requestbody: Add set option to replace request body (#5795)

Co-authored-by: Matt Holt <mholt@users.noreply.github.com>

modules/caddyhttp/requestbody/caddyfile.go

@@ -68,6 +68,12 @@ func parseCaddyfile(h httpcaddyfile.Helper) (caddyhttp.MiddlewareHandler, error)
 			}
 			rb.WriteTimeout = timeout
 
+		case "set":
+			var setStr string
+			if !h.AllArgs(&setStr) {
+				return nil, h.ArgErr()
+			}
+			rb.Set = setStr
 		default:
 			return nil, h.Errf("unrecognized request_body subdirective '%s'", h.Val())
 		}

modules/caddyhttp/requestbody/requestbody.go

@@ -18,6 +18,7 @@ import (
 	"errors"
 	"io"
 	"net/http"
+	"strings"
 	"time"
 
 	"go.uber.org/zap"
@@ -43,6 +44,10 @@ type RequestBody struct {
 	// EXPERIMENTAL. Subject to change/removal.
 	WriteTimeout time.Duration `json:"write_timeout,omitempty"`
 
+	// This field permit to replace body on the fly
+	// EXPERIMENTAL. Subject to change/removal.
+	Set string `json:"set,omitempty"`
+
 	logger *zap.Logger
 }
 
@@ -60,6 +65,18 @@ func (rb *RequestBody) Provision(ctx caddy.Context) error {
 }
 
 func (rb RequestBody) ServeHTTP(w http.ResponseWriter, r *http.Request, next caddyhttp.Handler) error {
+	if rb.Set != "" {
+		if r.Body != nil {
+			err := r.Body.Close()
+			if err != nil {
+				return err
+			}
+		}
+		repl := r.Context().Value(caddy.ReplacerCtxKey).(*caddy.Replacer)
+		replacedBody := repl.ReplaceAll(rb.Set, "")
+		r.Body = io.NopCloser(strings.NewReader(replacedBody))
+		r.ContentLength = int64(len(rb.Set))
+	}
 	if r.Body == nil {
 		return next.ServeHTTP(w, r)
 	}