httpcaddyfile: support matching headers that do not exist (#3909)

* add integration test for null header matcher

* implement null header matcher syntax

* avoid repeating magic !

* check for field following ! character

caddytest/integration/caddyfile_adapt/matcher_syntax.txt

@@ -31,6 +31,12 @@
 		query bar=baz
 	}
 	respond @matcher8 "query matcher merging pairs with the same keys"
+
+	@matcher9 {
+		header !Foo
+		header Bar foo
+	}
+	respond @matcher9 "header matcher with null field matcher"
 }
 ----------
 {
@@ -183,6 +189,24 @@
 									"handler": "static_response"
 								}
 							]
+						},
+						{
+							"match": [
+								{
+									"header": {
+										"Bar": [
+											"foo"
+										],
+										"Foo": null
+									}
+								}
+							],
+							"handle": [
+								{
+									"body": "header matcher with null field matcher",
+									"handler": "static_response"
+								}
+							]
 						}
 					]
 				}

modules/caddyhttp/matchers.go

@@ -470,13 +470,32 @@ func (m *MatchHeader) UnmarshalCaddyfile(d *caddyfile.Dispenser) error {
 	}
 	for d.Next() {
 		var field, val string
-		if !d.Args(&field, &val) {
+		if !d.Args(&field) {
+			return d.Errf("malformed header matcher: expected field")
+		}
+
+		if strings.HasPrefix(field, "!") {
+			if len(field) == 1 {
+				return d.Errf("malformed header matcher: must have field name following ! character")
+			}
+
+			field = field[1:]
+			headers := *m
+			headers[field] = nil
+			m = &headers
+			if d.NextArg() {
+				return d.Errf("malformed header matcher: null matching headers cannot have a field value")
+			}
+		} else {
+			if !d.NextArg() {
 				return d.Errf("malformed header matcher: expected both field and value")
 			}
 
 			// 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)
+			val = d.Val()
 			http.Header(*m).Add(field, val)
+		}
 
 		if d.NextBlock(0) {
 			return d.Err("malformed header matcher: blocks are not supported")