header: match subdirective for response matching

caddytest/integration/caddyfile_adapt/header.caddyfiletest

@@ -12,10 +12,14 @@
 	@images path /images/*
 	header @images {
 		Cache-Control "public, max-age=3600, stale-while-revalidate=86400"
+		match {
+			status 200
+		}
 	}
 	header {
 		+Link "Foo"
 		+Link "Bar"
+		match status 200
 	}
 	header >Set Defer
 	header >Replace Deferred Replacement
@@ -42,6 +46,11 @@
 								{
 									"handler": "headers",
 									"response": {
+										"require": {
+											"status_code": [
+												200
+											]
+										},
 										"set": {
 											"Cache-Control": [
 												"public, max-age=3600, stale-while-revalidate=86400"
@@ -136,6 +145,11 @@
 												"Foo",
 												"Bar"
 											]
+										},
+										"require": {
+											"status_code": [
+												200
+											]
 										}
 									}
 								},

modules/caddyhttp/headers/caddyfile.go

@@ -99,6 +99,16 @@ func parseCaddyfile(h httpcaddyfile.Helper) ([]httpcaddyfile.ConfigValue, error)
 			handler.Response.Deferred = true
 			continue
 		}
+		if field == "match" {
+			responseMatchers := make(map[string]caddyhttp.ResponseMatcher)
+			err := caddyhttp.ParseNamedResponseMatcher(h.NewFromNextSegment(), responseMatchers)
+			if err != nil {
+				return nil, err
+			}
+			matcher := responseMatchers["match"]
+			handler.Response.Require = &matcher
+			continue
+		}
 		if hasArgs {
 			return nil, h.Err("cannot specify headers in both arguments and block") // because it would be weird
 		}

modules/caddyhttp/headers/headers_test.go

@@ -143,6 +143,28 @@ func TestHandler(t *testing.T) {
 				"Cache-Control": []string{"no-cache"},
 			},
 		},
+		{ // same as above, but checks that response headers are left alone when "Require" conditions are unmet
+			handler: Handler{
+				Response: &RespHeaderOps{
+					Require: &caddyhttp.ResponseMatcher{
+						Headers: http.Header{
+							"Cache-Control": nil,
+						},
+					},
+					HeaderOps: &HeaderOps{
+						Add: http.Header{
+							"Cache-Control": []string{"no-cache"},
+						},
+					},
+				},
+			},
+			respHeader: http.Header{
+				"Cache-Control": []string{"something"},
+			},
+			expectedRespHeader: http.Header{
+				"Cache-Control": []string{"something"},
+			},
+		},
 		{
 			handler: Handler{
 				Response: &RespHeaderOps{