Fs (#242)

* play around with fs idea

* docs and fix test

* better docs

middleware/fs.go

@@ -0,0 +1,45 @@
+package middleware
+
+import (
+	"net/http"
+	"os"
+	"path/filepath"
+	"runtime"
+)
+
+// dir wraps http.Dir that restrict file access to a specific directory tree.
+type dir http.Dir
+
+// CoreDir is the directory where middleware can store assets, like zone files after a zone transfer
+// or public and private keys or anything else a middleware might need. The convention is to place
+// assets in a subdirectory named after the fully qualified zone.
+//
+// example.org./Kexample<something>.key
+//
+// CoreDir will default to "$HOME/.coredns" on Unix, but it's location can be overriden with the COREDNSPATH
+// environment variable.
+var CoreDir dir = dir(fsPath())
+
+// fsPath returns the path to the directory where the application may store data.
+// If COREDNSPATH env variable. is set, that value is used. Otherwise, the path is
+// the result of evaluating "$HOME/.coredns".
+func fsPath() string {
+	if corePath := os.Getenv("COREDNSPATH"); corePath != "" {
+		return corePath
+	}
+	return filepath.Join(userHomeDir(), ".coredns")
+}
+
+// userHomeDir returns the user's home directory according to environment variables.
+//
+// Credit: http://stackoverflow.com/a/7922977/1048862
+func userHomeDir() string {
+	if runtime.GOOS == "windows" {
+		home := os.Getenv("HOMEDRIVE") + os.Getenv("HOMEPATH")
+		if home == "" {
+			home = os.Getenv("USERPROFILE")
+		}
+		return home
+	}
+	return os.Getenv("HOME")
+}