Treat a clean EOF with unknown total as the whole asset

internal/provider/syncretry.go

@@ -62,7 +62,7 @@ func AssetStatusError(url string, resp *http.Response) error {
 
 // ReadPrefix reads up to n bytes of a 200 or 206 asset response. full reports
 // that the body is the whole object (per Content-Length or the Content-Range
-// total); a body ending before n bytes that is not the whole object is a
+// total, or a clean EOF before n when the total is unknown); a body ending before n bytes that is not the whole object is a
 // transient short read.
 func ReadPrefix(resp *http.Response, n int64) (body []byte, full bool, err error) {
 	body, err = io.ReadAll(io.LimitReader(resp.Body, n))
@@ -70,15 +70,18 @@ func ReadPrefix(resp *http.Response, n int64) (body []byte, full bool, err error
 		return nil, false, err
 	}
 	total := resp.ContentLength
+	unknown := resp.StatusCode == http.StatusOK && total == -1
 	if resp.StatusCode == http.StatusPartialContent {
 		total = -1
 		if _, t, ok := strings.Cut(resp.Header.Get("Content-Range"), "/"); ok {
 			if v, perr := strconv.ParseInt(t, 10, 64); perr == nil {
 				total = v
 			}
+			unknown = t == "*"
 		}
 	}
-	full = total == int64(len(body))
+	// net/http errors on a premature close, so a clean EOF before n with an unknown total is the whole object.
+	full = total == int64(len(body)) || (unknown && int64(len(body)) < n)
 	if !full && int64(len(body)) < n {
 		return nil, false, fmt.Errorf("short read: got %d of %d bytes", len(body), n)
 	}