caddyhttp: Add 'map' handler (#3199)

* inital map implementation

* resolve the value during middleware execution

* use regex instead

* pr feedback

* renamed mmap to maphandler

* refactored GetString implementation

* fixed mispelling

* additional feedback

modules/caddyhttp/map/caddyfile.go

@@ -0,0 +1,71 @@
+// 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 maphandler
+
+import (
+	"github.com/caddyserver/caddy/v2/caddyconfig/httpcaddyfile"
+	"github.com/caddyserver/caddy/v2/modules/caddyhttp"
+)
+
+func init() {
+	httpcaddyfile.RegisterHandlerDirective("map", parseCaddyfile)
+}
+
+// parseCaddyfile sets up the handler for a map from Caddyfile tokens. Syntax:
+//
+//     map <source> <dest> {
+//         [default <default>] - used if not match is found
+//         [<regexp> <replacement>] - regular expression to match against the source find and the matching replacement value
+//         ...
+//     }
+//
+// The map takes a source variable and maps it into the dest variable. The mapping process
+// will check the source variable for the first successful match against a list of regular expressions.
+// If a successful match is found the dest variable will contain the replacement value.
+// If no successful match is found and the default is specified then the dest will contain the default value.
+//
+func parseCaddyfile(h httpcaddyfile.Helper) (caddyhttp.MiddlewareHandler, error) {
+	m := new(Handler)
+
+	for h.Next() {
+		// first see if source and dest are configured
+		if h.NextArg() {
+			m.Source = h.Val()
+			if h.NextArg() {
+				m.Destination = h.Val()
+			}
+		}
+
+		// load the rules
+		for h.NextBlock(0) {
+			expression := h.Val()
+			if expression == "default" {
+				args := h.RemainingArgs()
+				if len(args) != 1 {
+					return m, h.ArgErr()
+				}
+				m.Default = args[0]
+			} else {
+				args := h.RemainingArgs()
+				if len(args) != 1 {
+					return m, h.ArgErr()
+				}
+				m.Items = append(m.Items, Item{Expression: expression, Value: args[0]})
+			}
+		}
+	}
+
+	return m, nil
+}