cmd/stunstamp: fix remote write retry (#12348)

Evaluation of remote write errors was using errors.Is() where it should
have been using errors.As().

Updates tailscale/corp#20344

Signed-off-by: Jordan Whited <jordan@tailscale.com>

cmd/stunstamp/stunstamp.go

@@ -509,20 +509,24 @@ func (r *remoteWriteClient) write(ctx context.Context, ts []prompb.TimeSeries) e
 
 func remoteWriteTimeSeries(client *remoteWriteClient, tsCh chan []prompb.TimeSeries) {
 	bo := backoff.NewBackoff("remote-write", log.Printf, time.Second*30)
+	// writeErr may contribute to bo's backoff schedule across tsCh read ops,
+	// i.e. if an unrecoverable error occurs for client.write(ctx, A), that
+	// should be accounted against bo prior to attempting to
+	// client.write(ctx, B).
+	var writeErr error
 	for ts := range tsCh {
 		for {
+			bo.BackOff(context.Background(), writeErr)
 			reqCtx, cancel := context.WithTimeout(context.Background(), time.Second*30)
-			err := client.write(reqCtx, ts)
+			writeErr = client.write(reqCtx, ts)
 			cancel()
-			// we could parse the Retry-After header, but use a simple exp
-			// backoff for now
-			bo.BackOff(context.Background(), err)
-			if err == nil {
-				break
-			}
 			var re recoverableErr
-			if !errors.Is(err, &re) {
-				log.Printf("unrecoverable remote write error: %v", err)
+			recoverable := errors.As(writeErr, &re)
+			if writeErr != nil {
+				log.Printf("remote write error(recoverable=%v): %v", recoverable, writeErr)
+			}
+			if !recoverable {
+				// a nil err is not recoverable
 				break
 			}
 		}