Add ensure-file parser package.

cipd/client/cipd/ensure/item_parsers.go

Index: cipd/client/cipd/ensure/item_parsers.go
diff --git a/cipd/client/cipd/ensure/item_parsers.go b/cipd/client/cipd/ensure/item_parsers.go
new file mode 100644
index 0000000000000000000000000000000000000000..7e6c8ac2a190df504d0a2538dd7285865a049876
--- /dev/null
+++ b/cipd/client/cipd/ensure/item_parsers.go
@@ -0,0 +1,47 @@
+// Copyright 2017 The LUCI Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+package ensure
+
+import (
+	"fmt"
+	"net/url"
+)
+
+// an itemParser should parse the value from `val`, and update s or
+// f accordingly, returning an error if needed.
+type itemParser func(s *itemParserState, f *File, val string) error
+
+// itemParserState is the state object shared between the item parsers and the
+// main ParseFile implementation.
+type itemParserState struct {
+	curRoot string
+}
+
+func rootParser(s *itemParserState, _ *File, val string) error {
+	if err := ValidateRoot(val); err != nil {
+		return err
+	}
+	s.curRoot = val
+	return nil
+}
+
+func serviceURLParser(_ *itemParserState, f *File, val string) error {
+	if f.ServiceURL != "" {
+		return fmt.Errorf("$ServiceURL may only be set once per file")
+	}
+	if _, err := url.Parse(val); err != nil {
+		return fmt.Errorf("expecting '$ServiceURL <url>' but url is invalid: %s", err)
+	}
+	f.ServiceURL = val
+	return nil
+}
+
+// itemParsers is the main way that the ensure file format is extended. If you
+// need to add a new setting or directive, please add an appropriate function
+// above and then add it to this map.
+var itemParsers = map[string]itemParser{
+	"@root":       rootParser,
+	"$serviceurl": serviceURLParser,
+}