fetcher: add tests for IsModified behaviour

In particular, add a failing test for the case where ETag changes but
Last-Modified does not.

internal/reader/fetcher/response_handler_test.go

@@ -0,0 +1,69 @@
+// SPDX-FileCopyrightText: Copyright The Miniflux Authors. All rights reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package fetcher // import "miniflux.app/v2/internal/reader/fetcher"
+
+import (
+	"net/http"
+	"testing"
+)
+
+func TestIsModified(t *testing.T) {
+	var cachedEtag = "abc123"
+	var cachedLastModified = "Wed, 21 Oct 2015 07:28:00 GMT"
+
+	var testCases = map[string]struct {
+		Status       int
+		LastModified string
+		ETag         string
+		IsModified   bool
+	}{
+		"Unmodified 304": {
+			Status:       304,
+			LastModified: cachedLastModified,
+			ETag:         cachedEtag,
+			IsModified:   false,
+		},
+		"Unmodified 200": {
+			Status:       200,
+			LastModified: cachedLastModified,
+			ETag:         cachedEtag,
+			IsModified:   false,
+		},
+		// This case is invalid per RFC9110 8.8.1, so ETag takes precedence.
+		"Last-Modified changed only": {
+			Status:       200,
+			LastModified: "Thu, 22 Oct 2015 07:28:00 GMT",
+			ETag:         cachedEtag,
+			IsModified:   false,
+		},
+		"ETag changed only": {
+			Status:       200,
+			LastModified: cachedLastModified,
+			ETag:         "xyz789",
+			IsModified:   true,
+		},
+		"ETag and Last-Modified changed": {
+			Status:       200,
+			LastModified: "Thu, 22 Oct 2015 07:28:00 GMT",
+			ETag:         "xyz789",
+			IsModified:   true,
+		},
+	}
+	for name, tc := range testCases {
+		t.Run(name, func(tt *testing.T) {
+			header := http.Header{}
+			header.Add("Last-Modified", tc.LastModified)
+			header.Add("ETag", tc.ETag)
+			rh := ResponseHandler{
+				httpResponse: &http.Response{
+					StatusCode: tc.Status,
+					Header:     header,
+				},
+			}
+			if tc.IsModified != rh.IsModified(cachedEtag, cachedLastModified) {
+				tt.Error(name)
+			}
+		})
+	}
+}