Add ensure-file parser package.

cipd/client/cipd/ensure/template.go

Index: cipd/client/cipd/ensure/template.go
diff --git a/cipd/client/cipd/ensure/template.go b/cipd/client/cipd/ensure/template.go
new file mode 100644
index 0000000000000000000000000000000000000000..d581095cc8d3b0feaf4acd3855b4546fc9433476
--- /dev/null
+++ b/cipd/client/cipd/ensure/template.go
@@ -0,0 +1,72 @@
+// 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 (
+	"regexp"
+	"strings"
+
+	"github.com/luci/luci-go/cipd/client/cipd/common"
+	"github.com/luci/luci-go/common/errors"
+)
+
+var templateParm = regexp.MustCompile(`\${[^}]*}`)
+
+var errSkipTemplate = errors.New("this template should be skipped")
+
+// expandTemplate applies template expansion rules to the package template,
+// using the provided platform and arch values. If err == errSkipTemplate, that
+// means that this template does not apply to this platform/arch combination and
+// should be skipped.
+func expandTemplate(template, platform, arch string) (pkg string, err error) {
+	skip := false
+
+	expansionLookup := map[string]string{
+		"platform": platform,
+		"arch":     arch,
+	}
+
+	pkg = templateParm.ReplaceAllStringFunc(template, func(parm string) string {
+		// ${...}
+		contents := parm[2 : len(parm)-1]
+
+		varNameValues := strings.SplitN(contents, "=", 2)
+		if len(varNameValues) == 1 {
+			// ${varName}
+			if value, ok := expansionLookup[varNameValues[0]]; ok {
+				return value
+			}
+
+			err = errors.Reason("unknown variable in ${%(contents)s}").
+				D("contents", contents).Err()
+		}
+
+		// ${varName=value,value}
+		ourValue, ok := expansionLookup[varNameValues[0]]
+		if !ok {
+			err = errors.Reason("unknown variable %(parm)q").D("parm", parm).Err()
+			return parm
+		}
+
+		for _, val := range strings.Split(varNameValues[1], ",") {
+			if val == ourValue {
+				return ourValue
+			}
+		}
+		skip = true
+		return parm
+	})
+	if skip {
+		err = errSkipTemplate
+	}
+	if err == nil && strings.ContainsRune(pkg, '$') {
+		err = errors.Reason("unable to process some variables in %(template)q").
+			D("template", template).Err()
+	}
+	if err == nil {
+		err = common.ValidatePackageName(pkg)
+	}
+	return
+}