From ca51529b81cc604bc5b57dfdbf3f67bdf61f7d5a Mon Sep 17 00:00:00 2001
From: Brad Fitzpatrick <bradfitz@tailscale.com>
Date: Fri, 12 Feb 2021 12:04:16 -0800
Subject: [PATCH] derp/derphttp: return nicer errors from Recv on Close

---
 derp/derphttp/derphttp_client.go | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/derp/derphttp/derphttp_client.go b/derp/derphttp/derphttp_client.go
index 87f11eca6..0c523f07e 100644
--- a/derp/derphttp/derphttp_client.go
+++ b/derp/derphttp/derphttp_client.go
@@ -709,10 +709,19 @@ func (c *Client) RecvDetail() (m derp.ReceivedMessage, connGen int, err error) {
 	m, err = client.Recv()
 	if err != nil {
 		c.closeForReconnect(client)
+		if c.isClosed() {
+			err = ErrClientClosed
+		}
 	}
 	return m, connGen, err
 }
 
+func (c *Client) isClosed() bool {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	return c.closed
+}
+
 // Close closes the client. It will not automatically reconnect after
 // being closed.
 func (c *Client) Close() error {