reverseproxy: Add handle_response blocks to reverse_proxy (#3710) (#4021)

* reverseproxy: Add `handle_response` blocks to `reverse_proxy` (#3710)

* reverseproxy: complete handle_response test

* reverseproxy: Change handle_response matchers to use named matchers

reverseproxy: Add support for changing status code

* fastcgi: Remove obsolete TODO

We already have d.Err("transport already specified") in the reverse_proxy parsing code which covers this case

* reverseproxy: Fix support for "4xx" type status codes

* Apply suggestions from code review

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

* caddyhttp: Reorganize response matchers

* reverseproxy: Reintroduce caddyfile.Unmarshaler

* reverseproxy: Add comment mentioning Finalize should be called

Co-authored-by: Maxime Soulé <btik-git@scoubidou.com>
Co-authored-by: Matt Holt <mholt@users.noreply.github.com>

modules/caddyhttp/responsematchers.go

@@ -0,0 +1,122 @@
+// Copyright 2015 Matthew Holt and The Caddy 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
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package caddyhttp
+
+import (
+	"net/http"
+	"strconv"
+	"strings"
+
+	"github.com/caddyserver/caddy/v2/caddyconfig/caddyfile"
+)
+
+// ResponseMatcher is a type which can determine if an
+// HTTP response matches some criteria.
+type ResponseMatcher struct {
+	// If set, one of these status codes would be required.
+	// A one-digit status can be used to represent all codes
+	// in that class (e.g. 3 for all 3xx codes).
+	StatusCode []int `json:"status_code,omitempty"`
+
+	// If set, each header specified must be one of the
+	// specified values, with the same logic used by the
+	// request header matcher.
+	Headers http.Header `json:"headers,omitempty"`
+}
+
+// Match returns true if the given statusCode and hdr match rm.
+func (rm ResponseMatcher) Match(statusCode int, hdr http.Header) bool {
+	if !rm.matchStatusCode(statusCode) {
+		return false
+	}
+	return matchHeaders(hdr, rm.Headers, "", nil)
+}
+
+func (rm ResponseMatcher) matchStatusCode(statusCode int) bool {
+	if rm.StatusCode == nil {
+		return true
+	}
+	for _, code := range rm.StatusCode {
+		if StatusCodeMatches(statusCode, code) {
+			return true
+		}
+	}
+	return false
+}
+
+// ParseNamedResponseMatcher parses the tokens of a named response matcher.
+//
+//     @name {
+//         header <field> [<value>]
+//         status <code...>
+//     }
+//
+// Or, single line syntax:
+//
+//     @name [header <field> [<value>]] | [status <code...>]
+//
+func ParseNamedResponseMatcher(d *caddyfile.Dispenser, matchers map[string]ResponseMatcher) error {
+	for d.Next() {
+		definitionName := d.Val()
+
+		if _, ok := matchers[definitionName]; ok {
+			return d.Errf("matcher is defined more than once: %s", definitionName)
+		}
+
+		matcher := ResponseMatcher{}
+		for nesting := d.Nesting(); d.NextArg() || d.NextBlock(nesting); {
+			switch d.Val() {
+			case "header":
+				if matcher.Headers == nil {
+					matcher.Headers = http.Header{}
+				}
+
+				// reuse the header request matcher's unmarshaler
+				headerMatcher := MatchHeader(matcher.Headers)
+				err := headerMatcher.UnmarshalCaddyfile(d.NewFromNextSegment())
+				if err != nil {
+					return err
+				}
+
+				matcher.Headers = http.Header(headerMatcher)
+			case "status":
+				if matcher.StatusCode == nil {
+					matcher.StatusCode = []int{}
+				}
+
+				args := d.RemainingArgs()
+				if len(args) == 0 {
+					return d.ArgErr()
+				}
+
+				for _, arg := range args {
+					if len(arg) == 3 && strings.HasSuffix(arg, "xx") {
+						arg = arg[:1]
+					}
+					statusNum, err := strconv.Atoi(arg)
+					if err != nil {
+						return d.Errf("bad status value '%s': %v", arg, err)
+					}
+					matcher.StatusCode = append(matcher.StatusCode, statusNum)
+				}
+			default:
+				return d.Errf("unrecognized response matcher %s", d.Val())
+			}
+		}
+
+		matchers[definitionName] = matcher
+	}
+	return nil
+}