net/routecheck: tweak routecheck documentation and coding style

This patch is a follow-up for PR #19639 that does some cleanups.

Updates #17366
Updates tailscale/corp#33033

Signed-off-by: Simon Law <sfllaw@tailscale.com>

net/routecheck/probe.go

@@ -137,8 +137,13 @@ func (c *Client) probe(ctx context.Context, nodes iter.Seq[probed], limit int, t
 				metricPingError.Add(1)
 				return nil
 			default:
-				c.vlogf("ping %s (%s): result: %f ms (err: %v)",
-					n.addr, n.ID(), pong.LatencySeconds*1000, pong.Err)
+				if pong.Err != "" {
+					c.vlogf("ping %s (%s): result: %f ms (err: %v)",
+						n.addr, n.ID(), pong.LatencySeconds*1000, pong.Err)
+				} else {
+					c.vlogf("ping %s (%s): result: %f ms",
+						n.addr, n.ID(), pong.LatencySeconds*1000)
+				}
 				metricPingReachable.Add(1)
 			}
 

net/routecheck/report.go

@@ -26,7 +26,7 @@ var (
 )
 
 // Report returns the latest reachability report.
-// Returns nil if a report isn’t available, which happens during initialization.
+// It returns nil if a report isn’t available, which happens during initialization.
 func (c *Client) Report() *Report {
 	metricReport.Add(1)
 	nm := c.nm.NetMapNoPeers()

net/routecheck/routecheck.go

@@ -195,9 +195,9 @@ func (c *Client) Close() error {
 		return nil
 	}
 
-	ch := c.hasNetMap.Swap(nil) // clear before waking anything up
-	if ch != nil && *ch != nil {
-		close(*ch)
+	hasNetMap := c.hasNetMap.Swap(nil) // clear before waking anything up
+	if hasNetMap != nil && *hasNetMap != nil {
+		close(*hasNetMap) // wake waitForNetMap
 	}
 
 	return nil