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// Copyright 2020 The LUCI Authors.
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// See the License for the specific language governing permissions and
// limitations under the License.
// Package vars implements a registry of ${var} placeholders.
// They can be used in config validation rule patterns and config set names
// and paths in place of a not-yet-known values.
package vars
import (
// Vars is the default set of placeholder vars used by the process.
// Individual packages may register vars here during init() time.
var Vars VarSet
// VarSet holds a registry ${var} -> callback that extracts it from a context.
// It is used by the config client to render config validation patterns like
// "services/${appid}". By using placeholder vars it is possible to register
// validation rules during process startup time before values of all vars
// are actually known.
type VarSet struct {
l sync.RWMutex
v map[string]func(context.Context) (string, error)
// Register registers a variable that can be used in templates as `${name}`.
// Such placeholder variable is rendered into an actual value via the given
// callback in RenderTemplate.
// The primary use case for this mechanism is to allow to register config
// validation rule patterns that depend on not-yet known values during init()
// time.
// Panics if such variable is already registered.
func (vs *VarSet) Register(name string, value func(context.Context) (string, error)) {
defer vs.l.Unlock()
if vs.v == nil {
vs.v = make(map[string]func(context.Context) (string, error), 1)
if vs.v[name] != nil {
panic(fmt.Sprintf("variable %q is already registered", name))
vs.v[name] = value
var placeholderRe = regexp.MustCompile(`\${[^}]*}`)
// RenderTemplate replaces all `${var}` references in the string with their
// values by calling registered callbacks.
func (vs *VarSet) RenderTemplate(ctx context.Context, templ string) (string, error) {
defer vs.l.RUnlock()
var errs errors.MultiError
out := placeholderRe.ReplaceAllStringFunc(templ, func(match string) string {
name := match[2 : len(match)-1] // strip ${...}
var val string
var err error
if cb := vs.v[name]; cb != nil {
val, err = cb(ctx)
} else {
err = fmt.Errorf("no placeholder named %q is registered", name)
if err != nil {
errs = append(errs, err)
return val
if len(errs) != 0 {
return "", errs
return out, nil