Set the LastModified header for raw files (#18356)

Although the use of LastModified dates for caching of git objects should be
discouraged (as it is not native to git - and there are a LOT of ways this
could be incorrect) - LastModified dates can be a helpful somewhat more human
way of caching for simple cases.

This PR adds this header and handles the If-Modified-Since header to the /raw/
routes.

Fix #18354

Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: 6543 <6543@obermui.de>

modules/httpcache/httpcache.go

@@ -37,18 +37,23 @@ func generateETag(fi os.FileInfo) string {
 
 // HandleTimeCache handles time-based caching for a HTTP request
 func HandleTimeCache(req *http.Request, w http.ResponseWriter, fi os.FileInfo) (handled bool) {
+	return HandleGenericTimeCache(req, w, fi.ModTime())
+}
+
+// HandleGenericTimeCache handles time-based caching for a HTTP request
+func HandleGenericTimeCache(req *http.Request, w http.ResponseWriter, lastModified time.Time) (handled bool) {
 	AddCacheControlToHeader(w.Header(), setting.StaticCacheTime)
 
 	ifModifiedSince := req.Header.Get("If-Modified-Since")
 	if ifModifiedSince != "" {
 		t, err := time.Parse(http.TimeFormat, ifModifiedSince)
-		if err == nil && fi.ModTime().Unix() <= t.Unix() {
+		if err == nil && lastModified.Unix() <= t.Unix() {
 			w.WriteHeader(http.StatusNotModified)
 			return true
 		}
 	}
 
-	w.Header().Set("Last-Modified", fi.ModTime().Format(http.TimeFormat))
+	w.Header().Set("Last-Modified", lastModified.Format(http.TimeFormat))
 	return false
 }
 
@@ -85,3 +90,33 @@ func checkIfNoneMatchIsValid(req *http.Request, etag string) bool {
 	}
 	return false
 }
+
+// HandleGenericETagTimeCache handles ETag-based caching with Last-Modified caching for a HTTP request.
+// It returns true if the request was handled.
+func HandleGenericETagTimeCache(req *http.Request, w http.ResponseWriter, etag string, lastModified time.Time) (handled bool) {
+	if len(etag) > 0 {
+		w.Header().Set("Etag", etag)
+	}
+	if !lastModified.IsZero() {
+		w.Header().Set("Last-Modified", lastModified.Format(http.TimeFormat))
+	}
+
+	if len(etag) > 0 {
+		if checkIfNoneMatchIsValid(req, etag) {
+			w.WriteHeader(http.StatusNotModified)
+			return true
+		}
+	}
+	if !lastModified.IsZero() {
+		ifModifiedSince := req.Header.Get("If-Modified-Since")
+		if ifModifiedSince != "" {
+			t, err := time.Parse(http.TimeFormat, ifModifiedSince)
+			if err == nil && lastModified.Unix() <= t.Unix() {
+				w.WriteHeader(http.StatusNotModified)
+				return true
+			}
+		}
+	}
+	AddCacheControlToHeader(w.Header(), setting.StaticCacheTime)
+	return false
+}