caddy/modules/caddyhttp/replacer.go

package caddyhttp

import (
	"net"
	"net/http"
	"os"
	"strings"
)

// Replacer can replace values in strings based
// on a request and/or response writer. The zero
// Replacer is not valid; it must be initialized
// within this package.
type Replacer struct {
	req    *http.Request
	resp   http.ResponseWriter
	custom map[string]string
}

// Map sets a custom variable mapping to a value.
func (r *Replacer) Map(variable, value string) {
	r.custom[variable] = value
}

// Replace replaces placeholders in input with the value. If
// the value is empty string, the placeholder is substituted
// with the value empty.
func (r *Replacer) Replace(input, empty string) string {
	if !strings.Contains(input, phOpen) {
		return input
	}

	input = r.replaceAll(input, empty, r.defaults())
	input = r.replaceAll(input, empty, r.custom)

	return input
}

func (r *Replacer) replaceAll(input, empty string, mapping map[string]string) string {
	for key, val := range mapping {
		if val == "" {
			val = empty
		}
		input = strings.ReplaceAll(input, phOpen+key+phClose, val)
	}
	return input
}

func (r *Replacer) defaults() map[string]string {
	m := map[string]string{
		"request.host": func() string {
			host, _, err := net.SplitHostPort(r.req.Host)
			if err != nil {
				return r.req.Host // OK; there probably was no port
			}
			return host
		}(),
		"request.hostport": r.req.Host, // may include both host and port
		"request.method":   r.req.Method,
		"request.port": func() string {
			// if there is no port, there will be an error; in
			// that case, port is the empty string anyway
			_, port, _ := net.SplitHostPort(r.req.Host)
			return port
		}(),
		"request.scheme": func() string {
			if r.req.TLS != nil {
				return "https"
			}
			return "http"
		}(),
		"request.uri": r.req.URL.RequestURI(),
		"system.hostname": func() string {
			// OK if there is an error; just return empty string
			name, _ := os.Hostname()
			return name
		}(),
	}

	// TODO: why should header fields, cookies, and query params get special treatment like this?
	// maybe they should be scoped by words like "request.header." just like everything else.
	for field, vals := range r.req.Header {
		m[">"+strings.ToLower(field)] = strings.Join(vals, ",")
	}
	for field, vals := range r.resp.Header() {
		m["<"+strings.ToLower(field)] = strings.Join(vals, ",")
	}
	for _, cookie := range r.req.Cookies() {
		m["~"+cookie.Name] = cookie.Value
	}
	for param, vals := range r.req.URL.Query() {
		m["?"+param] = strings.Join(vals, ",")
	}

	return m
}

const phOpen, phClose = "{", "}"