
+type VarDirInit struct {
+	Prefix string
+	Debug  bool
+	Logf   func(format string, v ...interface{})
+}
+
+// VarDirImpl is the implementation for the VarDir API's. The API's are the
+// collection of public methods that exist on this struct.
+type VarDirImpl struct {
+	init         *VarDirInit
+	mutex        *sync.Mutex
+	prefix       string
+	prefixExists bool // is it okay to use the prefix?
+}
+
+// Init runs some initialization code for the VarDir API.
+func (obj *VarDirImpl) Init(init *VarDirInit) {
+	obj.init = init
+	obj.mutex = &sync.Mutex{}
+	obj.prefix = fmt.Sprintf("%s/", path.Join(obj.init.Prefix, "vardir"))
+}
+
+// VarDir returns a directory rooted at the internal prefix.
+func (obj *VarDirImpl) VarDir(ctx context.Context, reldir string) (string, error) {
+	if strings.HasPrefix(reldir, "/") {
+		return "", fmt.Errorf("path must be relative")
+	}
+	if !strings.HasSuffix(reldir, "/") {
+		return "", fmt.Errorf("path must be a dir")
+	}
+	// NOTE: The above checks ensure we don't get either "" or "/" as input!
+
+	prefix, err := obj.getPrefix()
+	if err != nil {
+		return "", err
+	}
+
+	result := fmt.Sprintf("%s/", path.Join(prefix, reldir))
+
+	// TODO: Should we mkdir this?
+	obj.mutex.Lock()
+	defer obj.mutex.Unlock()
+	if err := os.MkdirAll(result, 0755); err != nil {
+		return "", err
+	}
+
+	return result, nil
+}
+
+// getPrefix gets the prefix dir to use, or errors if it can't make one. It
+// makes it on first use, and returns quickly from any future calls to it.
+func (obj *VarDirImpl) getPrefix() (string, error) {
+	// NOTE: Moving this mutex to just below the first early return, would
+	// be a benign race, but as it turns out, it's possible that a compiler
+	// would see this behaviour as "undefined" and things might not work as
+	// intended. It could perhaps be replaced with a sync/atomic primitive
+	// if we wanted better performance here.
+	obj.mutex.Lock()
+	defer obj.mutex.Unlock()
+
+	if obj.prefixExists { // former race read
+		return obj.prefix, nil
+	}
+
+	// MkdirAll instead of Mkdir because we have no idea if the parent
+	// local/ directory was already made yet or not. (If at all.) If path is
+	// already a directory, MkdirAll does nothing and returns nil. (Good!)
+	// TODO: I hope MkdirAll is thread-safe on path creation in case another
+	// future local API tries to make the base (parent) directory too!
+	if err := os.MkdirAll(obj.prefix, 0755); err != nil {
+		return "", err
+	}
+	obj.prefixExists = true // former race write
+
+	return obj.prefix, nil
+}