caddyhttp: Add support for listener wrapper modules

Wrapping listeners is useful for composing custom behavior related
to accepting, closing, reading/writing connections (etc) below the
application layer; for example, the PROXY protocol.

modules/caddyhttp/server.go

@@ -16,6 +16,7 @@ package caddyhttp
 
 import (
 	"context"
+	"encoding/json"
 	"fmt"
 	"net"
 	"net/http"
@@ -38,6 +39,10 @@ type Server struct {
 	// that may include port ranges.
 	Listen []string `json:"listen,omitempty"`
 
+	// A list of listener wrapper modules, which can modify the behavior
+	// of the base listener. They are applied in the given order.
+	ListenerWrappersRaw []json.RawMessage `json:"listener_wrappers,omitempty" caddy:"namespace=caddy.listeners inline_key=wrapper"`
+
 	// How long to allow a read from a client's upload. Setting this
 	// to a short, non-zero value can mitigate slowloris attacks, but
 	// may also affect legitimately slow clients.
@@ -106,6 +111,7 @@ type Server struct {
 
 	primaryHandlerChain Handler
 	errorHandlerChain   Handler
+	listenerWrappers    []caddy.ListenerWrapper
 
 	tlsApp       *caddytls.TLS
 	logger       *zap.Logger