Add more coverage for loader and strengthen type safety.

pkg/fs/confirmeddir.go

@@ -0,0 +1,58 @@
+package fs
+
+import (
+	"path/filepath"
+	"strings"
+)
+
+// ConfirmedDir is a clean, absolute, delinkified path
+// that was confirmed to point to an existing directory.
+type ConfirmedDir string
+
+// HasPrefix returns true if the directory argument
+// is a prefix of self (d) from the point of view of
+// a file system.
+//
+// I.e., it's true if the argument equals or contains
+// self (d) in a file path sense.
+//
+// HasPrefix emulates the semantics of strings.HasPrefix
+// such that the following are true:
+//
+//   strings.HasPrefix("foobar", "foobar")
+//   strings.HasPrefix("foobar", "foo")
+//   strings.HasPrefix("foobar", "")
+//
+//   d := fSys.ConfirmDir("/foo/bar")
+//   d.HasPrefix("/foo/bar")
+//   d.HasPrefix("/foo")
+//   d.HasPrefix("/")
+//
+// Not contacting a file system here to check for
+// actual path existence.
+//
+// This is tested on linux, but will have trouble
+// on other operating systems.  As soon as related
+// work is completed in the core filepath package,
+// this code should be refactored to use it.
+// See:
+//   https://github.com/golang/go/issues/18355
+//   https://github.com/golang/dep/issues/296
+//   https://github.com/golang/dep/blob/master/internal/fs/fs.go#L33
+//   https://codereview.appspot.com/5712045
+func (d ConfirmedDir) HasPrefix(path ConfirmedDir) bool {
+	if path.String() == string(filepath.Separator) || path == d {
+		return true
+	}
+	return strings.HasPrefix(
+		string(d),
+		string(path)+string(filepath.Separator))
+}
+
+func (d ConfirmedDir) Join(path string) string {
+	return filepath.Join(string(d), path)
+}
+
+func (d ConfirmedDir) String() string {
+	return string(d)
+}