From 9c8bbc7888d8886e1b47d1bf1fec151eecfd2b4d Mon Sep 17 00:00:00 2001
From: Colin Adler <colin1adler@gmail.com>
Date: Wed, 8 Jun 2022 19:22:16 -0500
Subject: [PATCH] wgengine/magicsock: fix panic in http debug server

Fixes an panic in `(*magicsock.Conn).ServeHTTPDebug` when the
`recentPongs` ring buffer for an endpoint wraps around.

Signed-off-by: Colin Adler <colin1adler@gmail.com>
---
 wgengine/magicsock/debughttp.go | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/wgengine/magicsock/debughttp.go b/wgengine/magicsock/debughttp.go
index 89164f21e..efdf01d96 100644
--- a/wgengine/magicsock/debughttp.go
+++ b/wgengine/magicsock/debughttp.go
@@ -172,6 +172,11 @@ func printEndpointHTML(w io.Writer, ep *endpoint) {
 				break
 			}
 			pos := (int(s.recentPong) - i) % len(s.recentPongs)
+			// If s.recentPongs wraps around pos will be negative, so start
+			// again from the end of the slice.
+			if pos < 0 {
+				pos += len(s.recentPongs)
+			}
 			pr := s.recentPongs[pos]
 			fmt.Fprintf(w, "<li>pong %v ago: in %v, from %v src %v</li>\n",
 				fmtMono(pr.pongAt), pr.latency.Round(time.Millisecond/10),