backend/http: support content-range response header

lib/rest/headers.go

@@ -0,0 +1,39 @@
+package rest
+
+import (
+	"net/http"
+	"strconv"
+	"strings"
+)
+
+// ParseSizeFromHeaders parses HTTP response headers to get the full file size.
+// Returns -1 if the headers did not exist or were invalid.
+func ParseSizeFromHeaders(headers http.Header) (size int64) {
+	size = -1
+
+	var contentLength = headers.Get("Content-Length")
+	if len(contentLength) != 0 {
+		var err error
+		if size, err = strconv.ParseInt(contentLength, 10, 64); err != nil {
+			return -1
+		}
+	}
+
+	var contentRange = headers.Get("Content-Range")
+	if len(contentRange) == 0 {
+		return size
+	}
+
+	if !strings.HasPrefix(contentRange, "bytes ") {
+		return -1
+	}
+	slash := strings.IndexRune(contentRange, '/')
+	if slash < 0 {
+		return -1
+	}
+	ret, err := strconv.ParseInt(contentRange[slash+1:], 10, 64)
+	if err != nil {
+		return -1
+	}
+	return ret
+}