echo/middleware/middleware.go

package middleware

import (
	"net/http"
	"regexp"
	"strconv"
	"strings"

	"github.com/labstack/echo/v4"
)

type (
	// Skipper defines a function to skip middleware. Returning true skips processing
	// the middleware.
	Skipper func(c echo.Context) bool

	// BeforeFunc defines a function which is executed just before the middleware.
	BeforeFunc func(c echo.Context)
)

func captureTokens(pattern *regexp.Regexp, input string) *strings.Replacer {
	groups := pattern.FindAllStringSubmatch(input, -1)
	if groups == nil {
		return nil
	}
	values := groups[0][1:]
	replace := make([]string, 2*len(values))
	for i, v := range values {
		j := 2 * i
		replace[j] = "$" + strconv.Itoa(i+1)
		replace[j+1] = v
	}
	return strings.NewReplacer(replace...)
}

func rewriteRulesRegex(rewrite map[string]string) map[*regexp.Regexp]string {
	// Initialize
	rulesRegex := map[*regexp.Regexp]string{}
	for k, v := range rewrite {
		k = regexp.QuoteMeta(k)
		k = strings.ReplaceAll(k, `\*`, "(.*?)")
		if strings.HasPrefix(k, `\^`) {
			k = strings.ReplaceAll(k, `\^`, "^")
		}
		k = k + "$"
		rulesRegex[regexp.MustCompile(k)] = v
	}
	return rulesRegex
}

func rewriteURL(rewriteRegex map[*regexp.Regexp]string, req *http.Request) error {
	if len(rewriteRegex) == 0 {
		return nil
	}

	// Depending how HTTP request is sent RequestURI could contain Scheme://Host/path or be just /path.
	// We only want to use path part for rewriting and therefore trim prefix if it exists
	rawURI := req.RequestURI
	if rawURI != "" && rawURI[0] != '/' {
		prefix := ""
		if req.URL.Scheme != "" {
			prefix = req.URL.Scheme + "://"
		}
		if req.URL.Host != "" {
			prefix += req.URL.Host // host or host:port
		}
		if prefix != "" {
			rawURI = strings.TrimPrefix(rawURI, prefix)
		}
	}

	for k, v := range rewriteRegex {
		if replacer := captureTokens(k, rawURI); replacer != nil {
			url, err := req.URL.Parse(replacer.Replace(v))
			if err != nil {
				return err
			}
			req.URL = url

			return nil // rewrite only once
		}
	}
	return nil
}

// DefaultSkipper returns false which processes the middleware.
func DefaultSkipper(echo.Context) bool {
	return false
}