caddyhttp: Merge header matchers in Caddyfile (#3832)

caddytest/integration/caddyfile_adapt/matcher_syntax.txt

@@ -18,6 +18,13 @@
 
 	@matcher6 vars_regexp "{http.request.uri}" `\.([a-f0-9]{6})\.(css|js)$`
 	respond @matcher6 "from vars_regexp matcher without name"
+
+	@matcher7 {
+		header Foo bar
+		header Foo foobar
+		header Bar foo
+	}
+	respond @matcher7 "header matcher merging values of the same field"
 }
 ----------
 {
@@ -127,6 +134,27 @@
 									"handler": "static_response"
 								}
 							]
+						},
+						{
+							"match": [
+								{
+									"header": {
+										"Bar": [
+											"foo"
+										],
+										"Foo": [
+											"bar",
+											"foobar"
+										]
+									}
+								}
+							],
+							"handle": [
+								{
+									"body": "header matcher merging values of the same field",
+									"handler": "static_response"
+								}
+							]
 						}
 					]
 				}

modules/caddyhttp/matchers.go

@@ -408,7 +408,16 @@ func (m *MatchHeader) UnmarshalCaddyfile(d *caddyfile.Dispenser) error {
 		if !d.Args(&field, &val) {
 			return d.Errf("malformed header matcher: expected both field and value")
 		}
-		http.Header(*m).Set(field, val)
+
+		// If multiple header matchers with the same header field are defined,
+		// we want to add the existing to the list of headers (will be OR'ed)
+		existing := http.Header(*m).Values(field)
+		if len(existing) > 0 {
+			http.Header(*m).Add(field, val)
+		} else {
+			http.Header(*m).Set(field, val)
+		}
+
 		if d.NextBlock(0) {
 			return d.Err("malformed header matcher: blocks are not supported")
 		}