rest: Rework handling HTTP2 zero-length replies bug

Add comment that the check is based on the stdlib HTTP2 client. Refactor
the checks into a function. Return an error if the value in the
Content-Length header cannot be parsed.

internal/backend/rest/rest.go

@@ -7,6 +7,7 @@ import (
 	"io"
 	"io/ioutil"
 	"net/http"
+	"net/textproto"
 	"net/url"
 	"path"
 	"strconv"
@@ -198,6 +199,44 @@ func (b *Backend) Load(ctx context.Context, h restic.Handle, length int, offset
 	return err
 }
 
+// checkContentLength returns an error if the server returned a value in the
+// Content-Length header in an HTTP2 connection, but closed the connection
+// before any data was sent.
+//
+// This is a workaround for https://github.com/golang/go/issues/46071
+//
+// See also https://forum.restic.net/t/http2-stream-closed-connection-reset-context-canceled/3743/10
+func checkContentLength(resp *http.Response) error {
+	// the following code is based on
+	// https://github.com/golang/go/blob/b7a85e0003cedb1b48a1fd3ae5b746ec6330102e/src/net/http/h2_bundle.go#L8646
+
+	if resp.ContentLength != 0 {
+		return nil
+	}
+
+	if resp.ProtoMajor != 2 && resp.ProtoMinor != 0 {
+		return nil
+	}
+
+	if len(resp.Header[textproto.CanonicalMIMEHeaderKey("Content-Length")]) != 1 {
+		return nil
+	}
+
+	// make sure that if the server returned a content length and we can
+	// parse it, it is really zero, otherwise return an error
+	contentLength := resp.Header.Get("Content-Length")
+	cl, err := strconv.ParseUint(contentLength, 10, 63)
+	if err != nil {
+		return fmt.Errorf("unable to parse Content-Length %q: %w", contentLength, err)
+	}
+
+	if cl != 0 {
+		return errors.Errorf("unexpected EOF: got 0 instead of %v bytes", cl)
+	}
+
+	return nil
+}
+
 func (b *Backend) openReader(ctx context.Context, h restic.Handle, length int, offset int64) (io.ReadCloser, error) {
 	debug.Log("Load %v, length %v, offset %v", h, length, offset)
 	if err := h.Valid(); err != nil {
@@ -249,16 +288,10 @@ func (b *Backend) openReader(ctx context.Context, h restic.Handle, length int, o
 
 	// workaround https://github.com/golang/go/issues/46071
 	// see also https://forum.restic.net/t/http2-stream-closed-connection-reset-context-canceled/3743/10
-	if resp.ContentLength == 0 && resp.ProtoMajor == 2 && resp.ProtoMinor == 0 {
-		if clens := resp.Header["Content-Length"]; len(clens) == 1 {
-			if cl, err := strconv.ParseUint(clens[0], 10, 63); err == nil {
-				resp.ContentLength = int64(cl)
-			}
-			if resp.ContentLength != 0 {
-				_ = resp.Body.Close()
-				return nil, errors.Errorf("unexpected EOF got 0 instead of %v bytes", resp.ContentLength)
-			}
-		}
+	err = checkContentLength(resp)
+	if err != nil {
+		_ = resp.Body.Close()
+		return nil, err
 	}
 
 	return resp.Body, nil